1884.] Microscopic Structure of some Hocks from the Andes. 243 



shall describe the two collections separately. It may be remarked 

 that the labels indicate that in the opinion of the native collectors 

 there is undoubtedly a crater on Guagna-Pichincha. 



Of these eighteen specimens, all but one would, without hesitation, 

 he described in general terms as " Trachytes," and that, though 

 rather a dark grey in colour, could hardly . receive any other name. 

 The others are either light grey inclining to a yellowish tint, or shades 

 of a pale red inclining to purple. 



Four of these specimens bear in common the designation Ouspide 

 del Bucu-Pichincha, August 24, 1879. Rucu-Pichincha, as has been 

 said, is the lower or middle summit, whose height, according to 

 Reiss and Stiibel, is 15,542 feet. Mr. Whymper informs me that the 

 term Cuspide in Ecuador is applied equally to the bond fide summit of 

 a mountain, or to the general culminating mass. These four speci- 

 mens have a compact-looking matrix of a pale-reddish colour, in which 

 are scattered numerous little crystals of a whitish felspar. All evidently 

 are rather decomposed. The first bears the additional label Tioloma 

 chiquito paso de la Guibrada " Bumi-huaico." The matrix is a warm 

 pinkish-grey in colour, containing numerous felspar crystals, seldom 

 exceeding 0*125 inch in longer diameter, and often less, together with 

 a few specks of a dark mineral. To the earlier stage of consolidation 

 belong the following minerals : (1) Felspar, commonly in crystals about 

 0*04 inch to 0"02 inch in longer diameter, frequeutly containing 

 numerous comparatively large, rounded and slightly lobed inclosures 

 of a brown glass, and an occasional speck of ferrite. Oligoclase, 

 sometimes showing zonal banding, is present, but the large extinction 

 angles of many of the crystals indicate that labradorite or anorthite, 

 probably the former, is also abundant. (2) A pyroxenic mineral in 

 well defined crystals ; some of these exhibit the usual outline, 

 cleavage, and characteristics of normal augite, but several in their 

 form, pleochroism, and mode of extinction agree with the orthorhombic 

 mineral, lately recognised as hypersthene.* The former are fre- 

 quently stained of a dark brown colour towards the exterior. 

 (3) Some ill-defined ferruginous spots may also be seen, which 

 sometimes appear to replace a crystal of augite, hornblende (in one 

 instance), or mica. 



To the later stage of consolidation belong fairly numerous micro- 

 liths of a plagioclastic felspar in a grey granulated base. This, by 

 remaining dark in all positions between the crossed Nicols, is proved 

 to be a glass, though somewhat modified by decomposition. 



The rock then is an augite-andesite with some hypersthene, and it 



* Messrs. Hague and Iddings, " Amer. Journ. Science," vol. xxvi, p. 222. Mr. J. 

 J. H. Teall, " G-eol. Mag.," Dec. ii, vol. x, p. 145, &c. Mr. Teall identifies the 

 mineral in the Cheviot porphyrites, and gives references to papers by Mr. W. Cross 

 and others. 



