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 -f- 30°. The felspar-lathes, 

 which average '06 — "08 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 "OS mm, and 

 ]five simple extinctions of acid andesine (5 — 10°). Interstitial glass 

 ixists in moderate amount. 



Species B. — Felspar-lathes 'i — *2 mm. in average length. 



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

 ' hat cannot be readily divided into groups according to their 

 1 )asicity. They form dykes and volcanic " necks " and are some- 

 limes scoriaceous. The small plagioclase phenocrysts, which are 

 1 tiost evident in the slide, present the two kinds above described 

 1 nder Species A. They give lamellar extinctions varying from those 

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

 I henocrysts, which are small and scanty, occasionally show inter- 

 \ rowths of rhombic pyroxene. The augite granules are generally 

 •< )2 to "03 mm. in size, and here and there a prism form gives 

 extinctions of -f 25°. The felspar-lathes which average 'ii to 



