ALKALINE ROCKS OF THE COOK AND SOCIETY ISLANDS. 199 



was almost a horizontal flow. Beneath the phonolite scoria, which 

 is 50 feet thick, there is basalt completely decomposed into red clay. 

 (Fig. 1). 



M^Tapio, 1000 Ft 

 Phonolite 

 ti Phonolite Scoria- 



"c^ °^ 



Probable basis of LorgI 

 limestone 



Fig. 1. 

 Geological Section Mt. Tapioi, Raiatea. 



A fine-grained basalt occurs south of the harbour of Avarua. 

 It outcrops at the sea-level. It contains a few phenocrysts of 

 labradorite. The greater part of the rock is extremely dense, and 

 consists mainly of microlites of labradorite with small crystals of 

 olivine stained yellow with limonite and minute grains of a colour- 

 less pyroxene. There is a great deal of magnetite. 



Another mile further south the ordinary coarse basalt or 

 dolerite so common in the Society Islands crops out at the sea- 

 level, but its relation to the other rocks could not be made out. 



The following is the composition of the phonohte from Mount 

 Tapioi, compared with that of r ocks from other South Sea localities : 



1. Summit of Mt. Tapioi, Raiatea, Society Islands. 



2. Boulders at Fare Harbour, Huaheine, Society Islands. 



3. Vairao Tahiti, Society Islands, Lacroix. 



4. Alkali Trachyte, Tafagaf;ai, Tutuila, Samoa, Weber. 



5. Signal Hill, Dunedin, New Zealand. 



6. Nephelinite, Black Rock, Raratonga, Cook Islands. 



