XXI ACID ANDESITES 299 



They have dark resorption borders and are sometimes deeply 

 corroded. They show in various stages the remarkable conversion 

 at the borders into fine pyroxene, which is described on page 306 .... 

 The pyroxene phenocrysts are more numerous in the Urata rock. 

 They are for the most part of the pale yellow feebly pleochroic 

 rhombic type that prevails in the island. A few phenocrysts of 

 pale augite (ext. 35°) may occur in the same slide ; whilst the two 

 pyroxenes may be associated as intergrowths. 



A crypto-crystalline variety of these rocks, where the felspar - 

 lathes and rhombic pyroxene prisms of the groundmass are only 

 in part differentiated, is found on the hills of Ndreke-ni-wai on the 

 shores of Natewa Bay (page 201). It is a pale-grey open-textured 

 rock, displaying numerous small macroscopic crystals of horn- 

 blende. 



Third Order of the Hornblende-Hypersthene- 

 Andesites 



{Felspars of the groundmass, short and broad, of the orthophyric 



type) 



These rocks occur generally as agglomerates and are more 

 particularly characteristic of the district between the Mariko Range 

 and the Salt Lake. They belong for the most part to the prism- 

 atic sub-order of the group and to the section with plagioclase 

 phenocrysts, and fall naturally into two divisions corresponding to 

 the two genera with glassy and opaque phenocrysts. The last 

 named would be regarded by some as porphyrites. The specific 

 gravity of the specimens ranges from 2'S2 to 27. 



The plagioclase phenocrysts, i to 2 mm. in size, give extinc- 

 tions indicating in some rocks oligoclase-andesine (10° — 15°) and 

 in others basic andesine (15° — 25°). Their opacity in the porphy- 

 rites is sometimes due to multiple macling, but more usually it 

 arises from the numerous fine cracks filled with decomposition 

 products that traverse them. The phenocrysts of dark brown 

 hornblende are generally abundant and give extinctions of i S degrees. 

 They as a rule have dark resorption borders in which the process 

 of conversion into fine pyroxene is in active operation. The 

 pyroxene phenocrysts are scanty and in most cases rhombic ; but 

 intergrowths with augite and separate crystals of the last-named 

 may occur. In the altered rocks or porphyrites they are largely 

 replaced by bastite and viridite. The felspars of the groundmass 



