246 



Prof. T. G. Bonney. Notes on the [Jan. 31, 



thicker than the rest, it is not in a very good state for examination, 

 and crystallites may be more numerous than appear. 



The collection made by Mr. Whymper himself consists of a series of 

 specimens taken during his ascent of the highest peak or cone, Gruagua- 

 Pichincha, including a specimen broken off the actual summit, and 

 •one specimen broken from the summit of Rucu-Pichincha, the middle 

 •or second peak of the mountain, by one of his guides, Louis Carrel. 

 The specimens from the slopes of Gruagua-Pichincha form one series 

 with the summit rock and with the second group, described above, 

 from the same part of the massif. Some, as might be expected, are 

 rather decomposed* others are vesicular — pumiceous or slaggy 

 varieties of the rock of which those already described are the more 

 compact form. There are minor differences in the quantity of felspar 

 and of the black pyroxenic mineral scattered through the matrix, but 

 these appear to me merely varietal; hence I have only had a slice 

 xyut from the specimen which was broken from the topmost crag of 

 Gruagua-Pichincha, for this, though generally resembling the specimens 

 described above, exhibits some slight differences. The external sur- 

 faces of the fragment have a slightly s coriaceous aspect, and where the 

 lichen- growth is chipped away, are of a dull grey to rusty brown 

 colour. The fractured faces show the matrix to be of a dull, but not 

 dark, colour, in places slightly vesicular, the walls of the hollows 

 being coated with a pellicle of iron-rust. In the matrix are scattered 

 pretty thickly whitish felspar crystals, not generally exceeding 0'2 

 inch in diameter, and granules or crystals of a black mineral, less than 

 0*125 inch in diameter, the former being rather larger, and the latter 

 rather smaller than the corresponding minerals in the group of speci- 

 mens already described from Guagua-Pichincha. Under the micro- 

 scope we find that (1) felspar crystals of very variable size are scattered 

 throughout the slide, the majority exhibiting polysynthetic twinning. 

 The boundaries generally are sharply rectilinear, but not a few of the 

 largest have evidently been broken, and one or two show signs of 

 external corrosion. Microlithic inclosures are frequent, but very 

 variable in amount. Some of the crystals contain a considerable 

 number of irregular bits of brown glass, with or without granules of 

 pyroxene ; others cavities, in which are bubbles, apparently fixed, 

 occupying about one-eighth of the whole volume ; others aggregates 

 of exceedingly minute granules, like the finest dust, possibly augite, of 

 which mineral, if I mistake not, belonites occur. As a rule, the 

 smaller crystals are clearer than the larger, and the inclosures are 

 more numerous in the inner than in the outer part, but there is 

 so much variation observable that no law can be established. It 

 is difficult to settle the precise species of the felspar, but the general 



* Mr. Whymper in all his collections endeavoured to bring away a fair repre- 

 sentation of what he saw on the mountain, rather than a series of choice specimens. 



