﻿I860.] 



FORBES BOLIVIA AND PERT. 



17 



fall of rain, is naturally not so characteristic in its development as 

 the two previously described formations, although at the same time 

 it presents some very striking features, and in some respects strikingly 

 reminds us of its supposed lagoon origin. 



3. Diluvial Formations of the Interior. — The saline deposits last 

 noticed are situated in the midst of what may be termed the great 

 Bolivian plateau, having an average altitude of fully 13,000 feet 

 above the sea, and bounded to the west by the Upper Oolitic rocks 

 of the coast-cordilleras, whilst to the east it abuts against the Si- 

 lurian range of the true Andes. This plateau is not uniform in its 

 mineralogical nature ; and when viewed in section from east to 

 west, it shows considerable diversity of composition, arising from 

 the ranges of hills which intersect it all bearing hearly north and 

 south, and thus dividing it into so many longitudinal valleys (see 

 PI. II.). 



These valleys are generally occupied by nearly level plains, formed 

 of the gravelly spoil produced by the wearing down of the bounding 

 ridges, with which they are consequently identical in lithological 

 composition. The ridges themselves seldom attain a greater eleva- 

 tion than 2500 feet above the plateau, and are generally under this 

 height ; but occasionally volcanic cones thrust themselves up to more 

 than 6000 feet above the plain, and consequently attain an eleva- 

 tion of fully 20,000 feet above the sea-level. 



The character of this plateau is well shown in the Sections Nos. 1 

 and 2 (PL II.), by a reference to which it will be seen that it may be 

 separated into three divisions— western, central, and eastern (Oolitic, 

 Permian, and Silurian, according to the nature of the rocks origina- 

 ting the diluvial accumulations which fill up the intermediate basins 

 or valleys). 



The most western of these is essentially composed of Upper Oolitic 

 detritus, with an occasional block of diorite, and in places abundant 

 volcanic debris from the neighbouring eruptions. They are covered 

 with but a very scanty verdure, if not entirely barren, and incrusted 

 with saline matter, and are generally either entirely destitute of 

 water, or possess some few springs at great distances from one 

 another and of abominable quality — frequently, as at Rio de Azufre 

 (Section jSTo. 2), not potable, and even causing death to the animals 

 which drink it, as sufficiently proved by the bones of mules, llamas, 

 &c, scattered along the banks. At most places the water generally 

 produces bad effects to those unaccustomed to it, even when it is 

 comparatively tasteless. 



Prom an examination of the waters from several localities, I may 

 observe that in one or two cases it was perfectly astonishing what an 

 amount of saline matter might be present in water which might be 

 termed " palatable," but which produced strong purgative effects ; 

 on examination, such a water from the desert of Atacama was found 

 to contain a very large amount of the sidphates of soda and mag- 

 nesia (Glauber and Epsom salts), associated with common salt and 

 carbonate and sulphate of lime ; and I can only suppose that the 

 bitter taste which the amount of sulphate of magnesia (Epsom salts) 



VOL. XVII. — part i. c 



