432 



Prof. T. G. Bonney. On the 



[Mar. 13, 



bat it is crowded with microliths of felspar, of a pyroxenic mineral, 

 of brown mica, ferrite, &c. The rock is thus a mica-hornblende- 

 andesite. The last specimen is labelled Cuspide del chusa longo. It is 

 a dark grey vesicular rock, the proportion of solid to cavity being 

 about two to one. The cavities commonly are not more than 0"2 inch 

 in longest diameter, irregular in form, slightly drawn out in one- 

 direction, and coated with brown iron oxide. The rock is compact in 

 structure, with a general resemblance to the matrix of the last 

 described, but contains only very minute crystals of whitish felspar, 

 rather irregular in form, and hardly more than 0'05 inch in diameter. 

 It is no doubt- an andesite, and is not unlike some of the scoriaceous 

 varieties of that rock which are obtained from the Auvergne volcanoes. 

 I have not thought it necessary to examine it with the microscope. 



The remaining ten specimens brought back by Mr. Whymper are 

 all representative of the highest part of Antisana. They were col- 

 lected from the upper part of a moraine, by the side of which he 

 encamped for the night, at an elevation of about 16,000 feet above the 

 sea, or 3,300 below the actual summit. The materials of this moraine 

 are derived from several rather small crags of rock which here and there 

 crop out from the snowy slopes above. None of them were touched by 

 Mr. Whymper during his ascent on the following day, for they are not 

 numerous and are generally in inaccessible positions. He was careful 

 to bring a specimen of every marked variety which caught his eye, so 

 that the series is probably a fair representation of the rocks which 

 constitute the Peak of Antisana. 



Of these specimens (1) and (2) are vesicular rocks of a dull reddish 

 colour, no doubt scoriaceous forms of a rock closely allied in composi- 

 tion to (4) and other dark varieties described below. (3) is a tuff r 

 consisting of a fine yellowish paste, in which are numerous fragments 

 up to the size of a small nut of a slightly vesicular, subvitreous,. 

 blackish rock, evidently closely allied to the next mentioned. (4) is a 

 blackish subvitreous rock, containing glassy-looking felspar crystals 

 up to about 0"2 inch diameter. A few minute vesicles are present. 

 The microscopic description is given below. (5.) A very similar rock,, 

 a little lighter in colour, also described more fully below. (6.) Closely 

 allied to the last, but paler, probably a little more decomposed. (7.) A 

 dark compact rock, with some small crystals of felspar ; very like the- 

 specimen from Antisanilla. (8.) A compact blackish rock, mottled 

 with small spots of dull gray, in the inner part of which a small 

 vesicle may be seen ; a very few crystals of felspar, not exceeding 

 0"1 inch diameter, are visible ; its microscopic structure is described 

 below. (9.) A rather vitreous, slightly vesicular, rock, a fiuidal 

 structure being indicated by reddish and blackish layers, containing 

 crystals of a whitish felspar, rarely exceeding 0*1 inch in diameter. 

 The microscopic structure is described below. (10.) A large fragment 



