TRUE VOLCANOES 283 



The length of this tract amounts to 628 geog. miles. Nearly- 

 double this length (occupying a space of 968 geog. miles) is 

 a tract of country free from volcanoes, from the Sangay, the 

 southern termination of the group of New Granada and 

 Quito, to the Chacani, near Arequipa, the commencement of 

 the series of volcanoes of Peru and Bolivia. So compli- 

 cated and various in the same mountain chain, must have 

 been the coincidence of the conditions upon which depend 

 the formation of permanently open fissures, and the unim- 

 peded communication of the molten interior of the earth 

 with the atmosphere. Between the groups of trachytic and 

 doleritic rocks, through which the volcanic forces become 

 active, lie rather shorter spaces, in which prevail granite, 

 syenite, mica-schists, clay-slates, quartzose porphyries, sili- 

 cious conglomerates, and limestones, of which (according to 

 Leopold von Buch's investigation of the organic remains 

 brought home by Degenhardt and myself), a considerable 

 portion belong to the chalk formation. The gradually in- 

 creased frequency of labradoritic rocks, rich in pyroxene 

 and oligoclase, announces to the observant traveller (as I 

 have already elsewhere shown) the transition of a zone 

 hitherto closed and non-volcanic, and often very rich in 

 silver in porphyries, destitute of quartz and full of glassy 

 felspar, into the volcanic regions, which still freely commu- 

 nicate with the interior of the earth. 



The more accurate knowledge which we have recently 

 attained of the position and boundaries of the five groups 

 of volcanoes (the groups of Anahuac or tropical Mexico, 

 of Central America, of New Granada, and Quito, of Peru 

 and Bolivia, and of Chili) shows that, in the part of the 

 Cordilleras which extends from 19^ north, to 46 south 

 latitude, (and, consequently, taking into account the curves 

 caused by alterations in the axial direction, for a distance 

 of nearly 5000 geog. miles,) not much 70 more than half 



70 The following is the result of the determination of the length and 

 latitude of the five groups of linear volcanoes in the chain of the Andes, 

 as also the statement of the distance of the groups from each other : 

 a statement illustrating the relative proportions of the volcanic and 

 non-volcanic areas. 



I. Group of the Mexican Volcanoes: The fissure upon which the 

 volcanoes have broken out is directed from east to west, frou? 



