xxi ACID ANDESITES 



297 



with specific gravity of 2-54—2-62, and displaying abundant por- 

 phyritic crystals of pyroxene, 2 — 3 mm. in size. In the slides they 

 show a large number of plagioclase phenocrysts together with those 

 of pyroxene in a relatively scanty groundmass, for the most part 

 orthophyric in texture and without residual glass. . . . The plagio- 

 clase phenocrysts, which are not usually over 2 or 3 mm. in size, are 

 often tabular and show distinct zone-lines. Though they are 

 traversed by minute cracks and have frequently a semi-saussuritic 

 appearance arising partly from change-products and partly from 

 the abundance of colourless inclusions, they yield clear lamellar 

 extinctions of medium and basic andesine (15° — 25 ). . . . The 

 pyroxene phenocrysts, which are not much altered, are in most 

 cases long pale-yellow rhombic prisms with rounded ends, behaving 

 optically as described on page 285 ; but intergrowths with mono- 

 clinic pyroxene may occur and even separate crystals of augite. . . . 

 The scanty groundmass, though in the main formed of short and 

 broad felspars, "12 mm. long, of the orthophyric type, displays in 

 places a rude mosaic, apparently of quartz and felspar. It also 

 shows abundant small pyroxenes in the form of small prisms 

 ('05 mm. in length), giving extinctions nearly always straight but 

 occasionally oblique (30 — 35 ). 



As examples of the felsitic order of these rocks, most of which 

 are altered like the propylites, I will first take the case of those 

 deep-seated rocks that are exposed in the river-bed above Nam- 

 buna in the Ndrandramea district. In the least altered state they 

 are dark grey and mottled, and have a specific gravity of 2 - 66 — 2 - 6c^ 

 In section they display tabular zoned plagioclase phenocrysts, 

 usually more or less occupied by alteration products, but at times 

 giving lamellar extinctions of basic andesine (20 — 25 ). The 

 rhombic pyroxene is more or less replaced by chloritoid pseudo- 

 morphs ; whilst the "grain " of the mosaic is often coarse ("03 mm.), 

 and much of it is evidently quartz. The more advanced stages of 

 alteration of these rocks are described in the account of the 

 district given on page 106. . . . Similar rocks, showing pyrites, 

 occur amongst the blocks of Vunimbua River ; but here the 

 rhombic pyroxene is mostly converted into bastite, and the 

 groundmass is in part trachytic as well as felsitic in texture. The 

 specific gravity is 27. 



(In the last survey of my collection I have found a solitary 

 specimen from an agglomerate in the Mbua-Lekutu "divide," 

 which must be referred to the order with felspar-lathes in flow- 

 arrangement. It is a pale grey rock showing abundant macro- 



