CHAP. XIX AUGITE-ANDESITES 267 



I. Genus of the Augite-Andesites 

 Formula. — Ati^, matr^ noti-Jlu, gran, phen^vitr. 



Characters. — In the groundmass the felspar-lathes and 

 prisms are not in flow-arrangement and the augite is granular. 

 Phenocrysts of glassy plagioclase. 



Description. — These rocks frequently form dykes ; and it is 

 probable that most of the instances where the nature of the 

 exposure could not be ascertained also fall into this category. 

 They are dark-brown or blackish, and their sp. gr. ranges, except 

 in the semi-vitreous rocks, from 27 to 2"83. They are sometimes 

 vesicular, and rocks with abundant interstitial glass are common. 

 They admit of grouping into two sub-genera according to the size 

 of the plagioclase phenocrysts : 



{a) Porphyritic, where the average size is 3 mm. or over. 

 {b) Non-porphyritic „ „ „ „ „ less than 3 mm. 



Nearly all the rocks in my collection belong to the second group. 



In the sections they display phenocrysts of plagioclase and 

 occasionally of pyroxene in a groundmass formed of a plexus of 

 felspar-lathes, augite granules, magnetite, and usually a fair amount 

 of smoky more or less opaque interstitial glass. . . . The felspar 

 phenocrysts, which are sometimes abundant, give lamellar extinc- 

 tions of andesine labradorite (15° to 30°). They are frequently 

 small (i to 2 mm.) and contain often many magma-inclusions. 

 Whilst the corroded aspect of some indicate that they belong to an 

 earlier period, the aggregate character and regular outlines of 

 others suggest that they have been produced in position. . . . 

 Pyroxene phenocrysts are absent in half the rocks. When present 

 they are generally small and of a pale augite which gives extinctions 

 of 30°. Their size does not usually exceed 2 mm. ; and they may 

 consist of single crystals (sometimes twinned) or of an aggregate of 

 smaller crystals. At times there is a suspicion of intergrowth with 

 rhombic pyroxene ; but no phenocrysts formed alone of that 

 mineral occur. . . . The augite granules of the groundmass as a rule 

 vary from "02 to '04 mm. in diameter. Occasional prism-forms 

 giving large extinctions occur. . . . The felspar-lathes vary much 

 in length in different rocks. In some they average as little as "05 

 mm., and in others as much as '2 mm. ; but the doleritic type with 

 yet longer lathes is not represented in the collection except among 



