272 REACTION OF THE INTERIOR OF THE EARTH 



cated and so diverse must have been, within the same 

 chain of mountains, the circumstances and relations 

 on which the formation of permanently open fissures, 

 and unobstructed communication between the molten 

 interior and the atmosphere, depend. Between the 

 groups of trachytic and doleritic rocks through which 

 the volcanic forces become active, there intervene some- 

 what shorter tracts, in which the prevalent rocks are 

 granite, syenite, mica schists, clay slates, quartzose 

 porphyry, siliceous conglomerates, and calcareous rocks, 

 of which last a considerable portion (according to Von 

 Buch's skilful examination of the organic remains 

 brought home by Degenhardt and myself) belong to chalk 

 formations. As I have elsewhere remarked, the gradually 

 increasing frequency of rocks containing labradorite, 

 pyroxene, and oligoclase announces to the attentive 

 traveller that he is passing from what had so far been a 

 distinct zone, non-volcanic and often rich in silver found 

 in porphyry without quartz and full of vitreous feld- 

 spar, into volcanic regions still in free communication 

 with the interior of the earth. 



The more exact knowledge which has been recently 

 acquired of the position and boundaries of the five 

 groups of volcanoes (i. e. the groups of Anahuac or 

 tropical Mexico ; Central America ; New Granada and 

 Quito ; Peru and Bolivia ; and of Chili) teaches us that 

 in the portion of the Cordilleras which extends from 

 19 north to 46 south latitude, making, with the 

 curves occasioned by variations in the direction of the 

 axis, a length of about 5000 geographical miles, very 

 little more than the half ( 394 ) has volcanoes. If we 

 examine the distribution of the non-volcanic spaces, we 



