XIX AUGITE-ANDESITES 271 



5. Genus of the Augite-Andesites 



Formula. — Aug, matr, non-flu, 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, I 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 -08 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 



