ON ITS EXTERIOK. VOLCANOES. 249 



necessary connection between the maximum of elevation 

 and a less degree of volcanic activity, or the nature of 

 the visible rock. Observations limited to particular coun- 

 tries might easily lead us into error in this respect. 

 For example, in the part of Mexico which lies within 

 the tropics, all the mountains which are covered with 

 perpetual snow (therefore, the culminating points of the 

 whole country) are volcanoes ; and this is also for the 

 most part the case in the Cordilleras of Quito, provid- 

 ing we permit ourselves to associate with the volcanoes 

 trachytic domes not open at the summit (Chimborazo 

 and Corazon): but on the other hand, in the eastern 

 chain of the Andes of Bolivia, the greatest mountain 

 heights are completely unvolcanic. The Nevados of 

 Sorata (21,288 feet) and Illimani (21,148 feet) consist 

 of graywacke slates, which have been broken through 

 by masses of porphyry ( 372 ), in which (as still remaining 

 evidences of the fact) fragments of slate are found 

 enclosed. Also in the eastern Cordillera of Quito, 

 south of the parallel of 1 35', the high snow-covered 

 summits of Condorasto, Cuvillan, and the Collanes, 

 which are opposite to the trachytic mountains, consist 

 of mica slate and slaty quartz. According to what 

 we now know (thanks to the meritorious labours of 

 Brian Hodgson, Jacquemont, Joseph Dalton Hooker, 

 Thomson, and Henry Strachey) of the mineralogical 

 constitution of the greatest elevations in the Himalaya, 

 it would appear that there also, what used to be called 

 the primitive rocks granite, gneiss, and mica slate 

 are the visible rocks, but not any trachytic formations. 

 Pentland, in Bolivia, found fossil shells in the Silurian 

 schists on the Nevado of Antacaua, 17,478 feet above 



