Marshall. — Geology of Earotomja and Aitutaki. 99 



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 

 laud v/hich Mr. Brown says is called locally " the dividing-ridge." This. 

 ridge, which at its highest point attains an elevation of 2,94:0 ft., appears to 

 show the features of rainfall erosion in tropical islands so well described by 

 Dana and Dutton in Haw^aii. Other specimens came from near the Toto- 

 koito Creek, and some Native weapons dug up in a plantation w^ere also 

 forwarded to me. 



The rocks from Muri Point and Black Point are a nephelinitoid phono- 

 lite. The former is rather the coarser type. It contains no feldspar, but 

 an abundance of nepheline. At a certain stage of growth ii'girine inclusions 

 were gathered into the nepheline, so definite crystalline forms of the mineral 

 are outlined. At a later stage the aegirine material was completely crys- 

 tallized, and pure nepheline fills the interspaces. With the exception of a 

 few crystals of apatite, the only other mineral is ppgirine-augite, with an 

 extinction-angle of 30''. In the specimen from Black Point there is a very 

 small amount of feldspar. 



Analysis. 



SiO., .. .. .. .. .. .. 54-60 



A1..0, 



Fe,03 



FeO 



TiO„ 



CaO 



MgO 



"-to^ 



NaaO 

 H,0 



17-48 

 5-72 

 1-02 

 0-80 

 3-10 

 1-44 

 9-32 

 5-61 

 1-56 



100-65 



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 oc- 

 currence 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 generalisationf in regard to the 

 association of andesitic rocks with the Pacific type of coast-line, and of alka- 

 line rocks with the Atlantic type of coast. Occurrences in New Zealand, 

 as pointed out by Gregory, J are also opposed to this view. 



* La Croix, "" Comptes Rendus," vol. cxxxix, p. 892. 

 t Prior, Rec. Antarctic Expedition : Geology. 

 t Nature 



