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 Hypersthene-Augite- 



Andesites 



{Felspar-lathes not in flow-arrangement?) 



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

 5 genus . . . {Vitr.) 



6 „ ... {Opac.) 



7 „ ... {Magn.) 



8 „ ... [Parv.), 



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 'i 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 



Yo^UVhA.— Hypersth-aug, matr, flu, gran, phen, vitr. 



Characters.— Glassy plagioclase phenocrysts. 



