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.
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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.