xix AUGITE-ANDESITES 283 



5. SUB-ORDER, PRISMATIC, OF THE AUGITE-ANDESITES 

 (Felspar-lathes in flow-arrangement. Aug, matr^flu^prism^) 



This sub-order includes dark-brown or blackish semi-vitreous 

 rocks, all but one of which belong to the genus below described. 

 Since the exception (which belongs to genus 17 of the synopsis) 

 differs only in the presence of plagioclase phenocrysts, its separate 

 description is not needed. 



20. GENUS OF THE AUGITE-ANDESITES 

 FORMULA. Aug, matr> flu, prism, non-phen^parv. 

 CHARACTERS. In the groundmass the felspar-lathes are in 

 flow-arrangement, and the augite is prismatic. Plagioclase pheno- 

 crysts are absent or are very small and scanty, and pyroxene 

 phenocrysts when present do not exceed 2 mm. in size. 



DESCRIPTION. These dark semi-vitreous rocks occur in 

 agglomerates and as " necks " and dykes, and are at times scoria- 

 ceous. They are usually compact and aphanitic, showing few if 

 any plagioclase phenocrysts and having a semi-conchoidal fracture. 

 The specific gravity varies, being generally 2*6 2*65, but according 

 to the degree of basicity and amount of glass it may be as low as 

 2'5 or as high as 277. In the less glassy condition, as in the case 

 of a rock from the ridge east of Na Raro, the felspar-lathes are 

 relatively scanty and the groundmass is mainly formed of augite 

 prisms in flow-arrangement. The lathes are generally small, less 

 than *i mm., and rarely over '2 mm. Their extinctions are those of 

 oligoclase and acid andesine. The pyroxene prisms, which give 

 the large extinctions of augite, have a pale muddy-brown hue and 

 are as a rule '03 "07 mm. long. Granular pyroxene, if present, is 

 subordinate in amount. The glass, which is always in good 

 quantity and is sometimes abundant, displays fibrous devitrification. 

 In a rock from the vicinity of Narengali a variolitic structure is 

 exhibited in the form of sheaf-like aggregates of fibre-like felspars 

 and skeleton prisms of pyroxene. 



6. SUB-ORDER, OPHITIC, OF THE AUGITE-ANDESITES 

 (Felspar-lathes in flow-arrangement. Aug, rnatr^flu, oph.) 



21. GENUS OF THE AUGITE-ANDESITES 

 FORMULA. Aug, matr.fiu, oph, phen, vitr. 

 CHARACTERS. In the groundmass the felspar-lathes are in 



