268 COSMOS. 



to 628 geographical miles. Nearly double this length (occu- 

 pying a space of 968 geographical miles) is a tract of country 

 free from volcanoes, from the Sangay, the southern termina- 

 tion 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 complicated and various in the 

 same mountain chain must have been the coincidence of the 

 conditions upon which depends the formation of permanently 

 open fissures, and the unimpeded communication of the molt- 

 en 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, quartz- 

 ose porphyries, silicious 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 increased frequency of labradoritic rocks, rich in 

 pyroxene and oligoclase, announces to the observant traveler 

 (as I have already elsewhere shown) the transition of a zone 

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

 ver in porphyries, destitute of quartz and full of glassy feld- 

 spar, into the volcanic regions, which still freely communi- 

 cate with the interior of the earth. 



The more accurate knowledge which we have recently at- 

 tained 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 Cordil- 

 leras 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 geographical miles), not much* more than half (calcu- 



* 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 vol- 

 canoes have broken out is directed from east to west, from the 

 Orizaba to the Colima, for a distance of 392 geographical miles, 

 between latitudes 19 and 19 20'. The volcano of Tuxtla lies 

 isolated 128 miles to the east of Orizaba, near the coast of the 

 Gulf of Mexico, and in a parallel (18 28') which is half a degree 

 farther south. 



