ALKALINE ROCKS OF THE COOK AND SOCIETY ISLANDS. 197 



approaches quite closely to the composition of Analcite Si02 54 "5, 

 Al 203 23-2, soda 14-1, H2O 8-2. Such chemical treatment as that 

 employed does not dissolve glass, and the chemical effect certainly 

 suggests that some easily decomposed isotropic mineral, probably 

 analcite, forms the base of the rock. A bulk analysis gave the 

 following result : — 



1. Hornblende type from Arue. 



2. Lacroi.x type L from Papenoo, Bull. Geol. Soc. tie Fra 



3. Lacro X type H. from Vairao. 



4. Me!f;, Rammclsberg, Zeilschr. d.d. Geol. Gesdl, XIII , p. 273 



1910, p. HE 

 1896. 



It will be seen that my analysis shows a negligible quantity of 

 sulphur and a distinct amount of chlorine. This indicates that the 

 blue isotropic mineral is sodalite, not hauyne, as it was identified by 

 Lacroix, whose analysis gives a higher percentage of sulphur. The 

 rock contains a considerably higher per centage of SiOg than those 

 quoted by Lacroix from other localities, and more than the 

 amount in the additional analyses collected by Rosenbusch 

 (" Elemente de Gesteinlehre," 2nd ed., p. 372). On the other 

 hand it is too basic for the ordinary rocks with sodalite. Even the 

 phonolitic types are much more acid. The lowest silica percentage 

 that I can find recorded in a sodahte-bearing rock is in that from 

 Mopanui (Dunedin, New Zealand), which has 56.40 per cent, of 

 silica, and another related rock from the same locality 55.66. 

 Lacroix, making use of the American classification, places his 

 hauynophyre in the Essexose sub-rang, where it is associated with 

 rhombporphyry, theralite, essexite and phonolite. 



Tahaa.— The only rock obtained from this island came from 

 Rei Point, on the north-east side. It is an extremely coarse 

 dolerite. The earliest mineral to form was olivine, but the grains 

 are usually rounded ; they are often stained with limonite. 

 Irregular grains of brownish augite and crystals of felspar (labra- 

 dorite) constitute the rest of the rock except for rather numerous 

 crystals of menaccanite. There is said to be a large outcrop of this 

 rock, which is of a light grey colour. 



