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



being rare, and there are two or three small crystals of a strongly 

 dichroic hornblende. Also one or two crystals of what appears 

 to be an iron mica. The predominance of hypersthene entitles this to 

 the name of a hypersthene-andesite. 



From the fifth camp fonr specimens have been brought. Of these 

 fonr are closely related, and are rocks presenting a general similarity 

 to the second specimen of the second camp, and to that from Pamas- 

 cucho in the Pichincha massif as well as one from Guagra-ialina, 

 Antisana, being compact greyish lavas, with small crystals of white 

 felspar. I have examined one microscopically, and find that it then 

 presents some differences. The felspar crystals, indeed, are similar to 

 those already described, except that perhaps they are a little more 

 crowded with microlithic enclosures, but there are no well-defined 

 crystals of augite, hornblende, or hypersthene. Instead of these are 

 rather numerous elongated or rounded bodies, one of the former attain- 

 ing '1 inch in length, consisting of an external zone of dusty opacite, 

 containing a less quantity of the same mineral associated with small 

 crystals or specks of a pyroxenic mineral. I have occasionally seen 

 these bodies (for one cannot give them a definite name), and I pre- 

 sume that they are analogous to the cases of replacement of augite 

 by magnetite, which are not uncommon, and are rude psendomorphs of 

 a mineral of the pyroxenic group, or in some cases possibly of mica.* 

 There are as usual scattered grains of magnetite, and the ground- 

 mass is a glassy base, crowded with lath-like felspar microliths and 

 granules of magnetite. The rock then is an andesite, but of a 

 slightly exceptional character. 



The second specimen, Mr. Whymper states, differs from any other 

 rocks which he saw upon the mountain. It is a rather crumbly rock 

 of very irregular fracture, having a very dark grey ground-mass, in 

 which crystals of glassy-white felspar, up to about '2 inch long, are 

 embedded. When examined microscopically it does not appear to 

 differ very materially from some of those already described, and 

 is very closely related to that forming the ridge of the mountain, 

 different only in the colour of the ground-mass, and, like it, being best 

 named a hypersthene-andesite. 



Mr. Whymper brought a large series of rocks collected at an eleva- 

 tion of about 18,400 feet to represent the materials of the cliff on the 

 southern face of the mountain beneath the second summit. They are 

 for the most part andesite lavas, more or less decomposed, varying 

 considerably in tint and in compactness, but evidently closely allied 

 lithologically. Several were not in a condition favourable for examiua- 



* Specimens in my collection from Auvergne, e.g., from a quarry about three 

 mile above Murat on the high road, from the cliffs of the Puj Cacadogne and the 

 Grande Cascade (Mont Dore) exhibit similar bodies. In some cases the bordered or 

 replaced mineral is hornblende. 





