﻿I860.] 



FORBES — BOLIVIA AND PERU. 



23 



account of the one being subaerial, or injected between the strata, 

 whereas the other has been subaqueous, and in consequence is fre- 

 quently met with as a tufaceous bed interstratified with other strata 

 of acknowledged sedimentary origin. As both such rocks might be 

 at one and the same time in course of formation, this difference can 

 hardly be looked upon as indicating a difference in geological age. 



The Sections Nos. 1 and 2 (PI. II.) are fully sufficient to show 

 how important a part volcanic action has taken in altering the con- 

 tours of the mountain-ranges here traversed : probably in no part of 

 the world do we find volcanic phenomena more energetically developed 

 or affecting so great a territorial area. 



As will be seen from the accompanying Map (PI. L), the volcanic 

 rocks forming at the north the active volcano of Ariquipa and others 

 in that neighbourhood are cut through in Section No. 1 (PI. II.) ; 

 and still further south, in Section No. 2, they form the volcano of 

 Tacora or Chipicani, 19,740 feet above the level of the sea ; still fur- 

 ther south they form the more or less active volcanos of Sajama 

 (22,915 feet), Coquina, Tutapaca, Tucalaya, Isluga, Calama, Ata- 

 cama, Licancau, Toconado, Llullayacu, and others intermediate, 

 which, in conjunction, form an almost continuous range of volcanos 

 into that part of the Desert of Atacama pertaining to Chile, through 

 which country we find this volcanic range appearing at intervals ; 

 and still more to the southward it is doubtless in connexion with 

 the volcanos of Patagonia, the north of Magellan's Straits, and Terra 

 del Fuego. As mil be seen from this, the general direction will 

 be nearly north and south ; and, from a study of the line of fracture 

 and position of the intruded rocks, it would appear that the sub- 

 terranean force here exerted had its centre to the west of this line, 

 and had acted at a high angle from the west towards the east. 



The beds of trachyte and trachytic tuff which are seen interstratified 

 in the raised beach at Tacna, Azapa, &c, and also in the great diluvial 

 formation of La Paz, have already been noticed. Further east, in 

 Section No. 1, at Tarocache, a very peculiar volcanic conglomerate and 

 tuff stratum was met with, remarkable for the columnar structure 

 which presents itself on a very large scale on the side of the nearly 

 perpendicular hill under which the road passes ; the columns are so 

 well developed, that, seen from the road, they look as regular as 

 similar basaltic formations. On Section No. 2, at Palca, and still 

 further east, at Questa Blanca, deposits on no great scale, of a white 

 crumbly trachytic tuff, composed of more or less decomposed felspar, 

 with quartz and hexagonal black or brown mica-plates and an occa- 

 sional speck of augite, are met with as more or less horizontal beds, 

 resting unconformably on the highly inclined strata of Liassic shales, 

 &c. ; both of these deposits, as well as the previously mentioned one at 

 Tarocache, appear to be remnants of some more extensive bed of sedi- 

 mentary origin formed of volcanic tuff and ashes from the volcanos 

 situated still higher up the range. 



In Sections Nos. 1 and 2, the great volcanic formation is seen a 

 little further to the east, forming a high ridge or range of peaks 

 averaging from 16,000 to 19,000 feet above the sea, visible from the 



