140 TRAVELS AMONGST THE GREAT ANDES. 



NOTE ON ROCKS FROM THE ANDES. 

 By Puofessor T. G. BONNEY, D.Sc, F.R.S. 



Duiiiig liis journey in the Ecnaddiian Andes, Mr. Wliyniper cullected a 

 large series of rocks, wliicli he has placed in my hands for mici'oscopic 

 examination.^ The collection has an exceptional interest from the fact that 

 so many of the specimens have been obtained from localities never visited 

 before Ijy travellers, and in not a few cases by any human being, for among 

 them are fragments from the highest ciags of seveial peaks hitherto nn- 

 ascended. Mr. Wliymper endeavoured to make his collection thoroughly 

 representative, so that the specimens are in various stages of jii'eservation ; 

 but there has always been a fair proportion in as good a condition for 

 examination as could reasonably be expected from localities where there are 

 no quarries and the rainfall is heavy. The number of specimens from the 

 different localities varies. From some mountains, such as Pichincha, Antisana, 

 and Chimborazo, it is large, — from others only two or three specimens have 

 been brought. This has been j)artly due to the exigencies of travel, and 

 yet more to the fact that there is such a general resemblance among the 

 igneous rocks of the whole district that it was in many cases obviously 

 useless to accumulate specimens which would be lithologically duplicates, 

 differing only in localities.^ 



With the exception of one mountain (Sara-urcu), which is composed of 

 metamorphic rocks, all the great peaks of the Andes ascended by Mr. 

 Whymper consist of one species of volcanic rock — that to which the name 

 Andesite has been given. This name, the derivation of which is obvious, was 

 first applied by Von Buch to the rock, as was the name Andedne to a species 

 of felspar which it was supposed largely to contain. This felspar is regarded 

 as a membei' of the plagioclastic group, and contains neaily equal amounts 



^ Descriptions of these liave been read before the Royal Society during tlie year 1S84, 

 and the five parts are published iu their Proceedings for that year. 



- The collection included specimens of the highest rocks of the four loftiest of the 

 Ecuadorian Andes, viz. — Chimborazo (20,498 feet), Cotopaxi (19,613 feet), Antisana 

 (19,335 feet), and Cayambe (19,186 feet) ; and others from the actual sumnuts of Cari- 

 huairazo (16,515 feet), Cotocachi (16,301 feet), Sincholagua (16,365 feet), Pichincha 

 (15,918 feet), Corazon (15,871 feet), and Sara-urcu (15,502 feet). 



