286 A NATURALIST IN THE PACIFIC chap. 



i. Sub-Order, Granular, of the Hypersthene-Augite- 



Andesites 



{Felspar-lathes not in fioiv -arrangement?) 

 FORMULA. — Hjpersth-a?tg, matr, non-flu, gran. 



i genus . . . {Vitr.) 



2 „ . . . . {Opac.) 



3 „ . . . . {Magn.) 



4 „ . . . . (Parv.) J 



See Synopsis, p. 247. 



Nearly all of the rocks of this sub-order that are represented in 

 my collection belong to the genus (1) with phenocrysts of glassy 

 plagioclase. They vary considerably in appearance and in colour 

 (black to grey), and occur under very different conditions, as in 

 " necks," old flows, large intrusive masses, dykes, agglomerates, 

 &c. Their specific gravity has rather a wide range according to the 

 degree of basicity. In the heavier rocks where the rhombic 

 pyroxene is scanty, it is usually 27 to 2 - 8. In the others, where 

 rhombic pyroxene is more predominant and where the felspar is 

 less basic, it is 2*6 to 27. 



In the slide small phenocrysts of plagioclase and pyroxene 

 occur in a groundmass of felspar-lathes and pyroxene granules, 

 whilst there is as a rule a fair amount of residual glass. The 

 plagioclase phenocrysts, which give extinctions in different rocks of 

 acid and basic andesine and contain abundant magma-inclusions, 

 are generally one to two mm. in size. The pyroxene phenocrysts 

 are small, and may be represented by separate crystals of the mono- 

 clinic and rhombic kinds, or by crystals displaying intergrowths of 

 the two sorts. The pyroxene granules vary much in size and are 

 evidently in great part of augite. In most of the rocks the felspar- 

 lathes are less than 'I mm. in length. In those where the length is 

 •1 to '2 mm. they are sometimes stout and display a few lamellae, 

 yielding extinctions corresponding to those of the phenocrysts. 



A singular dark grey almost holocrystalline doleritic rock (sp. 

 gr. 2*85) is exposed in the Thulanga Ridge (p. 211). It shows no 

 plagioclase phenocrysts, but those of pyroxene are numerous, which, 

 however, do not exceed 2 mm. in size, so that the rock would be 

 referred to genus 4 of this sub-order. It appears to be a doleritic 

 form of the plutonic rock found at Nawi in this neighbourhood 

 (p. 211). The pyroxene phenocrysts are mostly of brownish-yellow 

 augite, but rhombic pyroxene, either as separate crystals or as 

 intergrowths, is not uncommon. The plagioclase lathes are long 



