Art. II.—The Significant Features of Reef-bordered Coasts.
[Read before the Wellington Philosophical Society, 16th October, 1918; received by Editor 16th October, 1918; issued separately, 14th May, 1919.]
In recognition of the honour conferred by the New Zealand Institute in adding me to its list of honorary members, and in return for the kind reception given me at its meetings during my Pacific journey in 1914, I desire to offer the following notes for publication in its Transactions, in the hope that they may aid students of coral reefs in observing certain features of significance in connection with the origin of those extraordinary structures. References are appended to a number of my articles, the product of observation, reading, and reflection during five years past, where certain aspects of the coral-reef problem are treated more fully than they can be here.
Sea-level Coral Reefs are silent as to their Origin.—The corals and other organisms of a sea-level reef are truly of marvellous interest, and from a zoological point of view merit all the attention they have received; but when a reef is examined from a geological point of view its organisms are found to be reluctant, not to say incompetent, witnesses as to the manner of its formation. An observer may sail along the front of a reef, wander over its surface, or row about in its lagoon, and discover many facts regarding the varied forms of life there visible, and regarding the processes, organic and inorganic, now in operation; but, apart from such factors as the temperature and the depth of sea-water at which reef-building corals grow, he can learn little, if anything, about the past conditions under which the reef has been developed, so long as his study is directed to the reef alone.
On atoll reefs there are, indeed, no facts visible at the surface by which the various theories of the origin of coral reefs can be tested: it is only from borings in sea-level atolls or from natural sections of elevated atolls that competent testimony as to their origin can be gained. In this connection it may be noted that the interpretation of the Funafuti boring recently published by Professor E. W. Skeats, of Melbourne (1918),* gives a much better statement of its evidence as to the origin of that atoll than is to be found in the original report published by the Royal Society, which was almost silent as to the meaning of the facts that it set forth so minutely.
Fringing and barrier reefs are, on the other hand, associated with the coasts of land-masses, which may yield much information as to the past conditions and processes of reef-formation, if the geological structure and the physiographic development of the coastal slope are examined. For these reasons it is to the coasts of the land-masses which fringing or barrier reefs adjoin that attention is here chiefly directed.
Coasts of Emergence and of Submergence.
The general features of coasts on which coral reefs occur—either fringing reefs alone, or fringing reefs in the lagoons enclosed by barrier reefs—give helpful indications of the relative changes of level that the coasts have
[Footnote] * For references see p. 30.
suffered. Some coasts have a smooth seaward slope, and consist of imperfectly consolidated marine strata, dipping gently seaward, which have been little eroded since their emergence from the sea in which they were deposited: these are typical coasts of emergence. Other coasts, whatever their structure may be, exhibit forms of subaerial erosion, such as hills and valleys, the slopes of which appear to continue below sea-level, as if they had been partly submerged since they were eroded: these are coasts of submergence.
Coasts of Emergence. — Along coasts of emergence of the kind above specified the shore-line will generally be almost rectilinear or of simple curvature. The amount of emergence may be inferred from the altitude to which the marine strata rise along their inland border. It may be at once stated that coasts of this kind are seldom fronted by coral reefs, apparently because the loose sediments of their beaches and submarine slopes do not afford a suitable foundation for coral-growth: witness the Madras coast of India, the south coast of Java, and the west and south coasts of Borneo, all of which bear marks of sub-recent emergence. Another class of coasts of emergence, on which coral reefs abound, will be given special description below.
Young Volcanic Islands.—The coasts of young volcanic islands may be associated with coasts of emergence, especially if composed largely of ash and not of solid lava. They are frequently cliffed and beached, without reefs. Barren Island, east of the Andamans, in the Bay of Bengal, is somewhat cliffed, and but little fringed with corals. Réunion, in the western Indian Ocean, has reached a rather mature stage of erosion and abrasion, with a very imperfect development of fringing reefs, as will be further explained below. It therefore resembles certain strongly cliffed volcanic islands in temperate latitudes. Let it be noted that the cliffs of such islands are usually cut back by the waves at a faster rate than the valleys are cut down by their streams, so that the valleys are left hanging above sea-level, and their streams cascade down the cliffs to the beach.
Coasts of Submergence.—On coasts of submergence the shore-line will necessarily be irregular, advancing seaward around the outstanding points of partly submerged spurs and entering landward around the branching embayments of partly submerged valleys. Conversely, shore-lines of this kind indicate that the coasts which they border have been submerged, as Dana pointed out in 1849. Singularly enough, Darwin never perceived the value of this evidence in support of his theory (Davis, 1913).
The spur-ends of coasts of submergence in the coral seas usually offer excellent opportunity for the growth of fringing reefs, for their firm rocks are soon swept bare by the waves, and they are free from the detritus that accumulates in the bay-heads. If the submergence be slowly continued, a fringing reef, A (fig. 1), may be transformed into a barrier reef, B, by upward growth as the sea-level changes from S to T; but if the submergence be renewed at a more rapid rate, changing the sea-level from T to U, the barrier reef will be drowned, and, if a pause then occurs, a fringing reef of a new generation, G, will be formed, as will be more fully stated below.
Unconformable Reef Contacts.—In all cases of reefs bordering coasts of submergence the original fringing reef which forms the base of an upgrowing barrier reef, as well as the lagoon deposits within the barrier reef and the secondary fringing reefs that grow on the spur-ends of the lagoon shore, and also all fringing reefs of new generations, must rest unconformably on
an uneven foundation of subaerial erosion. This point has been too generally overlooked, although it is of the highest theoretical importance. Its converse is of practical value: reefs that rest unconformably on surfaces of subaerial erosion must have been initiated by submergence. Hence the nature of the contact of a reef and its foundation should be carefully observed, whether the reef be at sea-level or elevated above it.
Amount of Submergence.—The amount of submergence that an embayed coast has suffered is not well indicated by the depth of its embayments, for they may be much filled with sediments; the amount is better inferred by drawing a true-scale cross profile, as at P, fig. 2, of the spurs that enclose a bay-mouth, and continuing their slopes with decreasing declivity below sea-level until they meet. The visible cross-section of the valley above the
bay-head at Q should be taken as indicating the pattern of the submerged cross-section at the bay-mouth, P. The measure of submergence thus gained is only a minimum value, for, as shown in fig. 2, the depth of the submerged valley near the bay-mouth may be only about half the depth of the original valley-mouth, V.
Pre-submergence Period.—The duration of the pre-submergence period of subaerial erosion should be estimated as short, long, or very long, by comparing the actual form of the visible land-surface with its inferred initial form, due allowance being made for rock-resistance. In the case of dissected and embayed volcanic islands this comparison may often be made without much difficulty. On the coasts of continents and of continental islands the comparison may not be so easily instituted, but an attentive examination of the form of the coastal slopes will usually suffice to determine whether the cycle of erosion was in an early, middle, or late stage of its progress when it was interrupted by submergence.
Thus the submergence of the Queensland coast, in association with which the Great Barrier Reef of Australia and the discontinuous fringing reef in the broad lagoon were formed, did not occur until the rather resistant rocks which there prevail had been reduced to subdued forms of late maturity or even to the low relief of old age. The same may be said of much of the south-western coast of New Caledonia, except that the rocks there present near the shore are for the most part weaker than those of Queensland. In both these examples the pre-submergence period of sub-aerial erosion must have been of long duration.
In view of these various considerations it is evident that careful observation should be made of reef-bordered coasts from a physiographic as well as from a geological point of view, in order to determine whether the reefs have been formed in association with the submergence or the emergence of their foundation. It is also important that reef-free coasts in the coral seas should be similarly observed, in order to discover the conditions that do not favour reef-formation.
Rate of Submergence.—The ordinary statement of Darwin's theory of coral reefs implies that the rate at which reef-foundations have been submerged as a result of their own subsidence must not be greater, but may be less, than the rate of reef-upgrowth; and this has been held to be an improbable condition. Darwin's own statement of the problem made no such limitation as to the rate of subsidence, except where barrier reefs and atolls are actually found. For those reefs he stated that relatively rapid subsidences of small amount alternating with long stationary pauses probably represent the ordinary succession of events, and he believed that the average rate of submergence thus determined was not in such cases faster than the rate of reef-upgrowth.
This seems to hold true for the greater part of the open Pacific, where atolls and barrier reefs prevail, even though the submergence due to insular subsidence there has been accelerated by a sub-recent rise of ocean-level during the melting of the Pleistocene ice-sheets—a matter which has come into importance in recent years, as will be shown in more detail below. But exception to this statement is needed for an area to the north of the Fiji Group, where fifteen or more submarine banks, apparently submerged reefs or “drowned atolls,” have been discovered since Darwin's time; and also for the region of the Tonga Islands, where extensive submarine banks occur. In both these regions of the mid-Pacific, and in a few others, submergence appears to have taken place at a faster rate than reef-upgrowth. They thus correspond to a large part of the Indian Ocean, where submarine banks, apparently “drowned atolls,” prevail, as Darwin clearly understood.
Darwin on Fringing Reefs. — Furthermore, although Darwin regarded most fringing reefs as having been formed on stationary or on rising coasts, he clearly understood that rapid subsidence might drown earlier-formed reefs, whereupon the reefs that would grow on the new shore-line would be of the fringing class, as noted above. The statement of this point on page 124 of his Coral Reefs (1842) deserves attentive reading. True, inasmuch as Darwin did not understand that embayed shore-lines and unconformable reef contacts around spur-ends are sure signs of submergence, he discovered no examples of fringing reefs of this kind in the records that he studied, and all the fringing reefs on his chart are classed as occurring on stationary or rising coasts.
But his deductive expectation may now be confirmed, for the Australasian and other archipelagoes contain numerous examples of fringing reefs
unconformably contouring around the spur-ends of embayed coasts—witness Palawan, the south-westernmost member of the Philippines, and many other embayed islands in that group—well represented in recent charts of the United States Coast and Geodetic Survey; also the Andaman Islands, in the Bay of Bengal; for in all these examples the coast is elaborately embayed; and hence their fringing reefs must be unconformable, and their submergence must have taken place at a faster rate than reef-upgrowth. Many other examples of the same kind might be cited.
Fringing Reefs and Submarine Platforms.—Fringing reefs thus assume a much greater interest than is generally allowed to them: their relations to the features of the coasts they border deserve close attention. The breadth of the reefs should be noted as a means of estimating the time that has elapsed since the last movement of submergence took place. The off-shore soundings of reef-fringed coasts of submergence are also of importance, for they frequently reveal a submarine platform that in all probability represents a drowned barrier reef and its lagoon.
Such submarine platforms, several miles in width, are found in association with Palawan and the Andamans, although the sea-level fringing reefs of these islands are narrow. A well-developed submarine platform surrounds the greatly denuded “volcanic wreck” of Fauro, a small island with, narrow fringing reefs in the Solomon Group. A similar platform is shown by the latest surveys of the United States Hydrographic Office to surround the Samoan island of Tutuila; but the fact that the spur-ends of this island are rather strongly cliffed behind their fringing reefs distinguishes it from the other examples named. Submarine platforms occur around the Marquesas Islands also; but here, although the spur-ends are cliffed, as in Tutuila, they are not fronted by fringing reefs.
The depth of the submarine platforms off reef-fringed shores is not constant: along the west coast of Palawan the platform varies in depth from 25 or 30 fathoms near its southern end to 60 fathoms near its mid-length; the Fauro platform has depths of 70 or more fathoms; the Andaman platform is 30 or 40 fathoms in depth. On the other hand, part of the coast of Samar, in the Philippines, facing the open Pacific, has fringing reefs around its headlands, but its submarine slope descends rapidly to great depths. Now, let it be noted, first, that the three chief elements of the fringing-reef problem as here considered—duration of the pre-submergence period of subaerial erosion, rate and amount of submergence, and duration of post-submergence period of fringing-reef growth—have unlike values on different islands; secondly, that many other islands have well-developed barrier reefs which suggest slow submergence, and that some barrier reefs are broad and others are narrow, thus suggesting that the rate and date of their submergence are unlike; and, thirdly, that many elevated reefs occur at different altitudes and in different stages of erosion.
It thus becomes evident that the history of various reef-encircled islands must consist of unlike sequences of movements and pauses. Hence local movements of the reef - formations themselves, which may vary greatly, explain the varied facts much better than changes of ocean-level, which must everywhere be of the same rate, date, and amount. In order to learn how greatly the values of the various elements differ from place to place, their value for every coast should be determined independently. One of the most important of these elements is the
duration of the post-emergence or post-submergence stationary period, the estimation of which may now be considered in some detail.
Time since Emergence.—The shore-line of an emerged and thence-forward stationary coastal plain may be locally built forward, or “prograded,” by deltas if its rivers are of large volume and well charged with detritus from an elevated backland; and sand reefs enclosing shallow or marshy lagoons may be cast up by the waves between the deltas, and may advance seaward as the delta-fronts advance. Conditions of this sort appear to prevail along the Madras border of India, and around the south-west side of Borneo, thus proving that these coasts have been somewhat changed from their simpler initial form; but the littoral conditions are still manifestly unfavourable to coral-reef formation.
It is conceivable, however, that after a temporary supply of gravel and cobbles has been washed out by a flooded river to a certain part of the front of a delta that is for the most part composed of finer sediments the river may change its course, as rivers on deltas are prone to do. Then corals, attaching themselves to the larger cobbles, may spread sufficiently to form a small fringing reef, until a return of the river buries the corals. A buried reef of this kind will slant forward with the delta-front, and will lie conformably between the earlier and later foreest delta-beds. Such seems to have been the origin of a small elevated reef near Suva, Fiji: it lies on a local deposit of gravel, and both the gravel and the reef lie conformably in the slanting beds of volcanic mud, there known as “soapstone.”
The extent of the littoral lowland that is prograded along the border of a coastal plain will give some idea of the time that has elapsed since the plain emerged. But such lowlands are not always developed; for, if large rivers are wanting, the shore-line of a coastal plain may be cut back or retrograded farther and farther by the sea, as long as no change of level takes place. The farther it is cut back, the higher will be the resulting bluffs along the coastal-plain margin. The height of the bluffs along the shore of a retrograded coastal plain will therefore give an indication of the time during which it has been attacked by the sea. A more important point is that, however far such a stationary coast may be retrograded, a beach of loose detritus, continued off shore by a sheet of finer sediments, will, according to accepted physiographic theory, always cloak the abraded platform along the base of the retreating bluffs. No reefs are therefore to be expected on such a coast.
The Reef-free Coast of Madras.—It is important that the coasts of the coral seas should be examined with these principles in mind in order to test their correctness. As far as I have read, there is no published account of a strongly retrograded coast in the torrid seas that is still suffering abrasion in its original stand with respect to sea-level. It is interesting to note, however, that the high, hard-rock cliffs which, as described by Cushing, rise a short distance inland on the coast of Madras appear to have been cut back by the sea before the emergence of the present Madras coastal plain; hence the cliffs must, before the sub-recent movement of emergence by which a negative shift of the shore-line was caused, have exemplified a maturely retrograded, reef-free coast; and at the beginning of their abrasion the hard-rock land-mass must in all probability have been covered, near its shore-line at least, with the sediments of an ancient coastal plain of emergence, just as the emerged platform of marine abrasion which fronts the high cliffs is covered by a
modern coastal plain to-day; for otherwise it is difficult to understand why coral reefs should not have been formed there and have prevented the cutting of the high cliffs.
Cliffed Volcanic Islands.—It has been suggested above that the shore-lines of volcanic islands may be regarded as shore-lines of emergence, particularly if the island is largely composed of loosely compacted volcanic ash; for such a shore-line will be of comparatively simple outline, without pronounced salients or embayments, and the detritus washed down its slopes by its streams, added to that cut by the waves along the shore, will soon form a continuous beach, extending seaward in a sheet of loose sediments, on which reef-building corals cannot attach themselves (Davis, 1916B). Under such conditions the island will be continuously attacked by the waves, cliffs will be cut around its shore while valleys are eroded in its slopes, and if the island stand still long enough it will be completely truncated. Even then it may be difficult for corals to find a firm foundation for their growth until nearly all the loose detritus is swept off the surface of truncation.
According to Admiral Wharton, atolls were supposed to have been built up around the margin of truncated volcanic islands, no change of sea-level and no subsidence of the island being postulated. Darwin had previously considered this possibility and rejected it, because the resulting lagoons would be too shallow. According to Daly, atolls are supposed to have been built up on volcanic platforms that were abraded while the ocean was lowered and reef-building corals were killed, during the Glacial period. The best test of these suppositions involves a series of borings along the diameter of an atoll to a depth of 50 or more fathoms below present sea-level: the elevated atolls of the Loyalty Islands are to be recommended for such examination.
If it be true, as above suggested, that still-standing volcanic islands may, in the absence of protecting reefs, be cut away by the sea, a number of examples in different stages of abrasion should be found in the coral seas of to-day. Réunion is the best example of the kind, still in process of abrasion, that has come to my attention. Tahiti is an equally good example, but it has been somewhat submerged, and its shores are now defended by coral reefs, as will be more fully described below. Tutuila, in Samoa, and the Marquesas Islands probably, as noted above, belong to this series, but their place cannot be safely determined at present. Most volcanic islands in the coral seas are surrounded by barrier reefs, and their shore-lines are not cliffed. It is very desirable that all islands of the coral seas should be examined with the points here set forth in mind. The brief accounts now available of many such islands do not suffice to determine what stage of erosional and abrasional evolution they have reached.
Time since Submergence.—The headlands of coasts of submergence in temperate latitudes, not being defended by coral reefs, are vigorously attacked by storm waves; thus a cliff is formed rising high above sea-level, and a platform lying a little below sea-level. Coasts of submergence in the coral seas are as a rule fronted by barrier reefs or bordered by fringing reefs; hence they do not generally show the strongly cliffed spur-ends that characterize similar coasts in temperate latitudes. True, the spur-ends of such coasts are often cut off in low bluffs, B (fig. 4), 10 ft. to 50 ft. in height, forward from which one may see low-tide rock platforms 30 ft. to 100 ft. in breadth before one reaches the fringing reef, F, that is ordinarily found in such situations.
The absence of talus at the base of such bluffs shows that they are still washed by storm waves, but the strength of attack is not great. Indeed, it is probable enough that the greater part of the abrasion that these low spur-end cliffs attest was accomplished by waves that rolled in, little impeded, from the open ocean, shortly after the last period of submergence and before the present barrier reef was built up to sea-level. This implies that the submergence was rapidly accomplished and was followed by a pause. The form of the abraded rock platform on many spur-ends supports that view; for its outer border is about in line with the sloping ridge-crest above the cut-off bluff, as it should be if all the abrasion had been accomplished since submergence took place. If some abrasion had been accomplished during the progress of slow submergence the outer border of the platform could not be so nearly in line with the ridge-crest. Detailed observation of spur-end bluffs and platforms is therefore desirable.
If the above suggestion as to the origin of the spur-end bluffs be correct, their height will not be so good an indication of the time since submergence as may be found in the breadth of the fringing and barrier reefs, or in the area of the bay-head deltas. If the reefs are narrow and the deltas are small (due consideration being given the drainage-area and slope of their streams) the latest submergence must be recent; if the reefs are broad and the deltas are large enough to fill the embayments the submergence must be less recent, though by no means of ancient date. Careful record of all these features should be made.
The transformation of original discontinuous fringing reefs on the spur-ends of a slightly submerged coast into a nearly continuous barrier reef a mile or more outside of a more deeply submerged coast involves the circumferential extension of the fringing reefs as they grow upward, so that they shall close most of the broad breaches that would otherwise mark the sites of the original embayments. The barrier reef on the eastern side of Tahiti is a good example of discontinuous growth still interrupted by wide passages; on the western side the reef is more continuous. It may be noted in this connection that barrier reefs in Fiji are, unlike those of Tahiti, more generally interrupted on their leeward than on their wind-ward side: this may be due to the injury to coral-growth caused by the drift of fine sediment in the lagoon-waters.
The depth of a barrier-reef foundation, or the amount of submergence that has taken place since the barrier reef began its growth as a fringing reef, cannot be determined readily, because it is impossible to say whether the upgrowth of the reef has taken place vertically or on an inward or an outward slant. The rate of submergence appears to be the chief factor in determining the angle of upgrowth (Davis, 1916C). The vertical depth from a barrier reef to the underlying rock can be more safely estimated, because the submarine slope of the foundation mass can often be fairly well inferred.
It is not necessary here to proceed merely on the empirical principle, introduced by Dampier and followed by Darwin, to the effect that “a considerable degree of relation subsists between the inclination of that part of the land which is beneath the water and that above it,” for it is now possible to infer the declivity of a submarine slope more reasonably by observation of the structure and form of the land-mass
above sea-level. In the case of many volcanic islands it is not unreasonable to conclude that a barrier reef a mile from the island-shore has a vertical thickness of 1,000 ft. This conclusion evidently rejects the idea that a barrier reef is built upon a shallow platform of non-reef origin, which appears to me as improbable as that a volcanic island rests upon a shallow foundation of non-volcanic origin.
Mature Reef Plains.—Although coasts of recent submergence usually present favourable conditions for the growth of fringing or of barrier reefs, these conditions may not persist indefinitely on coasts that long remain stationary after a less recent submergence; for, if the land drained by the coastal rivers is of large-enough area, deltas, E (fig. 3), will in time not
only fill the drowned-valley embayments of a still-standing island, but will unite around the spur-ends and form a confluent alluvial lagoon plain, F, with a shore-line of comparatively simple pattern. As the advance or the progradation of such a plain continues, the fringing reefs on the spur-ends will be smothered with detritus. Such appears to have been the fate of many fringing reefs on the island of Tahiti, where an alluvial plain extends along much of the island-border. As the plain is still farther prograded, and as overwash of debris from the outer face to the inner slope of the outgrowing reef continues, the lagoon will be filled and converted into a mature reef plain, MP.
If the outwash of alluvium still goes on, the barrier reef, R, in spite of the width it may have then attained by outward growth, will be smothered, and its corals will be killed. Thereupon the sea will attack the reef and cut it away; and if this process be once begun there appears to be no reason for it to stop. The alluvial reef plain must in time be consumed, and then the central island will be attacked and cliffed; for as long as the island stands still, and as long as outwashed alluvium supplies material for a beach, coral growth cannot be re-established (Davis, 1917A). This sequence of events is evidently hypothetical in a high degree; nevertheless, the successive stages of such a sequence, and of all other reasonable sequences, should be carefully conceived by an observer of coral reefs while he is still on his voyage of investigation, in order that he may be able to confront the successive stages of the various sequences with the reefs that he sees, and thus discover which sequence gives the best history of their origin.
The danger of being buried and smothered in alluvium appears to threaten the barrier reef along the south coast of Viti Levu, Fiji, where the delta of the Rewa River has almost filled the lagoon; and a long stretch of the barrier reef on the south side of New Guinea appears to be
already smothered where the great delta of the Fly River has advanced far into the sea.
Most barrier reefs are, however, in no immediate danger of such a fate; their lagoons are far from being filled with alluvium; deltas, indeed, as a rule do not fill the embayments in which they head. Both the Rewa and the Fly are exceptional in being much larger than the rivers of most islands within barrier reefs. The prevalence of open lagoons shows that no such constant relation of land and sea level has been maintained as was provisionally postulated above, but that submergence has prevented lagoon-filling not only by providing new depths to be filled up, but also in the case of islands by diminishing the area and the altitude of the land from which part of the filling-material should come.
On the other hand, in view of the many possible changes of land and sea level, and of their many possible combinations with periods of rest, it is surprising that, among the many examples of reef-encircled islands, none are found with mature reef plains approaching or realizing the stage of smothering the corals on the reef-face. For if the development of barrier reefs depended only on the subsidence of their foundations, and if their foundations were of different ages and had subsided by different measures and at different dates, we should expect to see all stages of reef-development to-day—some close-set, discontinuous barriers; others broader and farther from their central island, the embayments of which should contain good-sized deltas; and so on through all the stages to a mature reef plain, the alluvium of which is just overlapping the reef; and then an old reef plain, much reduced from its original breadth by abrasion. But, with such exceptions as the Rewa and Fly deltas, no mature reef plains are known.
Combination of Island-subsidence with Changes of Ocean-level.—The absence of completed reef plains cannot be due to lack of detritus for their formation, for the amounts of detritus that have been discharged from many deeply denuded volcanic islands are vastly greater than the volumes of the lagoons enclosed by their barrier reefs. Hence the prevalence to-day of young barrier reefs with open lagoons must be taken as suggesting that some recent and widespread cause has produced a more general submergence than should be expected from island-subsidence alone; and this cause is perhaps to be found in the post-Glacial rise of ocean-level, for a rise of ocean-level combined with a prevalent but intermittent subsidence of reef-foundations would tend to maintain the barrier reefs of to-day in an early stage of their development and prevent the attainment of the more mature stage which they would reach during a long period of fixed levels of islands and ocean.
On the other hand, a fall of ocean-level, such as must have accompanied the oncoming of the last glacial epoch, would have tended to lessen or even to neutralize the submergence due to prevalent subsidence; hence during a glacial epoch lagoons may have been more generally filled than during an interglacial epoch or during the present post-Glacial epoch (Davis, 1915, p. 267; 1916A, p. 565). These somewhat transcendental aspects of the coral-reef problem are mentioned here in hopes that they may incite special observations, by means of which the possibilities here sketched may be assigned their proper values.
That the post-Glacial rise of ocean-level is not the entire or even the chief cause of the submergence under which barrier reefs as well as unconformable fringing reefs have been developed is proved by the great diversity
in the succession and the value of the movements and pauses recorded by the physiographic features of the islands within the reefs, as has already been briefly noted in an earlier paragraph, and as will be set forth more fully in a later one.
Origin of Lagoons.
According to Darwin's theory of intermittent subsidence, the lagoon occupying the depression or “moat” between an island slope and an upgrowing barrier reef is more or less completely filled by the outwash of detritus from the central island, by the inwash of debris from the reef-face, and by the accumulation of locally formed organic material. Recent observations warrant the assignment of a large value to the last-named process, which is further important because it is just as effective in aggrading a large lagoon as a small lagoon. If subsidence cease for a long period, the lagoon may be converted into a reef plain, as suggested in a preceding section.
This view of the relation between barrier reefs and their lagoon is the very opposite of that implied in Murray's theory of outgrowing reefs on non - subsiding foundations, for it is there postulated that lagoons are formed by the solution of the outgrowing reef along its inner border. There can be no question that sea-water flows into lagoons in sufficient quantity to dissolve away a large volume of limestone; but, as far as observational evidence goes, the loss thus occasioned is far overbalanced by the supply of new detritus from the various sources above mentioned.
According to Murray's solution theory, the inner slope of a barrier reef should consist of ragged and decaying limestone, and the lagoon-floor should be covered with insoluble silts (Davis, 1914, p. 641); but as a matter of fact the inner slope of barrier reefs usually consists of white coral sand, washed in from the outer reef-face; and the lagoon-floor is covered with accumulating calcareous deposits, except that near the deltas of large streams inorganic deposits preponderate. Detailed observations should be made by dredging in lagoons in order to test the generality of the above statements. Atoll lagoons deserve as much attention as barrier-reef lagoons in this phase of the problem.
Attention may here be called to Vaughan's view that many barrier reefs are built upon platforms which were produced by other than coral-reef agencies. Inasmuch as the supposed platforms beneath sea-level reefs are not open to direct observation, their existence as structures independent of reef-forming agencies is for the present only an inference. Any observable facts that bear on this aspect of the problem should be carefully noted. Among such facts pointed out by Vaughan, three may be noted: the first is that the exterior profile of most reefs shows a change from a moderate slope to a steep pitch at a depth of about 40 fathoms; the second is that reefs ocasionally stand a short distance back from the outer margin of a 40-fathom bench; the third is that where reefs are breached, as frequently happens on their leeward side, the lagoon-floor or “platform” continues.
It may, however, be reasonably urged that none of these facts necessarily leads to the conclusion that the production of a platform by some agency independent of reef-formation preceded the formation of the present reefs. As to the change from a gentle slope to a steep pitch in the exterior profile, many observers, including Darwin, Murray, and Gardiner, are agreed that this is the result of wave-action on reef detritus at present sea-level: the exterior slope of a reef is, in effect, a small “reef shelf,” corresponding
to the great continental shelves in origin, though not in size. The continuity of the “platform” where a surface barrier reef is wanting, especially on the leeward side of its circuit, may be readily explained as resulting from the continued action of reef-building and lagoon-flooring processes, during subsidences of varying rates and pauses of varying duration, under the influence of prevailing winds. The location of a reef a short distance back of a platform-margin is perfectly consistent with the production of the platform as a mature reef plain, afterwards submerged, and by no means demands that the platform shall have been made by processes in which reef-growth had no part. The discussion of this point by observers on reef-encircled islands is much to be desired.
Partly Emerged Coasts of Submergence.
A peculiar class of coasts of emergence, mentioned above as needing special consideration, includes mountainous land-borders that have partly emerged shortly after a greater submergence at too rapid a rate for the upgrowth of barrier reefs. They are characterized by irregular shore-lines with many salients and re-entrants, on which a comparatively thin cover of marine deposits accumulated during their brief submergence hardly conceals the hill-and-valley topography that was produced during a previous and much longer pre-submergence erosional period. On such a shore-line the unconsolidated marine deposits are soon worn away from the headlands during pauses in the emergence, so that unconformable fringing reefs may be formed there. If emergence continue intermittently, the fringing reefs will appear as terraces on the emerged slopes.
Coasts of this kind appear to be of importance in the coral-reef problem, because they are found to be of frequent occurrence on the deep-water shores of the Australasian region, where unconformable fringing-reef terraces are reported on many inlands that have embayed shore-lines. It is nevertheless a mistake to conclude, without further question, that all such terracing reefs have been formed during pauses in emergence, although this conclusion has nearly always been adopted by their observers. Such a conclusion tacitly postulates that the previous submergence took place at so rapid a rate that no reefs were formed during its progress. Yet it is evidently equally conceivable that the reefs may have been formed during pauses in a slow submergence and revealed by a rapid emergence (Davis, 1916A, p. 499). Discrimination between the two conditions of origin may be made if the structure of the emerged reef is laid bare by erosion, as will be shown in a later section. In any case, coasts of this kind merit special attention as indicating a pronounced instability.
Partly Submerged Coasts of Emergence.
Just as the rule that coasts of emergence are unfavourable to reef-growth is departed from in the case of steep coasts that are partly emerged after a brief submergence, so the rule that coasts of submergence are favourable to reef-growth is departed from in the case of gently sloping coasts that are moderately submerged after a long emergence, during which the adjoining sea-bottom was shoaled by the accumulation of sediments; for on such coasts the waves will sweep in so much sediment from the shallow bottom that any corals which may for a time attempt to grow on the headlands will soon be smothered and killed. The scarcity of reefs on those islands of the Australasian archipelagoes that have embayed shore-lines fronted by
shallow seas may perhaps be thus explained; but, as this aspect of the problem has been little considered by observers on the ground, the chief object of this paragraph is to stimulate local and critical observation rather than to announce an assured conclusion.
Clipped Coasts, partly submerged.
Since the proposal of the glacial-control theory of coral reefs (Daly, 1910; 1915) it has become important to note whether the spur-ends of embayed coasts inside of fringing or barrier reefs are cut off in cliffs that descend steeply below sea-level. That theory assumes that mid-Pacific volcanic islands have long stood still, and that their embayments occupy valleys which were eroded while the ocean was lowered during the Glacial period. It assumes furthermore that the lowered ocean was chilled sufficiently to kill the corals and other organisms of coral reefs, so that the reefs would be cut away, probably at some such level as 40 fathoms below the present ocean-surface.
Now, if these assumptions are correct, it follows that an embayed island, like Murea in the Society Group, which is now surrounded by a barrier reef about half a mile from the shore, must, after its corals were killed and its reef was cut away by the waves of the lowered sea, have been strongly cut back in cliffs; for, if the sea were actively abrading the island during a period long enough for the excavation of the open valleys now occupied by arms of the sea, the spurs between the valleys must have been cliffed. Be it remembered here that, according to the testimony of volcanic islands in the temperate oceans, the retreat of cliffs under the attack of sea-waves is more rapid than the deepening of valleys by streams, and hence all the more rapid than the slow widening of valleys by the weathering of their side slopes. Rock-resistance need not be considered, for it will affect cliff-cutting and valley-widening in similar fashion.
It is evident, therefore, that close attention should be given to the forms of spur-ends where they disappear in the lagoons of barrier reefs, particularly where the barrier reefs are not far off shore. If the spurs are cut off
in bluffs, B (fig. 4), from 10 ft. to 50 ft. in height, in front of which rock platforms extend from 30 ft. to 100 ft. forward, such bluffs and platforms must be attributed to wave-action at present sea-level, as has been explained above.
If, on the other hand, high spur-end bluffs or cliffs, LH, are found but no rock platforms are visible in front of them, and if (except for a narrow fringing reef) the lagoon has depths of 10 or 20 fathoms near the cliffed spur-ends, then the cliffs should be attributed to wave-action producing a profile HCP when the sea was lower or the land was higher than now. As
far as I can learn, low spur-end bluffs fronted by narrow rock platforms are of much more common occurrence than strong spur-end cliffs that plunge, except for their fringing reefs, into comparatively deep water. But fortunately certain islands possess strong spur-end cliffs fronting on deep lagoons: such islands deserve particular attention.
The Half-submerged Cliffs of Tahiti.—Tahiti, the largest island of the Society Group, is a volcanic doublet—that is, a larger and a smaller cone connected by an isthmus—now submaturely dissected by radial consequent valleys. In the central areas, where the valleys are close together and of great depth, the initial surface of the cones is lost; but it is well preserved in the peripheral areas, where the valleys are more widely separated and of moderate depth, as Dana long ago explained. The inter-valley spurs are, except at the north-western (or leeward) corner of the large cone, cut back in cliffs, which on the windward coasts rise 500 ft. to 1,000 ft. above present sea-level. Agassiz is, as far as I have read, the only observer of this beautiful island who has recognized the prevalence of cliffs around its shores.
Many of the smaller valleys are not cut down to present sea-level; their wet-weather streams fall in cascades from cliff-top notches. The larger valleys have been cut to a greater depth, for they descend below sea-level, and their mouths are occupied either by small arms of the sea or by delta-plains. The island is to-day bordered either by a fringing reef or by an alluvial plain, which is formed by the confluence of many delta-plains that have outgrown their valley-mouth embayments. Moreover, a somewhat discontinuous barrier reef now holds off the waves from most of the island circuit. Evidently, then, the cliffs have not been cut while the island has stood at its present level: they must have been cut when it stood relatively higher—that is, when the valleys were in process of deep erosion beneath present sea-level. Evidently, also, no reefs could have been present when the cliffs were cut.
The question then arises whether Tahiti stood still and had its cliffs cut while the ocean was lowered and chilled during the Glacial period, or whether Tahiti, besides experiencing changes of ocean-level in the Glacial period, itself subsided after cliffs had been cut around it, the cliffs having been formerly cut around Tahiti for the same reason that cliffs are now cut around Réunion—namely, because reef-forming corals cannot establish their colonies on the cobbles and gravels of the beaches that are ordinarily developed around the shore of a young volcanic island.
The latter alternative appears the more probable one of the two, for two reasons. First, the amount of the submergence by which the Tahitian valleys have been submerged appears to be 500 ft. or 600 ft. at least, and this is much more than the amount of lowering that the ocean is believed to have suffered during the Glacial period. Secondly, if the cliffs of Tahiti were cut around a still-standing island by the waves of the lowered and chilled ocean during the Glacial period, then the neighbouring island of Murea, as well as the other more distant members of the Society Group, should also be cut back in cliffs; but, apart from a few very exceptional cliffed spur-ends, that is not the case. The reefs of Tahiti should therefore be regarded not as having found their opportunity for upgrowth when the warming waters of the post-Glacial ocean were rising to their present level, but as having found their opportunity when submergence, caused in part at least by subsidence, embayed the island valleys so that the stream-washed detritus was pocketed in the embayments. In the absence of detritus the
wayes washed the cliffs and the rock platform in front of them bare, and this gave the corals a firm foundation on which to attach themselves.
If this interpretation be correct, the cliffs and reefs of Tahiti are not only beyond explanation by the glacial-control theory, but the island is a strong witness against the theory. It testifies that a reef-free island may be strongly cliffed in a time that suffices only for the erosion of steep-sided valleys: hence other islands, like Murea, Raiatea, and Huaheme, in the Society Group, in which the submerged valleys are much less steep-sided than those of Tahiti, ought to have been more strongly cliffed than Tahiti if they were reef-free while their now-submerged valleys were in process of erosion. The fact that they are not cliffed shows that they must have been protected by living reefs, and thus discredits the assumption that reef-corals were killed during the Glacial period. The possible less resistance of the lavas on Murea and the other islands than on Tahiti does not affect the argument, for if the Murean valleys are wider than the Tahitian valleys because the rocks of Murea are weaker than those of Tahiti, then for the same reason the spurs of Murea ought to be cut back in cliffs of greater height than those of Tahiti.
The reason for giving a special account of Tahiti is that, among the many reef-encircled volcanic islands of the Pacific, it is unique in being cut nearly all around its circuit by strong cliffs the bases of which are now below sea-level. Similarly, as noted above, Réunion is unique among islands in the coral seas in being cut all around by cliffs the bases of which are at the sea-level of to-day and are now undergoing attack by the sea in the absence of protecting reefs. Two intermediate stages are represented by the Marquesas and Tutuila (Samoa), which have submerged cliffs but are not surrounded by barrier reefs. Of these two stages, Tutuila is the later, because it has well-developed fringing reefs, while the Marquesas are reef-free. Search for other islands of the Réunion, Tahiti, and intermediate types is evidently desirable, for it is manifestly unsafe to generalize on a few examples. Yet, inasmuch as these few examples confirm each other, one is tempted to ask whether they do not show the typical stages of early development through which many deeply-dissected, reef-encircled volcanic islands have long ago passed—that is, whether many deeply-dissected, reef-encircled volcanic islands would not show reef-buried cliffs and platforms on their submarine slopes if they could be examined.
In a group of phenomena which offer few examples of early stages and many examples of later stages of development it certainly seems reasonable to regard the examples of later stages as having passed through the stages represented by the early examples, particularly when the early examples present the very features which a deliberate analysis of the problem leads one to regard as essential preliminaries to the features of the more advanced examples. This interpretation appeals strongly to me, because, instead of empirically entering the problem of reef-encircled islands at a middle stage of progress, the attempt is made to trace out all the stages of the problem from beginning to end.
On the other hand, many students of coral reefs may regard it as fanciful, not to say fantastic, to say that the cliffs which are still in process of abrasion on Réunion, and which are partly submerged and fairly well protected from wave-attack on Tahiti, are probably the counterparts of similar cliffs now completely submerged on the reef-buried lower slopes of many other volcanic islands. But another aspect of the problem deserves consideration before a decision on this question should be declared.
The Vanished Detritus of Deeply Denuded Islands.
Many volcanic islands, now deeply denuded in irregular forms, give clear indication of their initial conical form in the outward slant of their marginal lava-beds. It is in such cases a comparatively simple matter to reconstruct their original cone, VW (fig. 5), and to estimate the volume of detritus that has been removed in reducing the island to its present maturely denuded form, RM. Even if no submergence be assumed, the volume of detritus that has been carried away from so much of the initial volcanic mass as is now above sea-level is, as noted above, vastly greater than the volume of the lagoon waters, G, on all the reef-encircled islands that I have seen. How has this great volume of detritus been disposed of?
Let the island be supposed to have been formerly more emerged than now, and let it stand still with respect to sea-level, SC, during a period of deep dissection. Under these conditions the detritus washed out from its valleys would soon completely overwhelm any fringing reef that might by chance be established on its shores, and the waves would then cut cliffs, CL, all around its circuit, as is now the case on Réunion. This consideration alone is sufficient to discredit Murray's theory of outgrowing reefs on still-standing islands. Moreover, if the island stand still, cliff-cutting will continue and no opportunity for barrier-reef formation will be allowed. Under what conditions, then, is the formation of barrier reefs permitted?
An apparent escape from the difficulty of accounting for the vanished detritus around a still-standing island is found in changes of ocean-level during the Glacial period; for the detritus discharged while the ocean stood at a lower level than now would be deposited on the lower slopes of the island, and when the ocean rose again a barrier reef might grow up with it. But during the discharge of the detritus reefs could not flourish, and waves would then cut the island-shores back in cliffs; and if cliff-cutting endured through the time required to excavate the valleys now drowned in embayments the cliffs would surely be high enough to be still visible after the ocean has resumed its normal level. Hence the amount of submergence thus provided is insufficient for the needs of the problem. Moreover, all volcanic islands the eruptional growth of which was completed earlier than the beginning of the Glacial period should have had cliffs cut around their margin in pre-Glacial time, and some trace of these cliffs should now be found. Another supposition must therefore be made, as follows:—
If an island, VW (fig. 6), with sea-level originally at NV, does not stand still, it must subside to a great depth, NS, if no cliffs are to be cut around
its margin and if the larger part of its discharged detritus is to be deposited in the lagoon, G, of an upgrowing barrier reef, B; but in this case the early stages of subsidence must be so rapid, in order to provide sufficient lagoon-space for the deposition of detritus, that the upgrowth of a reef could hardly keep pace with it. It is not likely that the numerous barrier reefs of to-day have all survived so threatening a danger: hence a slower rate of early subsidence must be postulated.
Let the island, therefore, stand almost or quite still during a considerable period after its eruptive growth ceases. In this case the detritus supplied by the erosion of deep valleys, CY (fig. 5), and by the abrasion of high cliffs, CL, will be swept off shore in large amount, D, by vigorous waves, unimpeded by a barrier reef; then, if intermittent subsidence begin, placing sea-level at TE, the further discharge of detritus will be detained in the embayed valleys, E, and reef-upgrowth may begin. But, as under these conditions strong cliff-cutting will have accompanied the erosion of deep valleys, a considerable measure of subsidence, placing sea-level at UV, will be eventually necessary to submerge the cliff-tops, L, if they are not seen to-day. Whether this supposition represents the actual history of reef-encircled islands or not, it certainly provides a more reasonable condition for reef-growth than any other supposition here considered.
Various combinations of diverse conditions may be imagined. For example, the succession of events may be as follows: (1) Moderate cliff-cutting during a still-stand period before reefs are developed; (2) moderate submergence and reef-upgrowth; (3) a second still-stand period, resulting in the smothering of reefs by outwashed detritus, and renewal of cliff-cutting; (4) further subsidence and renewed reef-growth. Tahiti seems now to be approaching the third phase of this succession, for, if the present still-stand that is attested by the alluvial lowland around the island border endures as long as the earlier reefless period of valley and cliff-cutting, the lagoon will be overfilled, the smothered reefs will be abraded, and a new attack will be made by the waves on the cliffs at a higher level than before.
In any event, the only way of developing a barrier reef around a deeply dissected and non-cliffed volcanic island seems to be either to allow it to subside rapidly to a great depth while its reef is growing up, or to allow it to subside to a less depth after strong cliffs have been cut around its shore. And inasmuch as Réunion, Tutuila and the Marquesas, and Tahiti exemplify the second of these alternatives, the first alternative is regarded as the less probable of the two.
Many more observations of reef-encircled islands are needed before the questions here raised can be settled; and the observations must evidently be directed much more to the islands than to the reefs around them. The various possibilities here outlined, and as many others as can be invented,
should be critically reviewed by the observer while he is still on the ground, in order that he may give conscious attention to the details which are confirmatory of or contradictory to the different suppositions. The absence of records regarding significant details in many accounts of reef-encircled islands makes it impossible to use them in a settlement of the questions at issue.
Submergence by Ocean Rise or by Island Subsidence.
The changes of level involved in producing coasts of submergence or of emergence, and the changes in coral reefs therewith associated, may result from various causes. Chief among these are, first, a local movement of the earth's crust without significant alteration of ocean-level; secondly, an alteration of ocean-level due either to a distant movement of the earth's crust or to the general transfer of detritus from continents to ocean basins; and, thirdly, an alteration of ocean-level due to climatic change, whereby a considerable volume of water is withdrawn from or returned to the ocean in connection with the making or melting of continental ice-sheets.
As far as coral reefs alone are concerned, it is immaterial whether the changes of level upon which their formation or their emergence depend are caused by one of these processes or another; but when it is sought to assign coral reefs to their proper place in the history of the earth the causes of the changes of level with which they are associated must be determined as definitely as possible, and this is now the most difficult part of the coral-reef problem. In order to solve it, search must be made for the characteristics by which each kind of change of level may be recognized.
Crustal subsidence operating over large areas was accepted by Darwin and Dana as the whole cause of the subsidence with which coral reefs are so generally associated. Local subsidence of volcanic islands, as a result of their excessive weight, has been recently suggested by Molengraaff in explanation of mid-Pacific atolls. It may seem at first sight that either one of these processes would, if acting alone, cause a slight lowering of ocean-level, whereby coasts of emergence would be produced around continental shores; and in this case the resulting local submergence of the reef-encircled islands would be a little less than the local subsidence.
But a closer consideration leads to other conclusions: first, inasmuch as general crustal subsidence is presumably associated with compensatory uplifts of other areas, the changes in ocean - level thus caused may be neglected, and with all the more reason when it is noted that if a subsiding island is only partly submerged while a compensatory uplift of equal volume occurs on the ocean-floor without any emergence the result will be a small rise of ocean-level; secondly, if a large number of volcanic islands are built up in succession by eruption from the ocean-floor in such intervals of time that the earliest ones have subsided so far as to be crowned with atoll reefs when the latest ones are formed, the total effect on ocean-level will be not a fall, but a small rise (Davis, 1917B).
In this connection let it be noted that modern investigation gives little support to the old view that active volcanoes always occur in regions of elevation. There is much evidence to show that the reverse is often true. It is therefore desirable that the movements suffered by other islands in the neighbourhood of young volcanic islands should be independently worked out. It is certainly not legitimate to conclude, as has been done by an observer in the Australasian region, that a certain atoll could not have been formed by upgrowth during subsidence because an active volcano
stands near it, especially when the supposition of upheaval based on the occurrence of the volcano is contradicted by the occurrence of embayments in the shore-line of another island not far away.
The chief characteristics of crustal subsidences, as well as of crustal upheavals, are that the submergences or emergences they produce may vary from place to place in rate, amount, and date; and in these significant respects they will differ from the submergences or emergences due to other causes, which must involve universal changes of ocean-level, everywhere the same in date, amount, and rate, except where they are complicated by contemporaneous local crustal movements. Evidently, then, it is important to examine all structural and physiographic features of coral reefs and of their encircled islands from which inferences may be made as to the rate, amount, and date of the changes of level that they have suffered, in order to learn how far they are everywhere alike, or how far they vary from place to place.
Extravagant Deformation is demanded by Large Changes of Ocean-level.—A few examples of results already gained in this direction will be given below. But let it first be noted that in order to produce the submergence or upheaval of an island by 1,000 ft. a local subsidence or upheaval of the island by that amount in an ocean of essentially constant level is a much more economical movement than the vast crustal deformations involved in a rise or fall of the ocean-surface by the same amount around a still-standing island; for such a change of ocean-level can be brought about only by a change of the same measure in the entire ocean-floor (except around the still-standing island), or by a ten times greater change in a tenth of the ocean-floor.
Indeed, if a change of ocean-floor level over a tenth of its area involve roughly compensatory changes of a similar area elsewhere, then in order to cause a rise or fall of the ocean-surface by 1,000 ft. the failure of compensation must be of the order of 10,000 ft.; and, great as these movements are, their whole measure must be accomplished in the same period of time as that required for the much smaller measure of local upheaval or subsidence of the island under discussion. It thus appears that in seeking to account for a local submergence or emergence of 1,000 ft. an economy of vertical movements in a reef-encircled island involves an extravagance of movements elsewhere. Hence while small, slow, widespread, and synchronous changes in the relative level of land and sea may be plausibly ascribed to changes in the level of the ocean as a result of ocean-floor deformation, large, rapid, and local changes are best accounted for by movements of the island or coast where they are recorded.
Nevertheless, some students of coral reefs have attempted to throw the responsibility for large submergences or emergences of the islands that they have described upon other unspecified parts of the world. Thus C. W. Andrews says, regarding the emergence of Christmas Island, a little-dissected high-standing atoll, 1,200 ft. in altitude, in the eastern Indian Ocean, “It seems very probable that it is the general level of the surface of the sea that has been altered, and not merely a local upheaval of a limited land area that has taken place.” Inasmuch as in this case all islands and all continental shores that did not suffer emergence at the same time must have subsided with the ocean, an enormous terrestrial disturbance is involved in this method of accounting for the recently gamed altitude of a single small island.
Suess was somewhat more warranted in ascribing the emergence of a number of Pacific atolls to a sinking of ocean-level, for, according to the records that he quoted, their altitudes were about alike; but a closer examination of the facts shows not only that the altitudes of these emerged islands vary greatly, but also that the amount of post-emergence erosion that they have suffered is very unlike. Hence their present altitudes must be explained by local upheavals, varying in date as well as in measure.
Diverse Measures and Dates of Submergences and Emergences.—Local differences in the measures and the dates of emergences and submergences are the best indications of local movements, and evidence of such differences is found on islands in many parts of the Pacific and Indian Oceans. In Fiji, for example, the uplifted limestones, which reach an altitude of 650 ft. on Vanua Mbalavu, in the eastern part of the group, are greatly dissected; Vatu Vara, an elevated atoll thirty miles to the west, is hardly dissected at all, though its height is 1,030 ft.; Naiau, another elevated atoll, 580 ft. in altitude, forty miles to the south, is also little dissected; several other barrier-reef islands, one hundred miles or more to the west, show no signs of elevation.
Again, Viti Levu and Vanua Levu, the two largest islands of the Fiji Group, show fringing or close-set barrier reefs in association with slightly elevated reefs on parts of their southern coast, while to the north-west they have distant barrier reefs, enclosing broad lagoons. The barrier reef on the north-west of Viti Levu is well formed near the island, but fails to reach the sea-surface farther away, where the lagoon has the unusual depth of 58 fathoms. Such a combination of features can hardly be explained without assuming a gentle tilting of the islands.
A similar tilting would seem to be demanded by the features of the Pelew Islands as described long ago by Semper, although that zoological observer, who knew nothing of embayed shore-lines or of unconformable reef contacts, thought tilting too improbable a process to be believed in. New Caledonia shows abundant signs of recent submergence to some such measure as 80 or 100 fathoms, while the Loyalty Islands, not far away to the north-east, are recently elevated atolls. In the Solomon Group, Fauro, previously mentioned, is surrounded by a submarine platform which appears to represent a submerged barrier reef, while New Georgia, farther east in the same group, is bordered for part of its circuit by a remarkably good example of an emerged barrier reef.
It thus appears clear that diverse emergences and submergences at different dates are indicated in various island groups. Hence, even if changes of ocean-level from any cause have from time to time produced universal and synchronous emergences and submergences of moderate measure, local movements of much greater measure are also demanded by the features of various islands, and these local movements are probably the chief causes of the strong submergence which the drowned valleys and outstanding barrier reefs of many volcanic islands call for. Further observations on many reef-encircled islands should be made in order to learn the relative values to be assigned to the various causes of emergence and submergence; and from what has thus far been said it is clear that the observations should, in this aspect of the coral-reef problem also, be directed more to the islands than to the reefs which encircle them.
Although sea-level atolls are, by themselves, inscrutable structures, it sometimes happens that they occur at moderate distances from barrier-reef islands: then the changes of level demonstrated for the barrier reef may be plausibly extended to the atoll also. Thus it has been possible to show good reason for ascribing certain small atolls in Fiji to upgrowth during submergence, and to show also that the submergence was probably due to relatively local subsidence (Davis, 1916D). The large atoll of Ongtong Java, north of the Solomon Islands, can hardly have been formed according to any of the still-stand theories, because the Solomon Islands show many signs of diverse vertical movements. Similarly, the uplifted Loyalty atolls have probably suffered other movements than that of their last uplift, for they are not far distant from New Caledonia, which has had many disturbances.
It may, of course, be urged that the atolls here mentioned, standing near disturbed island groups, should not be taken to indicate the origin of the more numerous atolls in the mid-Pacific, but it may be answered that, while the mid-Pacific region has very probably been less disturbed by subsidences and upheavals than its western archipelagoes, nevertheless the atolls which are associated with barrier reefs resemble mid-Pacific atolls so closely in all essential particulars that the chief differences between them are probably to be found less in the diverse conditions of their origin than in the absence of neighbouring information-giving barrier-reef islands in the one case and their presence in the other.
It has been argued by some students of the coral-reef problem that the uniformity of the depth of atoll lagoons is better explained in connection with a rise of ocean-level everywhere of the same amount than by the subsidence of the atolls, which must vary somewhat from place to place. In so far as the post-Glacial rise of ocean-level can satisfy the demands of the problem this argument may be accepted; but inasmuch as the depths of atoll lagoons, as far as they are known, vary in a manner more suggestive of varying than of uniform measures of submergence, perfect stability of the atolls is improbable. Moreover, the reef-encircled volcanic islands that occur in close association with certain atoll groups demand a greater measure of submergence to account for their drowned valleys than can be provided by Glacial changes of ocean-level. Finally, the evidence of the Funafuti boring is, as noted above, strongly in favour of subsidence during the formation of its reef rock.
Recently-elevated atolls not dissected sufficiently to disclose their structure give little more testimony regarding their origin than can be obtained from sea-level reefs. But if a recently-elevated fringing or barrier reef lie unconformably upon its foundation, and if its limestones enter into valleys between the ridges of its central island, as is manifestly the case with the elevated reefs of Oahu, Hawan, submergence of an eroded land-surface must have taken place before the reef was formed. The measure of submergence can be inferred if the down-slope extension of the eroded land-surface beneath the reef can be determined.
If elevated reefs have been out of water long enough to suffer dissection, the details of their structure may be disclosed; but so abundant is the vegetation of tropical islands that observation of reef-structure is very
difficult. It is of importance that the observer who has opportunity of examining a dissected reef should locate the structural details that he may discover with respect to the total reef-mass; it is also important that he should bear in mind the expectable structures of reefs formed according to the several chief theories of reef-origin, as shown in figs. 7 and 8, for he will thus be led to make special search for critical structures in their appropriate locations.
Thus if the great body of an elevated reef consist, as in fig. 7, of steeply sloping layers of reef detritus mostly free from admixture with volcanic sands and gravels, resting conformably upon a non-eroded volcanic slope, T, and more or less complicated by slides, the reef should be explained as a product of outgrowth during a prolonged still-stand period. Darwin clearly recognized the possibility of reef-formation in this manner, but regarded it as seldom occurring, because it would not result in the formation of a reef-enclosed lagoon from 20 to 40 fathoms in depth. Murray attempted to overcome this difficulty by assuming, as Semper had before him, that the lagoon-cavity would be excavated by solution; but the assumption is not supported by the features of lagoons, as has been noted above.
On the other hand, an elevated reef may show a three-part structure, as in fig. 8. The steeply dipping, exterior strata, T, may be formed of detritus chiefly derived from the reef, but with some fine sands and silts
from the central island. The slanting layers may be sometimes complicated by slide-structure as in the preceding case; they may rest on a heavy deposit of volcanic detritus, D, which should be associated with a buried cliff. The intermediate wall-like structure, R, should contain much coral in place, as well as large and small fragments. The outward or inward slant of the wall appears to be dependent on the rate of subsidence during
its formation (Davis, 1916C). The nearly horizontal interior strata, L, may contain coarse sand near the reef-wall, fine lagoon deposits in the middle, and volcanic sands and gravels near the central island; and the whole mass, with the exception of some of the outer slanting layers, may lie unconformably on a rock surface of subaerial erosion. In such a case reef-upgrowth during prolonged submergence probably due to subsidence would be inferred. Irregularities in the reef-wall, as in fig. 9, would indicate changes in the rate of submergence. A horizontal outgrowth, H,
would occur during a long still-stand period, when delta plains, E, might almost fill the lagoon. The occurrence of a buried cliff and platform in the profile of the underlying rock, and an exterior detrital deposit, as shown in figs. 2, 4, 5, 8, and 9, would be of much theoretical interest.
In case dissected atolls are found, their structures should be studied with especial care; and if their rock foundation is disclosed it should be closely examined to see whether the atoll limestones lie on it conformably or not. Christmas Island, in the eastern Indian Ocean, merits renewed study in this respect, for basalt has been seen in ravines behind its limestones, but the nature of the limestone-basalt contact has not been fully described.
In case elevated reefs occur in terrace-like arrangement, one above the other, as on Cebú in the Philippines, and elsewhere, the structure of the successive terraces will indicate the sequence of formation of their reefs.
Following Gilbert's method of interpreting the terraces of Lake Bonneville, and assuming that the reefs rest unconformably on their rock foundation, a series of superposed reefs, one resting on the other, must have been formed during successive pauses in a long submergence and afterwards rapidly elevated; while a series of apposed reefs, one in front of the other, must have been formed during pauses in a long emergence, preceded by a rapid submergence. Such a structure as is shown in fig. 10 should be interpreted as meaning that reefs A and B were formed during pauses in submergence, while reefs C and D were formed during pauses in emergence. It is manifest that all details of reef-structure such as are here suggested should be critically observed.
It is singular that the coral-reef problem, which has been so long under discussion, should not have been already so far standardized as to make the suggestions contained in this article unnecessary; but, as a matter of fact, neither the special reports by various investigators of coral reefs, nor the leading text-books of geology and of physical geography, present the problem in such a form as to emphasize the matters that are of the greatest importance in its solution. Factors so essential as shore-line embayments and unconformable reef contacts often receive no mention whatever. The meaning of unconformable fringing reefs has been almost universally overlooked. The forms of spur-ends on reef-encircled islands are hardly ever described. The disposal of the waste from a deeply-dissected, reef-encircled island has received no discussion. Elevated reefs, even if unconformable with their foundation, have nearly always been interpreted as having been formed during pauses in the movement of uplift by which they were elevated, and no recognition has been given to the manifest possibility of their formation during pauses in a preceding subsidence.
Several reasons for the neglect of these essential considerations may be suggested. One is that the investigators of coral reefs have often been zoologists, untrained in geological inquiry. Another is that the physiographic principles which are involved in a critical study of the reef problem are not always familiar even to geological observers. A third and perhaps the most important reason is that few investigators of coral reefs appear to have taken the time necessary to think out the essential consequences of the several leading theories of reef-origin in order to discover which of the consequences are best supported by the facts. A fourth, as important as the third, is that observers have too often given their chief attention to the reefs, and have not attended sufficiently to the islands that they encircle. A fifth is that the origin of coral reefs is a very complicated matter, because many different factors may have a share in it, and many different solutions therefore appear possible.
It is in the hope of overcoming these deficiencies in the methods of reef-investigation that the preceding pages have been written. While it is recognized that the coral reefs constitute a wonderful field for zoological study, and that such study throws much light on the life-history of reefs in the past, it is urged that the geological and physiographic study of reef-encircled islands is necessary in order to discover the past inorganic conditions under which reefs were developed.
While it is fully understood that the observation of the visible zoological and geological facts of the present may absorb a large share of the attention of a reef-investigator, it is urged that he should frequently, while still in the field, take enough time from observational work to think out as carefully as possible the invisible conditions of the past according to each and every theory known to him, and that having done so he should return to an examination of the visible facts in order to discover which one of his theories they best support. New Zealand is favourably situated as a starting-point for the study of coral reefs; hence the scientific world must look to New Zealand students for new light on this old problem.
Daly, R. A., 1910. Pleistocene Glaciation and the Coral-reef Problem, Amer. Journ. Sci., vol. 30, pp. 297–308.
——, 1915. The Glacial-control Theory of Coral Reefs, Proc. Amer. Acad. Sci., vol. 51, pp. 157–261.
Davis, W. M., 1913. Dana's Confirmation of Darwin's Theory of Coral Reefs, Amer. Journ. Sci., vol. 35, pp. 173–88.
——1914. The Home Study of Coral Reefs, Bull. Amer. Geog. Soc., vol. 46, pp. 561–77, 641–54, 721–39. [The omission of the name of E. C. Andrews on p. 724 (eleventh line from bottom) of this article, which was printed during my absence on the Pacific, is a regretted oversight.]
—— 1915. A Shaler Memorial Study of Coral Reefs, Amer. Journ. Sci., vol. 40, pp. 223–71.
—— 1916A. Problems associated with the Origin of Coral Reefs, Sci. Monthly, vol. 2, pp. 313–33, 479–501, 557–72.
—— 1916B. Clift Islands in the Coral Seas, Proc. Nat. Acad. Sci., vol. 2, pp. 283–88.
—— 1916C. Extinguished and Resurgent Coral Reefs, Proc. Nat. Acad. Sci., vol. 2, pp. 466–71.
—— 1916D. The Origin of certain Fiji Atolls, Proc. Nat. Acad. Sci., vol. 2, pp. 471–75.
—— 1917A. The Great Barrier Reef of Australia, Amer. Journ. Sci., vol. 44, pp. 339–50.
—— 1917B. The Isostatic Subsidence of Volcanic Islands, Proc. Nat. Acad. Sci., vol. 3, pp. 649–54.
—— 1918. The Subsidence of Reef-encircled Islands, Bull. Geol. Soc. Amer., vol. 29, pp. 489–574
Skeats, E. W., 1918. The Coral-reef Problem and the Evidence of the Funafuti Borings, Amer. Journ. Sci., vol. 45, pp. 81–90; The Formation of Dolomite and its Bearing on the Coral-reef Problem, ibid., pp. 185–200.