1884.] Microscopic Structure of some Rocks from the Andes. 247 



results of my observations are in favour of the majority being 

 labradorite. 



(2.) Hornblende. This mineral occurs in crystals, sometimes rather 

 more than 0"05 inch diameter. It is usually of a strong olive-green or 

 brownish-green colour, showing a very marked dichroism, often not 

 conspicuously cleaved, but occasionally exhibiting in cross-sections a 

 very characteristic cleavage. Not seldom the crystals have their out- 

 lines rounded, as if worn or somewhat corroded, and are black- 

 bordered. Microlithic inclosures of felspar and of opacite are rather 

 frequent. (3.) A much paler and less dichroic mineral occurring 

 rarely, not black bordered. In one case a crystal of it appears to grow 

 on to the broken face of a hornblende crystal. It contains microliths 

 of opacite but not of felspar. The extinction angles indicate that this 

 also is hornblende, but I consider it to be formed subsequently to the 

 dark variety. (4.) Two or three clusters of a granular mineral of a 

 pale colour, which I believe to be augite. (5.) One or two crystals 

 which, from their dichroism and extinction, I consider hypersthene. 

 (6.) Granules of iron oxide, probably magnetite. The interstitial 

 matter is of a dusty grey-brown colour, and, when examined with 

 high powers, appears to consist of a transparent glass crowded with 

 minute earthy particles and specks of ferrite. The rock then which 

 forms the topmost crag of Guagua-Pichincha is, as our previous inves- 

 tigation would lead us to expect, a hornblende-andesite, though not 

 quite so normal a specimen as some of those already examined. 



The specimen broken from the highest point of Rucu-Pichincha is a 

 compact grey rock, in very fair preservation, containing scattered 

 crystals of a glassy felspar up to about 0'2 inch in diameter, and 

 smaller specks of a black pyroxenic mineral. Microscopic examina- 

 tion shows the felspars to be similar to those described in the last 

 specimen, except that cavities with bubbles are almost wholly want- 

 ing. Other inclosures are frequent, especially nearly colourless 

 belonites. The pyroxenic mineral is rather abundant : some is light 

 brown, the rest of a greenish tint. Some of the former is certainly 

 augite, some of the latter hypersthene ; probably the minerals may be 

 distinguished by their colours, but this of course cannot be fully 

 proved. Grains of magnetite occur as above. The matrix is often 

 darkened by the presence of specks of kaolin and ferrite, but differs 

 from the last in being crowded with very minute lath-like crystallites 

 of felspar. Probably, however, a colourless base of true glass is 

 present. The rock then is a hyperstheniferous augite- andesite, and 

 thus agrees with the specimens already described from this peak. 



The annexed analysis* is that of a rock, apparently from some dis- 

 tance up the peak of Guagua-Pichincha. From the description, how- 



* Koth. Die G-estems-analysen. 35. Analysis quoted from Abich. Yulk. Erscli. 

 184L. 58. 



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