xix AUGITE-ANDESITES 277 



SPECIES A. — Felspar-lathes -02- 1 mm. in average length. This 

 may again be sub-divided according to the degree of basicity of 

 the rocks : — 



(a) Sub-species of greater basicity. — Sp. gr. 276 to 2'82. . . • 

 Such rocks are represented in dykes and in the prevailing basic 

 agglomerates. They are at times scoriaceous. The small plagio- 

 clase phenocrysts, which are fairly numerous, give lamellar 

 extinctions of andesine labradorite (20 to 30°). Two kinds occur 

 which may or may not be represented in the same slide. In the 

 one the crystal is much corroded and contains abundant magma- 

 inclusions. It belongs in such a case to an earlier period. In the 

 other the outlines are clean and regular, and the crystal is often 

 cross-macled to such an extent that it may be inferred from its 

 unbroken condition to have been formed in situ. Augite 

 phenocrysts when present are small and scanty, pale-yellow, idio- 

 morphic, and giving extinctions of + 30 . The felspar-lathes, 

 which average '06 — "o8 mm. in length, give extinctions indicating 

 andesine labradorite. The augite granules are small ( # oi — "02 mm.). 

 Interstitial glass, generally scanty, is sometimes abundant when it 

 is smoky, showing fibrous devitrification, with irregular " lacunae " 

 filled with a brownish yellow opaque glass like palagonite. 



(b) Sub-species of lesser basicity. — Sp. gr. 2*65 — 270. . . . The 

 remarks on the plagioclase phenocrysts of the more basic sub- 

 species here apply, except that the lamellar extinctions indicate 

 medium andesine (12 — 20 ). The characters of the augite pheno- 

 crysts and granules are in both groups the same ; but in this case 

 there is more frequently a suspicion of intergrowth with rhombic 

 pyroxene. The felspar-lathes are very small, '04 or - 05 mm, and 

 give simple extinctions of acid andesine (5 — io°). Interstitial glass 

 exists in moderate amount. 



SPECIES B. — Felspar-lathes 1 — *2 mm. in average length. 



Blackish or dark -grey rather compact rocks, sp. gr. 275 — 279, 

 that cannot be readily divided into groups according to their 

 basicity. They form dykes and volcanic " necks " and are some- 

 times scoriaceous. The small plagioclase phenocrysts, which are 

 most evident in the slide, present the two kinds above described 

 under Species A. They give lamellar extinctions varying from those 

 of medium andesine to acid labradorite (15 — 30°). The augite 

 phenocrysts, which are small and scanty, occasionally show inter- 

 growths of rhombic pyroxene. The augite granules are generally 

 •02 to "03 mm. in size, and here and there a prism form gives 

 extinctions of + 25 . The felspar-lathes which average 11 to 



