xx HYPERSTHENE-AUGITE-ANDESITES 287 



and fairly stout, giving at times lamellar extinctions of 20 . Their 

 average length is 3 mm., and it is to their large size that the 

 doleritic texture is due. The pyroxenes of the groundmass are 

 similarly coarse (-2 mm.), and include both monoclinic and rhombic 

 forms, the latter infrequent. There is a slight tendency to semi- 

 ophitic behaviour in places ; but generally these pyroxenes are 

 irregular in shape or rudely prismatic. 



2. Sub-order, Prismatic, of the Hvpersthene-Augite- 



Andesites 



{Felspar-lathes not in flow-arra?igement.) 

 FORMULA. — Hypersth-aug, matr, non-flu, prism. 



" See Synopsis. 



This sub-order includes rocks varying much in appearance, but 

 all alike in the presence of prismatic pyroxene in the groundmass 

 and in the absence of flow-arrangement of the felspar-lathes. They 

 belong to the two genera, with glassy and opaque plagioclase 

 phenocrysts. These crystals are not usually over 2 mm. in size 

 and are of medium andesine (15 — 20 ). The pyroxene pheno- 

 crysts are small and may be entirely of monoclinic or of rhombic 

 pyroxene, or the two may be associated either as lamellar inter- 

 growths, or by displaying an eroded nucleus (generally rhombic) 

 around which a regular crystal of monoclinic pyroxene has grown. 

 The felspar-lathes are in some cases less than 1 mm. long, and in 

 others -i — - 2 mm. The pyroxene prisms of the groundmass 

 average "Oi — "04 mm. in length, and give both straight and oblique 

 extinctions, the last prevailing. The specific gravity ranges from 

 2*55 to 275 according to the degree of basicity and amount of 

 interstitial glass, which is usually in fair quantity. 



4. Sub-order, Granular, of the Hypersthene-Augite- 



Andesites 



{Felspar-lathes in flow-arrangement.) 



13. Genus 

 FORMULA.— Hypersth-aug, matr, flu, gran, phen, vitr. 

 Characters.— Glassy plagioclase phenocrysts. 



