290 A NATURALIST IN THE PACIFIC chap. 



more or less devitrified. The specific gravity varies considerably, 

 but is as a rule between 2-55 and 275, the more basic rocks con- 

 taining augite in preponderance and basic andesine, whilst the 

 less basic possess a large proportion of rhombic pyroxene and dis- 

 play oligoclase-andesine. Sometimes, as in the case of a rock 

 composing an agglomerate east of Nanduri, where the porphyritic 

 plagioclase is opaque and there is some degree of alteration, the 

 rock looks very like a porphyrite. 



Third Order, Orthophyric, of the Hypersthene-Augite- 



Andesites 



Formula. — Hypersth-aug, matr, orth. 



Characters. — Felspars of the groundmass short and broad. 



Since the material is insufficient for the separate description 

 of each genus, a general account of the order is alone given. 

 These rocks are often represented in agglomerates or they occur 

 as large blocks, either lying on the surface or imbedded in tuffs. 

 Many of them are somewhat altered. 



They are for the most part dark grey dull-looking rocks, with 

 a specific gravity of 27 to 2'8, showing macroscopic plagioclase 

 together with conspicuous pyroxene phenocrysts. The plagio- 

 clase phenocrysts are usually small (i to 3 mm.), and give extinc- 

 tions of medium andesine (15° — 20°) and in some rocks of acid 

 labradorite (30°). They are as a rule corroded and are penetrated 

 by numerous fissures, whilst they contain a considerable amount 

 of altered magma-inclusions with sometimes other alteration pro- 

 ducts. The pyroxene phenocrysts are from 2 to 4 mm. in size. 

 Brownish-yellow augite, giving extinctions of over 30°, and pale- 

 yellow rhombic pyroxene of the type before described occur 

 generally in the same slide, and are frequently associated as inter- 

 growths in the same crystal. They may have regular outlines 

 or dark eroded borders, and at times they exhibit abundant dark 

 opaque inclusions. The broad felspars of the groundmass are 

 sometimes rectangular and give lamellar extinctions of medium 

 and acid andesine (12° — 17°). They vary in length in different 

 rocks from 'OS to '2 mm. and more. The pyroxene of the ground- 

 mass is generally granular and coarse ('02 — '05 mm.). As indi- 

 cated by the extinctions of occasional prism-forms it is composed 

 of both augite and rhombic pyroxene, the former prevailing. The 

 prismatic sub-order is also represented, and here the pyroxene of 

 the groundmass is in great part prismatic, the length of the prisms 



