Art. XVII.—Geology of Rarotonga and Aitutaki.
[Read before the Otago Institute, 10th November, 1908.]
But few statements have hitherto been made as to the geological nature of this island, and they are very general. Mr. Percy Brown has lately been good enough to forward me specimens of rock from several localities on the island, and from the study of these, as well as the statements of Mr. Brown, Mr. James Allen, M.P., and those in the Government Year-book, the following description has been compiled.
Relatively few soundings have been made near Rarotonga, but there is at present no reason to doubt that it is surrounded on all sides by water between 2,000 and 3,000 fathoms in depth. It is situated to the east of the deep trench which extends from New Zealand almost to Samoa.
The volcanic rock appears to rise directly from the ocean-shore without any intervening fringe of raised coral rock, though the island is surrounded with a fringing reef of coral. This appears to prove that no change in elevation has taken place since the volcanic action ceased.
In the neighbouring island of Tonga Mr. Lister has proved an elevation of 1,000 ft., while the atoll of Palmerston and others seem to prove consider-
able depression in their neighbourhood, if the brilliant results acquired by a study of Funafuti borings can be extended to other isolated atolls.
The rocks of Rarotonga appear never to have been described. Those that were sent to me were obtained from Muri Point, in the north-west, and Black Point, in the south-east. Between these two points is the high land which Mr. Brown says is called locally “the dividing-ridge.” This ridge, which at its highest point attains an elevation of 2,940 ft., appears to show the features of rainfall erosion in tropical islands so well described by Dana and Dutton in Hawaii. Other specimens came from near the Toto-koito Creek, and some Native weapons dug up in a plantation were also forwarded to me.
The rocks from Muri Point and Black Point are a nephelinitoid phonolite. The former is rather the coarser type. It contains no feldspar, but an abundance of nepheline. At a certain stage of growth ægirine inclusions were gathered into the nepheline, so definite crystalline forms of the mineral are outlined. At a later stage the ægirine material was completely crystallized, and pure nepheline fills the interspaces. With the exception of a few crystals of apatite, the only other mineral is ægirine-augite, with an extinction-angle of 30°. In the specimen from Black Point there is a very small amount of feldspar.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
From Totokoito Creek a rolled fragment of dolerite was sent. The augite and olivine are in large crystals, as well as the iron-ore, which is probably titaniferous. Feldspar is restricted to the groundmass, where it is associated with augite and magnetite.
This rock recalls the descriptions of basalts from Tahiti,* though the specimens which I possess are of a wholly different character. The occurrence of alkaline rocks in this island is of great interest, and perhaps serves to suggest a relationship with Tahiti, where nepheline syenites have been recorded by La Croix. They are quite different from anything yet found in New Zealand.
The occurrence of such rocks in a mid-Pacific island appears to throw doubt upon the accuracy of Dr. Prior's generalisation† in regard to the association of andesitic rocks with the Pacific type of coast-line, and of alkaline rocks with the Atlantic type of coast. Occurrences in New Zealand, as pointed out by Gregory,‡ are also opposed to this view.
[Footnote] * La Croix, “Comptes Rendus,” vol. cxxxix, p. 892.
[Footnote] † Prior, Rec. Antarctic Expedition: Geology.
[Footnote] ‡ Nature
The rock here described from Muri Point would, from the complete absence of feldspar, be classed as a nephelinite according to Rosenbusch's scheme. It is retained here amongst the phonolites because its chemical and mineralogical composition show much closer relationship to the phonolites than to the basalts.
The Native weapon forwarded by Mr. Brown is of quite different material from the rocks described. The absence of olivine and the abundance of feldspar place it among the augite-andesites. It is quite conceivable that it was made from one of the Tongan rocks.
Mr. C. Cameron, R.M., kindly forwarded me samples of rock from this island. The following notes were made by him on the specimens: “Sample A, from the mainland, is fairly common, sometimes in large blocks, and was occasionally used in ancient times for building round the priests' marae. It is easily shattered by fire. Sample B is from a small island called Rapoka, in the lagoon, about a mile from the mainland. It is often used by the women for their native ovens, as it stands fire better than A, though not entirely well.”
A. A relatively coarse basaltic rock, with much olivine and an abundance of idiomorphic augite, and but little feldspar, which is labradorite. The colour of the augite shows that it is strongly titaniferous. No nephelin could be distinguished in section, but the rock-powder gelatinises readily on treatment with dilute HCl, and crystals of salt are formed when the solution is evaporated.
B. Very fine grained compact rock. Olivine is very abundant, and is often stained with limonite on the margin. Augite granular, and restricted to the groundmass, where it is associated with much magnetite and a predominance of minute crystals of nepheline, without sharply crystalline boundaries. The rock is clearly a nephelinite or nepheline-basalt.
The accompanying map, copied from the Admiralty chart, shows the form of the island. It is at once seen that the mainland is at the northern side of the interior of the circular coral reef, and is situated near the margin. Rapoka Island is near the south-east portion of the interior of the reef.
The form of the island suggests that submergence has taken place, but it may be that the coral reef has arisen on the margin of a shoal formed by wave-action dispersing the material of a loose scoria cone, as has been exemplified in the case of Falcon Island, in the Tonga Group. It will be remembered that this origin has been suggested by Mr. Lister to explain the occurrence of atolls in the southern portion of the Tonga Group of islands, although there is clear evidence of elevation of as much as 1,000 ft. in the north-east portion of the group.
The rocks from Aitutaki are not closely related to any types that I have seen in the south-west Pacific. They resemble the Auckland basanites more closely than any of the others.
Explanation Of Plates III And IV.
Sketch-maps of Rarotonga and Aitutaki.
Fig. 1. Micro. section of nepheline-phonolite, Rarotonga The clear portion is nepheline; the dark material is ægirine. Magnified 30 diameters, ordinary light.