xix AUGITE-ANDESITES 271 



5. Genus of the Augite-Andesites 

 FORMULA. — Aug, matr, nonflu, prism, phen, vitr. 



CHARACTERS. — In the groundmass the felspar-lathes and 

 prisms are not in flow-arrangement and the augite is for the most 

 part prismatic. Plagioclase phenocrysts glassy. 



DESCRIPTION. — Except as regards the prismatic pyroxene 

 these rocks do not differ much from the " granular " augite-andesites. 

 Those before me show phenocrysts of plagioclase and sometimes 

 of augite in a groundmass displaying a mesh-work of felspar-lathes, 

 prismatic pyroxene, and much interstitial glass. . . . The plagioclase 

 phenocrysts, 1 to 3 mm. in size, show abundant magma-inclusions 

 arranged either zone-wise or parallel to the twinning-planes. 

 They are often eroded. . . . The phenocrysts of augite, which give 

 extinctions of over 30 , are often rounded and display glass and 

 other inclusions. . . . The prismatic pyroxenes of the groundmass 

 vary in average length from '03 to - o8 mm. They have the peculiar 

 pale muddy brown hue characteristic of the prismatic augite in 

 these rocks, and give oblique extinctions up to 30 and over. They 

 may be short and broad or long and slender, and when there is 

 much glass in the rock they resemble the felspar-lathes in their 

 forked and imperfect extremities. Granular pyroxene occurs, but 

 is subordinate. . . .The felspar-lathes, 'i mm. long, are rarely 

 lamellar, and give extinctions measured from their long axis of 20° 

 (basic andesine). 



6. Genus of the Augite-Andesites 

 FORMULA. — Aug, matr, non-flu, prism, phen, opac. 



CHARACTERS. — In the groundmass the felspar-lathes are not in 

 flow-arrangement and the augite is for the most part prismatic. 

 The plagioclase phenocrysts are opaque. 



DESCRIPTION. — Light and dark grey rocks displaying abundant 

 opaque plagioclase phenocrysts not exceeding 2*5 mm. They 

 are somewhat altered, one of the specimens having a sp. gr. 

 of 2-68. 



In the section they exhibit phenocrysts of plagioclase, and 

 occasionally of augite in a groundmass of felspar-lathes, pyroxene 

 prisms and granules (the former predominating), with a fair amount of 

 altered interstitial glass. . . . The plagioclase phenocrysts owe their 

 opacity to the great number of fine and sometimes parallel cracks 



