Art. LIII.—The Gem Gravels of Kakanui; with Remarks on the Geology of the District.
[Read befor the Otago Institute, 14th November, 1905.]
There have long been in the collections of the Otago Museum and of the Otago School of Mines specimens of sands from Kakanui, labelled “gem sands.” The late Professor Ulrich, Director of the Otago School of Mines, was of opinion that gems would some day be found at Kakanui, evidently being struck by the association of minerals which these sands contained. Their investigation seems desirable both from a theoretical and a practical point of view. In the investigation of
their matrix there has been occasion to make some general remarks on the geology of the district. For the purposes of more convenient reference hereafter, it has been thought desirable to keep detailed descriptions of the collected fossils for a separate paper.
Kakanui (Maori; “large kaka”) is the name of a range of mountains, a river, and a township, all situated in the northeast corner of Otago. The name has been applied by Captain Hutton to the upper part of the Wanaka system, of early Palæozoic age, typically developed in the Kakanui Mountains. The Kakanui River derives its gravels largely from these and younger submetamorphic rocks. The country between the river and the sea is composed mostly of the Oamaru formation of early Tertiary age. It may be roughly described as consisting inland of conglomerates overlaid by limestones, near the coast of ash-beds capped by limestones.
There appears to be no reference, in the many geological papers dealing with the neighbourhood, bearing directly on the “gem gravels” as such. The Oamaru district is the typical locality for the Oamaru system in New Zealand geology, and has been frequently explored by geologists, chiefly for the sake of determining the age of the system (which by Sir James Hector is included in his Cretaceo-tertiary system, and by Captain Hutton is ascribed to Oligocene). The papers on this subject occur mostly in the “Reports of the Geological Survey of New Zealand” and the “Transactions of the New Zealand Institute.” These will be referred to in due place.
Mr. Mantell, the first geologist to visit the district, noted the “volcanic grit” of Kakanui, which has proved to be the matrix of the “gem sands.” His observations on these minerals seem to have been entirely overlooked by all subsequent observers, although his paper is frequently referred to in discussions on the age of the rocks. The following is the paragraph referred to: “The volcanic grit of Kakanui. contains a great variety of crystalline volcanic products, as hornblende, augite, garnets, &c.”*
After the fullest inquiries from residents in the district, and personal examination of many localities, both in the valleys of the Kakanui River and its tributary the Waiareka, it has been found that there are three distinct occurrences of stones that may be possibly of value as gems. The first is of sapphire,
[Footnote] * Mantell, Quart. Journ. Geol. Soc., 1850, p. 325.
identified as such by Professor Ulrich. Mr. Charles Reid, of Windsor, has cut some specimens of these. They were obtained from a gold-miner who worked in Slaty Creek, a tributary of the Kakanui, which enters it some fifteen miles from its mouth. A superficial search revealed no indication of them in the gravels of this stream. The stones are of fair size, but of poor colour. The second mineral is of a jasperoid nature. It has been traced to Red Hill, near Elderslie, in the property of Mr. Menlove. Green, yellow, and red bands may be obtained, and no doubt polished ornaments might be made of them. The mineral seems to form a large dyke, but more work is necessary on this point. The third occurrence, the “gem gravels” proper, were found to occur chiefly on the beaches south of Kakanui Heads. A recent bed of marine gravel covers large areas of land, and the “gem sands” may be obtained by panning off this gravel. They have been found as far north as Oamaru Cape. No trace of them has been found up the Kakanui or Waiareka Rivers. These sands are similar to the Museum specimens, and may now be briefly described. Besides pebbles (quartz), they contain small cleavage fragments of hornblende and feldspar, clear green minerals (presumably pyroxenes or amphiboles), red minerals (presumably garnets); the two latter minerals being angular and fractured, but showing no crystalline form. On the beach between this stream and Kakanui South Head the sea has concentrated the sands so that they consist almost entirely of garnet and magnetite, forming a thin layer on the top of the ordinary beach sand in a similar manner to the black sands (magnetite) occurring at many places on the east coast of Otago. The beach is periodically worked by a gold-miner, who scrapes off the top layer and washes it in a cradle for gold. It was believed by many of the residents of the district that occasionally rubies were obtained with the gold, and that they fetched a good price. The matrix was found to be a volcanic breccia occurring both on the North and South Head at the mouth of the Kakanui River. All the minerals of the “gem sands” were found in it, with the exception of the magnetite and quartz. These, however, are of such universal occurrence on the east coast of Otago that it is unnecessary to specially consider them here. A general description of the breccia and its relations is given. Detailed accounts of the minerals and fossils are withheld till further investigation has been made.
The breccia, for convenience called “mineral breccia,” occurs at four or five separated points in the neighbourhood
of Kakanui Township. On the North Head it is found for about half a mile on the sea-shore, forming low cliffs capped with gravel, and generally forming a beach of marine denudation for many feet seawards from the cliffs. Proceeding northwards on the beach it is seen to be underlaid unconformably by another volcanic breccia carrying no large crystals, for convenience called “barren breccia.” The latter forms the cliffs for about three-quarters of a mile, and then gives way to a conformably overlying very fine tuff, full of cleavage flakes of biotite. The “barren breccia” in its uppermost layer is fossiliferous, having apparently been covered with a coral reef. The fine tuff is only 15 ft. thick, and is overlaid conformably by a limestone which forms the cliffs on the beach for the next half-mile. It is overlaid conformably by a glauconitic greensand, and the whole is covered unconformably by thick beds of gravel and clay.
Further north the limestone gives place, on the cliffs, to the “mineral breccia,” which underlies it conformably, and this forms the cliffs for half a mile and then dips under the more recent beds of clay and gravel which form the Awamoa beach and stretch to Oamaru Cape.
I. Diagrammatic Section And Sketch, North Head, Kakanui.
a. Mineral breccia. b. Barren breccia. c. Fine micaceous tuff. d. Limestone. e. Greensand. f. Gravel. g. Quarry limestone.
The “mineral breccia” is also exposed on the west side of the hill forming the North Head, on cliffs near the Waiareka River. Here its relations are not clearly seen, as there is a slide between it and the “barren breccia” on the south, and it is lost under clay to the north. There is also an exposure of the “mineral breccia” about two miles north of Kakanui in a road-cutting (not shown in map); it is overlaid conformably by limestone.
On the South Head there are two exposures of breccia bearing the minerals, but their occurrence is somewhat different. On the cliffs exposed on the river-bank on the south shore the following section occurs:—
II. Section At Cliff On River-Bank, South Head, Kakanui.
a. Limestone. b. Marl. c. Coralline limestone. d. Tuffs and breccia. e. Hard cap of breccia. f. Limestone. g. Clay. h. Gravel.
A limestone of unknown thickness forms the base; this is covered conformably by a marl or calcareous mud, 26 ft. thick, the upper 6 in. being coralline limestone; then follows 60 ft. of rather fine volcanic tuff containing small cleavage fragments of hornblende, the top layer of 1 ft. being set in crystalline calcite, rather hard, and containing most of the minerals found elsewhere in the breccia. This is covered conformably by at least 40 ft. of limestone, containing many fossils. Clay then forms the cliffs to the mouth of the river. An isolated exposure of limestone occurs just at the river-mouth, its relations being obscured by gravel and an artificial breakwater erected there.
A few yards further on, towards the east, the “mineral breccia” occurs again, and forms the cliffs right round the South Head for about three-quarters of a mile. It contains fossils in its upper layers, and going southwards disappears conformably under a limestone 12 ft. thick. This limestone is covered conformably by a glauconitic greensand, and then gravel forms the cliffs for a mile to the south.
III. On Kakanui South Head. (Distance, ¼ mile.)
a. Mineral breccia. b. Limestone. c. Greensand. d. Gravel. e. Clay.
A bluish clay forms the beach for this mile to the south, but does not form cliffs, being only a few feet above sea-level. Its actual contact with the greensand cannot be seen.
The dips of the breccia and the associated rocks vary very rapidly from place to place. At the south end of the North
Head the breccia forms a small anticline, or, rather, elongated dome, as can be seen by tracing the outcrop-lines exposed on the beach.
The breccia is very calcareous and is roughly bedded, suggesting a submarine origin. The size of the included minerals diminishes on proceeding from the axis of the anticline outwards. The following are the largest dimensions of the rocks and minerals exposed here (the two dimensions refer to the two largest in cross-section exposed on the surface of the breccia): Hornblende, 100 mm. by 75 mm.; black augite, 115 mm. by 90 mm.; feldspar, 70 mm. by 40 mm.; garnet, 12 mm. by 10 mm.; green augite, 4 mm. by 5 mm.; basalt inclusions, 25 mm. by 30 mm.; holocrystalline inclusions, 75 mm. by 40 mm.
IV. Sketch-Plan Of Outcrops Of Mineral Breccia On The Beach, North Head, Kakanui, Showing Periclinal Fold.
The thickness of the breccia is rather difficult to determine, but at least 100 yards across the strike is exposed here, which at an average dip of 26° gives a minimum thickness of 130 ft. Further north, along the beach, the dip changes to the opposite direction, showing evidently a synclinal curve. The “barren breccia” also varies considerably in dip, and appears to form an anticline. It is slightly faulted at one place, the displacement being 18 in. to the northwards, and the hade of the fault 60° N. Near the fault is a fissure, filled evidently from above with the materials that form the “mineral breccia”; this filling, withstanding erosion a little better than the “barren breccia,” stands out like a dyke. Similar vertical cracks in a tuff at Oamaru are filled with limestone, and at first sight seem to be vertical beds of limestone. The “barren breccia” becomes flatter to the northwards in its upper layers, the dip
being 14° at its contact with the fine tuff. The top layer, as stated, is fossiliferous, and appears to have formed the sea-bottom for a sufficient time to permit of a growth of corals.
The chief fossils found here were: Corals—Flabellum circulare and other species; Trochocyathus, sp.; Deltocyathus, sp.; Amphihelia intricata; and Graphularia, sp. Brachiopods—Magellania lenticularis and M. sinuata; Liothyrina, n. sp.; Notothyris suessi; Terebratella gualteri; and Rhynconella squamosa. Scaphopods—Dentalium mantelli. Gasteropods—Scalaria brownii; Siphonalia nodosa; Mitra, sp.; Gibbula, sp.; Turbo, n. sp.; Natica, sp. Lamellibranchs—Cardita, n. sp.; and Pecten sectus. Cephalopods—Nautilus, sp.
The fine micaceous tuffs are poor in fossils, and apparently represent a period during which the ocean-floor was being covered with fine detritus from some neighbouring volcano. The “mineral breccia” contains very little mica; none was found on the sea side of the North Head, and but little on the side next the river. More was found on the South Head in both exposures, but the small amount renders it improbable that these tuffs could have come from the eruptions causing the “mineral breccia.”
The limestone overlying the micaceous tuff is, on the beach, poor in shells, being made up largely, as microscopic sections show, of Foraminifera and Bryozoa. The following genera occur: Textularia, Rotalia, Nummulina, and Globigerina.
On the top of the hill forming the North Head, however, a limestone of a different nature, but evidently stratigraphically the same, occurs. It appears to be composed almost entirely of Brachiopod shells, the shells themselves and the spaces between them, however, being filled with the same genera of Foraminifera and Bryozoa as occur on the beach limestone. It is thus referred to in the Geological Survey Reports (1883, p. 63) by Mr. McKay: “Kakanui Limestone: This forms an isolated hill near the Township of Kakanui. The limestone is full of shells, mostly Brachiopoda, and thus differs greatly from the ordinary Ototara stone, which is usually poor in such forms. In former reports it was referred to as a Tertiary rock, but closer examination and a large series of fossils collected show it to belong to the lower part of the Ototara stone. The Brachiopoda collected in great part belong to Terebratula, Terebratulina, and peculiar forms of Terebratella not found in New Zealand Tertiary rocks.”
The limestone here appears to be slightly more crystalline than in other parts, and breaks more readily across the middle of the Brachiopod shells, thus exposing the brachial arms and allowing of certain determination of the genera. Besides this,
in the blasting to which the stone is subjected in quarrying, the matter inside and around the shells is shattered, and allows the arms to be exposed by picking. The following fossils were obtained, bearing out Mr. McKay's reference to the genera: Liothyrina (Terebratula), n. sp.; Notothyris (Terebratulina) suessi; Magellania lenticularis; Rhynconella squamosa; Terebratella, n. sp.; Pecten sectus; Pecten hutchinsoni; Ostrea, sp.; Pleurotomaria tertiaria; and Aturia australis. This limestone, from its pure nature and hardness, is well adapted for burning for lime, and is used for that purpose. On the beach it is softer and contains more impurities of the nature of volcanic-ash inclusions.
The limestone on the beach has at the south a dip in a northerly direction, but it becomes flatter to the north and then dips to the south, thus forming a syncline. It is covered, wherever marine denudation has not denuded it, with a layer about 6 ft. thick of glauconitic greensand containing many fossils. The following were obtained: Isis, n. sp.; Graphularia, sp.; Magellania sinuata; Turitella, sp.; and Fusus, sp. (casts); Dentalium mantelli; Pecten hutchinsoni; numerous echinoid spines, and sharks' teeth. The limestone is underlain to the north by the “mineral breccia,” which is here finer than on the first exposure.
The general relation of the foregoing section is seen in the diagrammatic section (Section I) on p. 485.
Captain Hutton, in a paper on the “Geology of North-east Otago,”* refers thus to these rocks: “Kakanui Volcano: The Kakanui River runs into the sea between two low hills formed of scoriaceous sandstone overlain by the Ototara limestone, here generally more compact than usual. The sandstones of the northern hill form a periclinal curve, which extends across the river so as to include the rocks seen in the river-bed between the bridge and the sea.” On comparing the sections it will be seen that the folding is rather more complex than he has indicated.
The cliffs on the Waiareka River call for little special detailed mention. They are mostly formed of the “barren breccia,” and exhibit a great deal of irregularity in stratification, perhaps due to current bedding. In one place a few fossils were found, but they were too weathered for identification. The “mineral breccia” contains all the minerals found in the first exposure, and biotite in addition. The breccia is slightly less coarse than at the other exposure, the largest fragments being 75 mm. in
[Footnote] * Trans. N.Z. Inst., 1886, pp. 415–30.
diameter. Garnet, both in combination with other minerals and free, is more abundant here than elsewhere.
The exposure two miles north of Kakanui showed a limestone resting on the “mineral breccia.” The limestone in its upper layers has the characters of the ordinary Ototara stone, but the lowest few feet are more fossiliferous and resemble the Kakanui quarry limestone. This goes to prove that the Kakanui limestone is, as Mr. McKay remarks (loc. cit.), a local development of the lower layers of the Ototara stone. The fossils found here were: Flabellum, sp.; Isis dactyla; Graphularia, sp.; Terebratella gualteri; Liothyrina, n. sp.; Rhynconella squamosa; Pecten hutchinsoni; and Cidaris spines.
The sequence on the river-bank on the South Head is important as showing that a limestone underlies the volcanic rocks, the limestone forming wherever the sea-bottom was not subjected to a shower of cinder from the neighbouring volcanoes. The basement limestone is similar in microscopic structure (sections) to the others in the locality. It contained a few fossils, those collected being Liothyrina, n. sp.; Pecten, sp.; and Lamna, sp.
The overlying marl has few fossils. It would probably be a very suitable material for cement-making when mixed with some pure “white” limestone, but its small thickness and the over-lying gravels would prevent it from proving a cheap material to work.
The “mineral breccia” in this exposure is much finer in grain than in the others. It contained most of the minerals found on the opposite side of the river, and black mica in addition. The overlying limestone is rich in fossils; the following were obtained: Liothyrina, n. sp.; Terebratella gualteri, Terebratella aldingi, Magellania lenticularis, Rhynconella squamosa, Pecten hochstetteri, Pecten yahlensis, Pecten hutchinsoni, Dentalium mantelli, and numerous Cidaris spines.
The isolated exposure of limestone at the mouth of the river appears to be flat. It may lie on top of the “mineral breccia” exposed a little further east. If, however, it should be the same as that just mentioned, some displacement of the strata must be assumed.
The development of the “mineral breccia” at the South Head is somewhat like that on the North Head, but the outcrop lines cannot be traced on the beach, because of its steeper and more shingly nature. The inclusions are largest about the middle point of the cliffs. The dimensions obtained were: Hornblende, average, 12 mm.; black augite, 70 mm. by 25 mm., 60 mm. by 75 mm. by 35 mm.; feldspar, 40 mm. by 25 mm.; biotite, 40 mm. by 40 mm. by 12 mm.; holocrystalline inclu-
sions, 150 mm. by 100 mm., 125 mm. by 100 mm., 175 mm. by 200 mm., 25 mm. by 40 mm. A basalt block 5 ft. by 4 ft. by 1 ft. contained—Hornblende, 75 mm. by 50 mm.; black augite, 50 mm. by 40 mm.; biotite, 25 mm. by 12 mm.; green augite, 6 mm.; holocrystalline inclusions. This large block of basalt suggests proximity to the crater. It is possible that the sudden changes of dip may be due to the opposite directions of dip inside and outside of the crater.
One point regarding the origin of the inclination of the rocks might perhaps be discussed here. Captain Hutton in the paper referred to* states his conclusion that the Ototara lime-stone is the remains of several old coral reefs built up round small volcanic islands near the coast, and that it retains its original plane of deposition. This explanation seems to be pretty well borne out by the observations on the locality, but the syncline of the “mineral breccia” shown in the section of the North Head requires a hypothesis of folding.
The fossils obtained in the layers of the breccia to the south were as follows: Nothothyris suessi and Waldheimia lenticularis. The overlying limestone was rather glauconitic below, but passed into a hard coral reef above. The surface is easily denuded of the overlying greensand, and much eroded. It was formed chiefly of corals, especially Flabellum circulare and Isis dactyla, and Lamna, sp. The overlying greensand is very fossiliferous, and contains Isis dactyla; Isis, n. sp.; Magellania sinuata; Terebratella, n. sp.; and Fusus, sp. (casts).
There was no contact here with the blue clay seen further south. Captain Hutton (loc. cit.) says, “The unconformity between it and the underlying limestone is plainly to be seen in the coast section, not only in the difference of dip, but also in the denuded surface of the limestone (Section iv).” This appears inconclusive, for the denuded surface of the limestone is due to recent removal of the greensand and subsequent weathering. The difference of dip is more conclusive, but, compared with the general run of dips in the locality, is not very surprising. Still, there is a probability of unconformity from the fact that no contact is seen, and from the general flat nature of the blue clay. The fossils obtained in it were Flabellum circulare, Dentalium mantelli, Turritella rosea, Scalaria rotunda, Solonella australis, and Cucullœa, sp. A whale was also found. Of these T. rosea and S. funiculata are not recorded elsewhere from the Oamaru system, and go to prove that the beds belong to the Pareora and not to the Oamaru system.
[Footnote] * Trans. N.Z. Inst., 1886, p. 421.
Remarks On Stratigraphy.
From the above description, the matrix of the Kakanui “gem gravels” is seen to be a submarine volcanic breccia occurring on or near the sea-coast at the mouth of the Kakanui River. It is covered in all exposures by a limestone having the same nature and bearing the same fossils as the Ototara limestone, so that its stratigraphical position is in the Oamaru system. From the nature of the fossils in the breccia, and the occurrence of glauconite in and above the limestone, the formation of the breccia must have taken place at a considerable depth below sea-level. Should this breccia prove to be a matrix for gems, there would be three or four exposures where it might be quarried, and a surface-area of some acres. As there must have been one or more craters from which the breccia was erupted, there must be pipes running down to a considerable depth, and most likely containing larger fragments of mineral and less calcite. These could be worked by mining, although the permeable nature of the breccia might allow of a large influx of water. Should gems occur, they would probably be of a sufficient size in these pipes to repay working.
Identifications Of The Minerals And Rocks Contained In The Breccia.
Complete and detailed descriptions of these minerals will be of considerable petrological interest, but are withheld till further work has been done. It will be sufficient here to state that, after a considerable amount of chemical and microscopic examination, the following were identified:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Hornblende||in large fragments.|
|Feldspar, near oligoclase|
|Biotite||in smaller fragments.|
|Sandstone, limestone, greywacke.|
|Quartz, mica-schist, granulite, and garnet gneiss.|
|Various basic plutonic rocks, which may be grouped in two classes—(1) Iherzolites, eulysites, and wehrlite (containing fine spinels); (2) garnetiferous peridotites.|
The basalt contains inclusions of almost all the other rocks and minerals found in the breccia.
There is no trace of any gem of value in either the “gem sands” or the breccia, although the garnets and green augites would make pretty stones when cut. There are many points of geological interest, however, about the breccia which make a more complete examination of interest. In the first place, the presence of granulite and gneiss point to the existence, at no great depth, of such rocks as form the Wanaka formation, of the west coast of Otago.
The fact that so many of the minerals have a rounded form and high polish wants explanation.
The similarity, both in occurrence and composition, of the holocrystalline rocks to those of Kimberley at once raises the interesting question, Is it probable that diamonds will be found at Kakanui? This similarity is probably the reason that Professor Ulrich considered there was great probability that gems would be found at Kakanui. It will be of interest to discuss this similarity. Both occurrences are volcanic breccias; both are calcareous, the Kimberley breccias being now only seen in pipes or the necks of old craters, the Kakanui being well stratified and originally submarine; there must, however, be necks, and probably one occurs on the South Head. Both consist mostly of ultra-basic minerals and rocks. At Kimberley no volcanic rock is present, at any rate in a recognisable state; at Kakanui there is a limburgite or feldspar-free basalt, whose explosion may be assumed to have caused the eruption.
The amount of alteration that the Kimberley breccia has undergone has almost disguised its original character, and it is sometimes taken as a rock species. It has been shown by Bonney, however, that this alteration has had nothing to do with the formation of diamonds. Both breccias contain rounded rock-fragments.
The following isolated minerals are recorded from Kimberley: Enstatite,*† topaz,* chrome-diopside,* diallage,* ironpyrites,* ilmenite,*† olivine,† smaragdite,† chrome-diopside (“omphacite” of some authors),† a brown mica,† garnet (mostly pyrope, but more than one variety observed),† magnetite,† chromite.†
The following rock - fragments are recorded: Garnet-green pyroxene (omphacite) rock = eclogite,* bright-green pyroxene, zoisite purple-garnet rock,* pyrope-chrome diopside (olivine) rock = eclogite,† garnet-diopside-mica-enstatite rock = enstatite eclogite,† garnet-bastite rock,† olivine-bastite rock = saxonite,† garnet - enstatite - chrome - diopside - olivine rock = granatiferous
[Footnote] * Bonney, “Geological Magazine,” 1891, pp. 413, 414.
[Footnote] † Ibid, 1899, p. 309.
The Kakanui breccia, then, contains six rock — forming minerals also found at Kimberley—viz., olivine, diopside, diallage, garnet (two varieties), smaragdite, and spinel (cf. chromite). Both contain lherzolites and eulysites, and each contains two granatiferous periodotites—viz., eulysite and eclogite at Kimberley, and eulysite and garnet-pyroxene as Kakanui.
The diamond at Kimberley is found included in the eclogite and in the pyrope. Dr. Bonney considers the garnet very important: “As the ordinary varieties of the latter mineral [garnet] seem to be produced at a high temperature, the association [of diamond with it] may be significant.”
The origin of the diamond at Kimberley has been traced to a pyrope in an eclogite. The diamond is included in the pyrope as well-formed crystals. Its ultimate origin is obscure, as the parent rock has so high a percentage of oxygen that, according to Dr. Bonney, it is difficult to understand how so small an amount of carbon escaped oxidation. It seems as if the diamond formed in some more basic magma, and had been either taken up by the eclogite magma, or the magma containing it had mixed with one or more acid to form the eclogite magma. In any case, under the present amount of knowledge, the presence of diamond in an eclogite cannot be taken as a sign that diamond occurs only in eclogite, or that all similar eclogites contain diamonds. Many eclogites occur elsewhere without diamonds.
Under these circumstances, to establish a resemblance between the breccias at Kakanui and at Kimberley will give a possibility that diamonds occur at the former place, but not a very high degree of probability. The presence of two distinct garnetiferous peridotites in the former and of several in the latter creates a great similarity, and shows a similarity of conditions—magmas with little alkalis and crystallizing at a high temperature. Such a similarity is of sufficient interest to justify a thorough examination of the New Zealand locality, both from a theoretical and practical point of view.
The occurrences resemble one another also in that both are volcanic breccias. This can only bear on the subject in that, as diamonds would only be likely to occur in ultra-basic rocks, and as such rocks generally occur at a great depth in the crust, there would be greater probability of their occurrence in a rock formed from deep-seated eruptions than in any other way.
In regard to the other point of similarity, the occurrence in both breccias of rounded fragments, Dr. Bonney has proved to
his own satisfaction that the rounded form is due to the inclusion of waterworn fragments from deep-seated conglomerates. In the present case there is no evidence to show that such has happened. The rounding and polishing is so fine that it can only have been caused by friction, and the only suggestion that has occurred to me is that which Dr. Bonney rejects in the Kimberley case—viz., that the fragments have been rolled up and down in the crater. This point, however, has no bearing on the probability of the occurrence of diamonds.
In conclusion, from the similarity of the Kakanui breccia to that of Kimberley, it would excite no surprise if they were discovered, but it could hardly be predicted that diamonds occur at Kakanui.
The following papers refer, en passant, to the neighbourhood of Kakanui:—
1850. Mantell, “On the Geology of New Zealand,” Quart. Journ. Geol. Sci. (1850), vi, p. 324.
1870. Charles Traill, “On the Tertiary Series of Oamaru and Moeraki,” Trans. N.Z. Inst., vol. ii, p. 166.
1877. “Oamaru and Waitaki Districts,” Progress Reports, Geol. Surv. of N.Z., p. 41.
1883. “The North-eastern District of Otago,” Progress Reports, Geol. Surv. of N.Z., p. 63.
1886. Hutton, “On the Geology of the Country between Oamaru and Moeraki,” Trans. N.Z. Inst., vol. xix, pp. 415–30.
On diamonds: Carvill-Lewis, Geol. Mag., 1887, p. 22. Bonney and Miss Raisin, Geol. Mag., 1891, p. 413. Bonney, Geol. Mag., 1899, p. 309; Proc. Roy. Soc., 1900, lxvii, p. 475; Geol. Mag., 1900, p. 246.