xix AUGITE-ANDESITES 281 



(2) Least basic sub-species . . . Sp. gr. 255 — 275. Dark com- 

 pact aphanitic rocks especially characteristic of the Ndrawa 

 district. When plagioclase phenocrysts are present, they are very 

 scanty and not generally over a millimetre in size, possessing 

 rectangular clean outlines and showing but few inclusions. They 

 may display carlsbad twinning and zoning, or albite twinning, 

 when they give extinctions of oligoclase andesine (io° — 15°). 

 Pyroxene phenocrysts are either absent, or scanty and small, being 

 usually of pale yellow augite with occasional indications of inter- 

 growth with rhombic pyroxene. The felspar-lathes as a rule 

 average "07 or "oS mm. and present a dense fluidal arrangement. 

 They rarely display lamellar twinning and give extinctions 

 measured from the long axis of oligoclase and oligoclase andesine 

 (2 — 8°). The pyroxene granules are very small, averaging "Oi mm. 

 and less. There is also fine magnetite. A little interstitial glass is 

 usually present. When abundant it is not generally smoky but 

 shows clear fibrous devitrification. . . . One of the specimens, 

 which is semi-vitreous, exhibits tube-like steam-pores drawn out to a 

 length of 1 — 1 1 centimetres. The felspar microliths are only "02 mm. 

 in length. The copious glass has the character above described. 



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



This species may also be sub-divided into two sub-species 

 (more basic and less basic). Since, however, all but one of the 

 fifteen rocks belonging to the species are of the more basic kind 

 my remarks will mainly apply to them. They are dark-brown or 

 dark-grey compact aphanitic rocks, occasionally banded or streaky, 

 in appearance, and ranging in specific gravity from 275 to 2-84. 

 They occur in several districts, but are especially characteristic of 

 the Ndrawa district. The plagioclase phenocrysts, if present, are 

 very scanty and small (1 or ih mm.). They contain inclusions of 

 the magma and give lamellar extinctions of andesine labradorite 

 (20 — 30°). Pyroxene phenocrysts do not generally occur. When 

 present, they are small and of pale yellow augite yielding large 

 extinctions . Occasionally micro-porphyritic augite is well repre- 

 sented. The felspar-lathes, which exhibit a well-marked flow- 

 arrangement, are generally -13 to -15 mm. long. Lamellar twinning 

 is uncommon, the extinctions measured from the long axis indicat- 

 ing basic andesine (io° — 20°). The augite granules are abundant 

 and small ( - oi — - 02 mm.). Occasional prism-forms yield large 

 extinctions. Magnetite is abundant, its grains corresponding in 

 size to the augite granules. There is as a rule a little residual 

 glass, which shows fibrous devitrification and is not smoky. The 



