Art. LXXI.—Remarks as to the Cause of the Warmer Climate which existed in high Northern Latitudes during former Geological Periods.
[Read before the Wellington Philosophical Society, 18th August, 1877.]
Although nearly all modern geologists have been willing to admit that the phenomena of volcanos and earthquakes must be directly connected with the passage of heat from the interior to the surface of our globe, they have at the same time been indisposed to allow that this internal heat could have had any influence, even during the remotest geological times, upon the climatic conditions which affect the existence of life. Indeed, Sir Charles Lyell, who may be taken to have been the exponent of the views of the most advanced geologists of the day, more than once expressly denied the existence of any such influence, and sought otherwise to explain the remarkable fact that, within the arctic regions, where the present climatic conditions are almost opposed to the existence of terrestrial life at all, there occurred, in past geological times, a flora as rich as that which now occupies the hottest parts of the tropics.
It will, unquestionably, appear presumptuous in me to attempt to refute the opinions of Sir Charles Lyell and those who have followed him upon this point, but the recent investigations of physicists have led me to doubt whether those opinions are altogether tenable. Indeed, I think I shall be able to show that many of the facts mentioned by Sir Charles Lyell himself are only consistent with the proposition that the climatic conditions suitable to the maintenance of a luxuriant flora in arctic latitudes during early geological times were chiefly due to heat radiated from the interior of our globe.
In order that my line of argument may be understood I must, in the first place, call attention to the received opinions of all leading physicists as to the original condition of the material of our globe. Now, whatever doubts might formerly have been entertained as to the existence of nebulous matter, these doubts have been set at rest by the use of the spectrum analysis, and the beautiful theory propounded by Laplace in regard to the formation of our planetary system, has thus received a very strong confirmation. His theory is that the sun was at one time the centre of a nebula, whose diameter extended vastly beyond the orbit of the most distant of our planets,
and which revolved round its centre of gravity during the process of condensation. That, from time to time, extensive rings of this nebulous matter were left by the central condensing mass at points in the circumference where the centrifugal and gravitating forces became exactly balanced, and that these rings, still circulating round the central nucleus, broke up into masses which became endued with a motion of rotation and assumed a spheroidal form. These masses, acting in the same manner as the parent mass, and abandoning similar rings of outlying matter, led to the formation of the satellites of the various principal planets. During this process of condensation, which, of course, took place in obedience to the all-pervading law of gravitation, the motion of the condensing particles of which each planet was composed, was converted into heat, and that to such a degree as would result in the fusion of the whole into one mass. Meyer, indeed, remarks, in a paper on Celestial Dynamics (as quoted by Nasmyth and Carpenter in their great work on the moon), that “the Newtonian theory of gravitation, whilst it enables us to determine, from its present form, the earth's state of aggregation in ages past, at the same time points out to us a source of heat powerful enough to produce such a state of aggregation,—powerful enough to melt worlds; it teaches us to consider the molten state of a planet as the result of the mechanical union of cosmical masses, and to derive the radiation of the sun and the heat in the bowels of the earth from a common origin.”
Sir Charles Lyell, however, though he did not dispute the opinions of Laplace and others as to the effect of the condensation of cosmical matter, appeared unwilling to admit the continued existence of internal heat to the extent contended for by leading physicists, and inclined rather to the opinion advanced by Poisson, “that in cooling by radiation to the medium which surrounded the earth, the parts which were first solidified sunk, and that by a double descending and ascending current the great inequality was lessened, which would have taken place in a solid body cooling from the surface.” I am here quoting directly from Poisson, and not from Sir Charles' work, as I wish to show how completely at variance Poisson's opinions are with the laws which govern heated matter passing into the solid condition. Nasmyth and Carpenter point out, with special reference to the opinions of Poisson and of those who held similar views, “that fusible substances are (with a few exceptions) specifically heavier whilst in their molten condition than in the solidified state, or, in other words, that molten matter occupies less space, weight for weight, than the same matter after it has passed from the melted to the solid condition,” and they point to the remarkable facts, amongst others, that cold iron floats upon molten iron, cold silver upon molten silver, cold slag upon molten
slag, and a variety of other cases of the same kind, in order to show how untenable are the propositions of Poisson and his followers, that if the globe had ever passed from a liquid to a solid state, in consequence of the loss of heat by radiation, the cooling and consolidation of the nucleus would have begun at the earth's centre.
Now, assuming that the views of Poisson are untenable, and that the cooling of the globe commenced at the surface, and extended towards the centre, it is palpable that the loss of heat resulting from radiation must have been greatest in the earlier periods of the earth's revolutions, and must have decreased in proportion as the solidified crust extended in depth. But, in our globe three different modes for the transmission of heat have been distinguished, the first being periodic and affecting the temperature of the crust according to the different positions of the sun and the seasons of the year; the second, also due to the sun, namely, that a portion of the heat of the sun which penetrates the crust in the equatorial regions moves through the crust towards the poles, where it escapes into the atmosphere; the third being derived from the secular cooling of the earth, and from the primitive heat still being disengaged from the surface. This latter has for many ages been very insignificant, owing chiefly to the fact that it is interrupted in its passage by an enormous thickness of sedimentary and other strata, which are very bad conductors of heat. Laplace has shown, by reference to astronomical observations taken in the time of Hipparchus, that within the last 2000 years no sensible contraction has taken place in the globe by cooling, but it must be borne in mind that such a period as 2000 years, vast as it may seem when taken in reference to ordinary historical events, is but as a fleeting second in the eras which have passed since our globe was condensed into its original fluid mass, and that it is therefore highly improbable that in so comparatively short a time any appreciable change in the length of a day arising from such a cause, could have been ascertained.
There is, however, a matter of very considerable importance in connection with the present distribution of heat in the crust of the globe, to which I must call your attention, namely, the periodic changes of temperature occasioned on the earth's surface by the sun's position and by meteorological processes. Now, it has been ascertained, by carefully conducted experiments, that these changes are continued in the crust of the earth, though to an inconsiderable depth, but that they are, nevertheless, such as even now to exercise a very marked influence on vegetation, and, indeed, on life generally. The slow conducting power of the ground, which checks the loss of heat in winter, is favourable to the growth of deep-rooted trees. “Points that lie at different depths on the same vertical line,” says Humboldt, “attain the maximum and minimum of imparted temperature
at very different periods of time. The further they are removed from the surface the smaller is this difference between the extremes. In the latitude of our temperate zone (between 48° and 52°) the stratum of invariable temperature is at a depth of from 59 to 64 feet, and at half that depth the oscillations of the thermometer, from the influence of the seasons, scarcely amount to half a degree. In tropical climates this invariable stratum is only one foot below the surface, and this part has ingeniously been made use of by Boursingault to obtain a convenient, and, as he believes, certain determination of the mean temperature of the air of different places. The mean temperature of the air, at a fixed point, or at a group of contiguous points on the surface, is, to a certain degree, the fundamental element of the climate and agricultural relations of a district, but the mean temperature of the whole surface is very different from that of the globe itself.” We have no data for determining the depth at which the stratum of invariable temperature lies within the arctic regions, but looking to the increase which takes place between that at which it is found within the tropics, and that at which it occurs some 20° further north, we may assume it to lie at a depth of little under 200 feet in the former region. I am not aware of the mean temperature of the air in the arctic regions, but it must be so low as to be absolutely antagonistic to all but the most stunted and hardy forms of vegetable life.
It must not be supposed, however, from what I have already said, that the supposed gradual diminution of the primitive heat of the globe has not been resorted to by geologists to account for alterations in climate. This is not the case, but, unfortunately for the earlier propounders of the theory, the condition of our knowledge did not afford them sufficient evidence in support of it, and, indeed, it is only within the last few years that the investigations of physicists have supplied grounds which would justify the proposition. The authority of Sir Charles Lyell, which was arrayed against it, tended moreover, to check further investigation, but although (as I before observed) I may be treated as presumptuous in endeavouring to set up this theory in opposition to his views, I feel that recent discoveries justify further discussion on the subject.
I now propose to consider briefly the nature of the surface conditions of our globe after the condensation of the nebular matter had been completed. We have in the present surface conditions of our own satellite, some evidence of what that of our globe would have been but for the presence of a controlling element, to which I shall hereafter allude. The researches of Nasmyth and Carpenter on the moon, published in 1874, have given to the world the clearest possible view of the present condition of her surface, indicating, as that condition does in the most unmistakable manner, its
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origin as a mass of diffused cosmical matter, condensed into a planetary mass by the natural gravitation of its particles, such condensation resulting in the generation of heat sufficient to reduce the whole mass to a molten condition. But no sooner had this mass been completely formed, than it began to cool at the surface, by the radiation of its heat into space, and the surface must then have presented the peculiar aspect described by the authors referred to, as that exhibited by the surface of a pot of molten metal drawn from a melting furnace. But, as the process of cooling continued, a definite thickness of the surface would pass from the fluid into the solid condition, undergoing, whilst doing so, that expansion which is observed to take place immediately upon solidification, and causing, by such expansion, and the subsequent contraction which accompanies the cooling of a solid body, great irregularities upon the surface of that globe. During this first period the heat radiated would be sufficient to maintain, in a highly rarefied state, all the elements of our atmosphere, including aqueous vapour. But when this cooling had proceeded until the heat of the surface of the crust had been reduced to a point below the boiling point of water at the then rate of atmospheric pressure, the aqueous vapour would be condensed upon that surface in the form of water, which would gradually increase in quantity until the cooling had proceeded far enough to admit of the existence of life. We know that the divellent energies common to ordinary gaseous bodies, are even more conspicuous in bodies which assume the gaseous form at high temperatures. Thus water under the ordinary pressure of our atmosphere becomes thoroughly a gas only when heated to 212°, and retains this gaseous form, within certain limits, above its boiling point. But below 212° the case is different, the elastic force of aqueous vapour (as steam is more properly termed below 212°) rapidly diminishes, so that at 32° (the freezing point of water) its elastic force is found to be scarcely equal to onefifth of an inch of mercury, and a given volume to weigh only 1/150 of what steam ought to weigh, supposing water could exist as a perfectly gaseous body at 32° under a-pressure of 30 inches of mercury. Hence the molecules of aqueous vapour at 32° must be five or six times further apart than in the perfectly gaseous form of steam, and so feeble in their repulsive force that, even when thus separated, the aqueous molecules cannot be approximated by slight increase of cold or of pressure without partial coalescence and the formation of water or ice. But we are told that the self-repulsive force exerted by the molecules of water in the liquid and even in the solid form, though feeble, is not annihilated, and that hence, when the atmosphere surrounding water or even ice is dry, the superficial molecules of the water or ice assume their self-repulsive character, and fly off until the surrounding atmosphere is saturated. The quantity of vapour which
can thus be held in solution in the atmosphere at any given temperature is fixed and invariable. But the quantity has not been found to be at any known temperature, and increases rapidly with increase of heat. Since, however, the quantity of water which the air can hold in solution at any temperature is fixed, it follows that when that temperature is reduced the superfluous water must be yielded up and deposited in the liquid form, such deposition, in the large scale of nature, usually constituting rain.
The question, however, naturally arises, whence has our globe derived its large supply of water? Now, this question has been fully considered by Mr. W. Mattieu Williams, in his admirable work “The Fuel of the Sun,” and I make no apology to you for giving the substance of what he says on the subject almost in his own words. After pointing out his reasons for believing in the existence of an infinite atmosphere, of which the atmosphere surrounding our planet is but a denser portion, and after discussing the difference of atmospheric pressure at the surface of the various bodies constituting our solar system, he asks whether we are to consider the water which covers the lower valleys of the earth as planetary or atmospheric matter? Whether it is one of the special constituents of our globe or only a portion of the general atmospheric matter which the earth's gravitation has condensed round it? He then proceeds to discuss these questions by reference to those known properties of water, to which I have already alluded, which show that the position occupied by water on our own or any other planet is entirely dependent on comparatively moderate variations of temperature and pressure. “If,” as he observes, “the temperature of the earth were raised or the pressure diminished in a sufficient degree, the whole of the water of the ocean would rise from its present bed and take its place in the atmosphere as one of its constituent gases, and would there exist in a state corresponding to the carbonic acid of our actual atmosphere.” Indeed, after fully considering the matter, he comes to the conclusion,—a conclusion so fairly demonstrated as to be, in my opinion, irresistible, “that the water upon our earth is but a portion of the matter which its gravitation has collected from the all-pervading medium of the universe,” and he adds that there is good reason to believe that gaseous water is one of the most important constituents of that general atmospheric medium, and probably constitutes a considerable percentage of the whole. He further observes that the spectrum analysis has afforded the strongest possible confirmatory evidence of an universal distribution of water, for that, whether directed to the sun, to the stars, to the nebulæ, or to the luminous matter of comets or meteors, the general reply is, “Water, water, water everywhere;” Professor Graham having even found occluded hydrogen in meteoric stones that have reached the earth, Mr. Williams
therefore says that “he will assume that water belongs to the atmosphere, and, in the present order of things, should be found as a constituent of the atmosphere of all the orbs of space; the state of its existence, whether solid, fluid, or gaseous, whether combined as water, or separated into its constituents of free hydrogen and free oxygen, being dependent on the physical conditions to which it is subjected.”
Now, it is scarcely necessary for me to remark that all life, as it is known to us, is dependent upon the existence of water. This is a fact which we learn from any elementary work on physiology, and we are, therefore, justified in assuming that until water existed on our globe, at a temperature not inconsistent with life, no life could be developed upon it. You have before you on the table a series of specimens taken from the hot waters of springs in the Rotomahana district, showing, in all probability, the very highest temperatures compatible with the existence of living organisms, and we may look back to a period counted, probably, by hundreds of millions of years, when such low forms of life were the only ones which were to be found on the surface of our planet. And this brings me to the immediate subject of this paper, namely, had the heat radiated from the interior of our globe, any effect upon climate during the earlier periods of life brought under our notice by the geological records? I venture, for reasons which I will proceed to explain, to agree with the older ideas on the subject, in spite of the positive opinions expressed by Sir Charles Lyell.
It is clear that long before the surface conditions of our globe were such as to permit of the condensation of aqueous vapour upon it, it revolved round the sun in the orbit which it now occupies, and that even then the heat of its equatorial regions received a large increase from the latter body. It will have been observed that the depth to which the surface is now permanently heated by the rays of the sun diminishes with great rapidity as we approach the equatorial regions, but there is no reason for supposing that during the gradual cooling of the globe, the radiation of heat would, even supposing the absence of any check due to the sun's rays, have been greater from the equatorial regions than from the polar ones. The contrary must, in effect, have been the case, and the polar regions of our globe were doubtless the earliest to present surface conditions fitted to retain water upon them. If this were so, then certainly life must, in its earliest stages, have had its origin in arctic latitudes, gradually extending towards the tropics as the surface of the latter regions became sufficiently cool to permit water to accumulate there also. It will, of course, be understood that the accumulation of water on our globe was very slow, and I cannot but think that the arguments brought forward by Mr. Mattieu Williams, in the work alluded to, as to the materials which constitute the fuel of the sun, apply
to the presence of water on our planet. Our planet, in common with the sun and the other members of the system, is moving through space at the rate of four to five hundred thousand miles a day, and must, if the hypothesis of an universal atmosphere be correct, to the extent of its gravitation force, daily obtain from this universal atmosphere a fresh supply of water. Now this supply will be equal to that which can be obtained from the contents of a cylinder of this atmosphere, the length of which is from four to five hundred thousand miles by nearly eight thousand in diameter.
This is an interesting question, into which, however, it is unnecessary for me to enter, even if I possessed the elements necessary for the calculation of the probable quantity of water, if any, annually added to that upon our globe from this source.
Let me now enquire how far geological evidence can be adduced in support of the view that heat, radiated during the cooling of our globe, affected climate during the earlier periods of life upon it.
In dealing with this subject I propose to accept what has been given to us by Sir Charles Lyell in his great work already alluded to, in regard to the character of the life forms during past geological periods.
In the tenth and eleventh chapters of the tenth edition, published in 1867, the characters of the climate during the several periods extending from times immediately anterior to the historical, up to the Silurian period, is very fully discussed, and the author commences by commenting upon the objections which had been raised to the theory which endeavours to explain past geological changes by reference to causes now in action, pointing out that one of those objections is founded on the former prevalence of climates hotter than those now experienced in corresponding latitudes. I have before observed, however, that Sir Charles entirely repudiated the idea that during any portion of the time which has elapsed since life appeared on our globe, climate was affected by the heat radiated from the globe itself as the result of the cooling of its mass; and his whole argument is founded upon the assumption, that the changes which had evidently taken place were due to other causes than the one referred to.
I will not trouble you with references to times prior to the Pliocene period. With regard to that period, however, and more especially to its lower strata, in common with those of the Upper Miocene, we learn that the fauna and flora of the whole of Central Europe afford unmistakable evidence of a climate approaching that which is now only experienced in sub-tropical regions; and it is a matter of no small interest to know that when the climate of Europe was sub-tropical, a still greater heat prevailed nearer the equator, as specially evidenced by the investiga-
tions of Dr. Falconer, and Sir Proby Cautley, in the Sewalik Hills. Sir Charles remarks that these and other investigations lead irresistibly to the opinion that there was a much greater analogy in those ages than there is now between the temperature of the West Indies in latitude 18° and that of Europe in latitude 48°. But he also says, which is much more significant for the purposes of my contention, that if we pass from the equatorial to the arctic latitudes of the Northern Hemisphere, we find an assemblage of fossil plants resembling in many respects that of Œninghen, in Switzerland, in which Professor Heer detected the leaves, fruits, and sometimes flowers of about 500 species of plants, in which he found a near resemblance to the flora of the Carolinas and other southern states of the American Union. Of the Lower Miocene flora he says “that it has been traced from Italy northwards to Devonshire, and even to Iceland. In these high latitudes, however, the tropical and sub-tropical genera disappear, though the vine and tulip-tree and some other forms indicate a temperature 15° to 20° Fahr. warmer than that now belonging to the same countries,” quoting Heer and Gaudin, “Climat du Pays Tertiare,” pp. 174–207, in support of his statement.
Further on he says (still speaking of the Lower Miocene flora), “In Spitzbergen, in latitude 78° 56° N., no less than ninety-five species of plants are described by Heer, many of them agreeing specifically with North Greenland fossils. In this flora we observe Taxodium of two species, a hazel, poplar, alder, beech, plane-tree, lime (Tilia), and a Potamogeton, which last indicates a fresh-water formation, accumulated on the spot. Such a vigorous growth of fossil trees, in a country within 12° of the pole, where there are now scarcely any shrubs except a dwarf willow and a few herbaceous and cryptogamous plants, most of the surface being covered with snow and ice, is truly remarkable.”
With regard to the Eocene fauna and flora of Central Europe, we learn that it possesses species and genera having a great affinity to Lower Miocene forms, but departing further than these do from the modern European type, and resembling, in many respects, those of the tropical regions of India and Australia, and that, especially in the London clay of the Isle of Sheppey, fossil fruits of the cocoa-nut, screw-pine, and custard-apple remind us of the hottest parts of the globe. In the beginning of the eleventh chapter, Sir Charles specially calls his reader's attention to the fact, that an examination of the Pliocene, Miocene, and Eocene strata, viewed successively in the order of their higher antiquity, affords evidence of a temperature continually increasing in proportion as we recede further from the glacial epoch. (The italics are mine.) Passing now to the secondary formations generally, the same law as that traced in the tertiaries is found
to obtain, the peculiar reptilian fauna which characterizes this period indicating unmistakably the existence of a warm temperature in the seas, lakes, and rivers, whilst the flora supports the hypotheses of Heer, Adolphe Brogniart, and others, that the climate of Europe, even within 12(½)° at the Pole, must have resembled that of the West Indies at the present day.
I might repeat the same language with regard to the Triassic, Jurassic, Carboniferous, Devonian and Silurian periods, all of which afford similar evidence, but I prefer quoting the following general remarks on the subject, with which Sir Charles Lyell concludes the eleventh chapter of his work:—“The result, then, of our examination in this and the preceding chapter, of the organic and inorganic evidence relating to the climate of successive geological periods, is in favour of the opinion that a warmer temperature generally prevailed in the northern hemisphere from the 30th parallel of latitude to the pole than that now experienced. In the Pliocene era the fauna and flora of Central Europe were sub-tropical, and a vegetation resembling that now found in Northern Europe extended into the arctic regions as far as they have been yet explored, and probably reached the pole itself. In the secondary or Mesozoic ages, the predominance of reptile life, and the general character of the fossil types of the great class of vertebrata, indicate a warm climate and an absence of frost between the 40th parallel of latitude and the pole, a large Ichthyosaurus having been found in latitude 17° 16°, and the general character of the Mollusca and corals, as well as of the plants, being in perfect accordance with the inferences deduced from the fossil reptiles. If we carry back our retrospect to the primary or Paleozoic ages, we find an assemblage of plants that imply that a warm, humid, and equable climate extended in the Carboniferous period uninterruptedly from the 30th parallel of latitude to within a few degrees of the pole, or to northern regions where at present the severe winter's frost and the almost universal covering of snow, lasting for many months, preclude the existence of a luxuriant vegetation. In rocks older than the Carboniferous the evidence of plants, insects, and fish fails us; but the invertebrate fauna has such a resemblance to that of the latter primary and the older secondary periods as to force us to believe that the climate of the temperate and arctic regions was very analogous to that which generally prevailed in these subsequent epochs.”
As before observed, however, Sir Charles, and those who follow him, decline to admit that heat radiated from the surface of our globe during its secular cooling from an original heated condition, had any influence in producing these observed differences in climate in the northern regions, and attribute them entirely to successive changes in the distribution of land
and water. It is certainly true that the Gulf Stream at present exercises a considerable influence on the climate of the localities on which it impinges, but this influence does not produce, outside of tropical regions, such effects as those which would be necessary in order to account for the existence in arctic regions of plant forms of the classes above alluded to; and it is scarcely possible to conceive any distribution of land and water which could result in such effects, in the past, upon the climate of a part of the globe, the present climatal conditions of which are admittedly almost opposed to the existence of any vegetation whatsoever. But if we find that the polar regions were those which were first fitted for the retention of water, we may fairly assume that it is in those regions that we must search for the first indications of life on our globe, the temperature of those portions which lie between the tropics, and for several degrees on each side of them, being necessarily maintained for a much longer period at a heat too great to permit water to lie upon the surface. It is curious in this connection to observe that Sir Charles has given us maps (now shown to involve serious fallacies) showing the relative distribution of land and water which would be calculated to produce, in the present day, at all events, the maximum of heat and cold on the surface of the globe, that which applies to the former showing the bulk of land lying for about 45° on each side of the equator, and that which applies to the latter showing the bulk of the land extending for a similar distance from each of the poles. On the whole, therefore, I feel that there is some justification for believing that the climate of the arctic and antarctic regions of our globe during the past geological epochs to which I have referred, was directly influenced by heat radiated from it during its secular cooling from the condition of a molten mass of aggregated cosmical matter, and that the first appearance of life took place when portions of its surface became sufficiently cool to admit of water resting upon it at a temperature not exceeding 120° Fahr.
I do not apologize for bringing these views under your notice, as I agree with Mr. Mattieu Williams that it is fortunate for the human race that men who study pure science are so far raised by its moral influence above prejudice and personality, that their perception of truth is not obscured by the medium through which it is conveyed, and that it is accepted as frankly, fairly, and courteously from the humblest outside student as though presented by the highest constituted authorities. If the views contained in this paper have no soundness in them they will be calmly refuted, or suffer death from deserved neglect. If, on the other hand, they are at all sound or suggestive, they will receive acceptance even from those whose preconceptions may have been opposed to them.