120 Prof. T. G. Bonney. On the [June 19, 



are generally fairly sharply defined, but a portion of the crystal often 

 has a fractured outline. There is much variation in the amount 

 of enclosures ; in some of the crystals they are almost absent, in 

 some they abound. They are grannies and microliths of pyroxene, 

 scales of iron-glance, often apparently formed along lines of 

 fracture, opacite, and brown glass. The outer part of the crystals is 

 generally free from enclosures ; the interior is sometimes full of 

 them the latter case, however, is rare. 



The slide also contains a considerable number of grains and crystals 

 of augite and of .hypersthene. These seldom exceed about '01 in 

 diameter, and are often less. Some of the latter miueral are very 

 well characterised by form, dichroism, and extinction. In one part of 

 the slide is a cluster of several augite crystals, with some smaller 

 of felspar, a little iron glance, and a grain or two which I suspect to 

 be olivine (it is colourless, while the other two minerals are tinted, 

 and is more granular in texture, but, unfortunately, does not offer 

 anything wherefrom to measure the extinction angle). There are 

 several granules of iron peroxide scattered throughout the slide. 

 The ground-mass consists of a colourless glassy base, densely crowded 

 with microliths of felspar about "001 long, probably for the most 

 part oligoclase. The rock accordingly is an hyperstheniferous augite- 

 andesite. 



The most interesting specimen from the vicinity of Mr. Whymper's 

 first camp is a compact subvitreous rock, almost black in colour with 

 a few light-coloured specks, having a slightly rough subconchoidal 

 fracture ; in short, macroscopically, a very typical augite-audesite of 

 the dark type which so often contains hypersthene. 



The microscopic examination fully bears out the above inference. 

 Scattered about in the ground-mass are numerous crystals of plagio- 

 clastic felspar similar to those described above, probably labradorite, 

 seldom exceeding '03 inch in greatest length ; also numerous very 

 well-characterised crystals of hypersthene and some of augite, with 

 granules of magnetite. The ground-mass consists of very minute 

 elongated felspar microliths and pyroxenic granules, with much dusty 

 opacite, crowded in a glassy base. This rock also is a typical hyper- 

 stheniferous augite-andesite. 



According to Mr. "Whymper, comparatively little rock in situ was 

 met with upon the route which he followed on his ascent of the cone 

 of Cotopaxi, the surface being principally covered by debris and ash, 

 and the above are the only two specimens which he collected. But of 

 the more fragmental materials he has brought back several examples. 



Three specimens of a dark purple-grey slightly vesicular lava, 

 speckled with small crystals of a glassy felspar, were collected at an 

 elevation of about 12,000 feet. They are rounded in form, and range 

 in size, according to Mr. Whymper, from a diameter of about 4 feet 



