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Prof. T. G. Bonney. On the 



[Mar. 13, 



elongated form or in small granules. Owing to the smallness and 

 rather indefinite character of the pyroxenic constituents it is difficult 

 to speak very positively about them ; augite, however, is certainly 

 present, and possibly hypersthene. Minute crystals and grains of an 

 iron oxide, probably magnetite, as we might expect from the colour of 

 the rock, are rather abundant. The ground-mass appears to consist 

 of a clear glass, faintly tinged with brown, and densely crowded with 

 microliths. These are lath-like crystallites of felspar, generally not 

 exceeding 0"005 inch long, not seldom composed of two or three 

 individuals, and belonites, of a very faint green tinge, not exceeding 

 about 0*001 inch by 0*0002 inch, probably hornblende. The evidence as 

 to the felspar is conflicting ; probably both oligoclase and labradorite 

 are present, but my observations tend to the conclusion that the latter 

 is the more abundant species. The rock on the whole agrees best 

 with augite- andesite. Its specific gravity, determined for me by 

 Mr. J. J. H. Teall, is 2-656. 



A second specimen from the same locality resembles the former in 

 structure, but is of a dull india-red colour. It is so evidently the 

 same rock, differently coloured by conversion of the black oxide of 

 iron into the red oxide, that I have deemed it needless to examine its 

 microscopic structure. The two other specimens are simply scori- 

 aceous varieties of the latter rock. 



It is difficult to fix the precise localities of most of the specimens 

 obtained by Mr. Whymper from the collector at Quito, as the places 

 mentioned on the labels are not known to the former, and in most 

 cases, he thinks, are of no more general acceptation than the names 

 attached by Alpine herdsmen to the crags and pinnacles in the 

 vicinity of their chalets. All, he believes, are from the south-western 

 or western side of the mountain and from localities whose height 

 above the sea is not likely to exceed about 13,000 feet. Three are pro- 

 bably derived from some one subsidiary crater on the south-western 

 flank of Antisana, named Guagra-ialina, though there is a slight 

 variation in the spelling. The first of these is labelled Corriente 

 de lava de Guagrahialina volcan, Lado 8.0.0. Antisana. It is a dark 

 grey rock, of scoriaceous aspect, with many small vesicles, usually less 

 than 0*1 inch in diameter, and several specks of whitish felspar. It 

 resembles some of the dark grey lavas of Auvergne, and, like them, is 

 no doubt an augite-andesite. As the specimen presents no features 

 of special interest, I have not examined it with the microscope. The 

 next is simply labelled Antisana, Guagrayalina volcan. It is a com- 

 pact dull grey rock, with a slight purplish tinge, containing occa- 

 sional crystals of glassy felspar, sometimes rather more than 0*1 inch 

 in length. These, on examination with the microscope, prove to be 

 a plagioclastic felspar, but there is so much variation in the extinc- 

 tion angles that it is impossible to decide upon the species. They con- 



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