266 E. 0. Thiele: 



basio ones, the formei- largely predominating. Tiiey show an 

 infinite variety of texture and colour, but chemically all are 

 closely similar. 



The commoner varieties show a base varying in colour from 

 light gi'een to pink and dark chocolate-brown. Phenocr^^sts con- 

 sist of quartz and orthoolase felspar, the latter varying from 

 white to pink. 



One type from the western slopes of Wellington, in the 

 vicinity of Lake Kamg, is an attractive rock with a light 

 greenish base and moderate sized phenocrysts, of white ortho- 

 clase and quartz. Tbin sections show a fine perlitic structure. 



In general from a structural point of view two divisions can 

 be made : — 



(1) Those showing marked flow structure. 



(2) Tbo'se of the type of normal quartz porphyries. 



Thin sections of the latter often show the phenocrysts em- 

 bayed by the devitrified magma, but occasionally sections show 

 no such features, and sharp, angular fragments of quartz and 

 felspar form a gi-anular base, suggesting a pyroclastic origin, 

 but this feature is hard to establish. These rocks are of great 

 extent and thickness, and their relation and distribution has 

 been briefly referred to in a former paper. 



SiO., - 



Al-A - 

 TiO., - 



"^^.O, - 



FeO - 



MgO - 



CaO - 



Na,0 - 



K.,0 - 



P.O, - - - 



Water combined - 



Water hygroscopic 



100.33 100.54 



