1884.] Microscopic Structure of some Rocks from the Andes. 245 



ment, sometimes are collected in the inner part of crystals which 

 have a clear external band. They are mostly a brownish glass, in 

 which gas cavities are rather frequent; others are belonites of a 

 pale colonr; (2) hornblende in fairly regular crystals, with well 

 denned cleavage, mostly of a rather dark greenish-brown colour, but 

 occasionally paler; (3) granules, rather rounded in outline, of an 

 iron oxide. In the second stage of consolidation are microliths of a 

 plagioclastic felspar in a glassy base stained with opacite. 



The second specimen presents only varietal differences from the 

 above, but is less well preserved. We may accordingly name this 

 series hornblende -andesite. 



The next two specimens are labelled Sistemade Pichincha (massif of 

 Pichincha), and are distinguished from the other by having a dull-grey 

 matrix, in which crystals of whitish felspar, not exceeding 0'2 inch 

 (and generally less), are scattered. The better suited for examina- 

 tion has the additional indication Penoneo de Pamescucho, abajo de 

 Nina-urcu* In microscopic structure this rock is not unlike the first 

 described : it has a glassy base, rather darkened with ferrite and 

 opacite, and crowded with minute crystallites of felspar, in which 

 are scattered crystals of a plagioclase, probably labradorite, with augite 

 and hypersthene. It is, therefore, like the rock from Rucu-Pichincha, 

 a hyperstheniferous augite-andesite. 



The other specimen from Pichacho de los Ladrillos has a general 

 resemblance to the last, except that it is more scoriaceous and 

 decomposed. I have not thought it needful to have a section made. 

 It is probably an augite-andesite, and very possibly contains a little 

 hypersthene. The last two specimens are both from Nina-Urcu. They 

 resemble one another and each of the other groups in some respects. 

 The matrix is compact, is a dull reddish colour, and in it are scat- 

 tered crystals of a whitish felspar, as in the last group, and of a pyro- 

 xenic mineral, somewhat as in the second, the general colour coming 

 nearer to that of the first group. 



The better preserved one, which I have examined microscopically, 

 is labelled Gerro de Candela, Encillada entre Pucu y G-uagua-Pichincha, 

 the other Encillada al Rucu y Guagua-Pichincha. In the former are 

 seen, in the first stage of consolidation, (1) crystals of a plagioclastic 

 felspar, most, if not all, of which, from its extinction angles, is 

 probably labradorite ; (2) crystals of normal augite ; (3) crystals of 

 hypersthene, as above described. Cavities and what appear to be 

 glass inclosures are rather numerous in both the felspar and the 

 augite. The ground-mass appears to be a dark-brown glass, all but 

 opaque ; but as the slide has offered difficulties to the workman, and is 



* Pamascucho, below Mna-urcu. The latter name is applied by Eeiss and Stiibel 

 to a point in the ridge connecting G-uagua-Pichincha with Eucu-Pichincha, the 

 elevation of which they give as 14,472 feet. 



