1884.] Microscopic Structure of some Rocks from Ecuador. 131 



tion, and as I feel convinced of their close relationship, I have only 

 made a microscopic examination of two of the best preserved speci- 

 mens. These are somewhat intermediate between the two specimens 

 mentioned above from Pichincha and Antisana ; being compact greyish 

 lavas, with scattered crystals of white felspar. The redder specimen 

 which most resembles the above rocks, does not, after what has been 

 said, need a detailed description ; it is a hypersthene-andesite with a 

 little hornblende. The duller-coloured specimen is an augite-andesite 

 with some hypersthene. The base of each is a clear glass, containing 

 many very minute microliths, probably of felspar, and' irregularly 

 clouded with a grey dust and opacite. 



Only one specimen remains to be described, the highest rock 

 obtained by Mr. Whymper on Chimborazo, at an elevation of about 

 19,300 feet. It is a slightly scoriaceous lava, rough to the touch, 

 almost purple-black in colour, with numerous very minute specks of a 

 glassy felspar. Except that the base is rendered rather more opaque 

 by disseminated opacite, it does not differ very materially from several 

 already described. There are the usual crystals of felspar, one or two 

 being much rounded and very full of dull glassy enclosures ; there is 

 a fair amount of augite, but no well-characterised hypersthene ; so 

 that the rock may be named an augite-andesite. 



Thus the rocks of Chimborazo appear to be andesites, and rather 

 closely related ; the only variation of any importance being in the 

 amount of hypersthene and the occasional presence of hornblende. 



VII. "Notes on the Structure of some Rocks from the Andes of 



Ecuador, collected by E. Whymper. No. IV. Carihuairazo, 



. Cayambe, and Corazon." By Professor T. (1. BONNET, 



"D.Sc., F.R.S. Received June 19, 1884. Read June 19, 



1884. 



I have been favoured by Mr. Whymper with some short notes on 

 the structure and physical features of the three volcanic mountains 

 whose rocks are investigated on this occasion, and have prefixed them 

 to my lithological descriptions. It is remarkable what a general 

 uniformity there is in the products of these summits of the Equatorial 

 Andes, and this, as Mr. Whymper informs me, was so obvious that he 

 made but small collections from the mountains which were visited 

 daring the latter part of his journey. 



Carihuairazo. 



" This forms the northern part of the massif of Chimborazo. It is 

 separated on its south side from its great neighbour by the depression 



K2 



