﻿I860.] 



FORBES BOLIVIA AND PERU. 



19 



above clown to the Alameda of La Paz, was found by measurement 

 to be 1650 feet, consisting of alternating beds of grey, bluish, and 

 fawn-coloured claj's, gravel, and shingle-beds, along with boulders of 

 clay-slate, grauwacke, and granite, frequently of enormous size, and 

 well rounded as if by the action of water ; the beds are nearly hori- 

 zontal, or dip to the south. About 300 feet below the surface of the 

 plain, there is seen in this section a bed of trachytic tuff, evidently 

 volcanic, and about 20 to 30 feet in thickness. This is visible at a 

 great distance as a white band, running along the precipices encir- 

 cling this valley or ravine, and appears to be contemporaneous with 

 the beds of clay, gravel, and boulders, which, with this solitary 

 exception, form the rest of this diluvial accumulation, and which, 

 except in the uppermost beds, do not contain, as far as I examined, 

 any volcanic detritus. 



No trace whatever of volcanic activity being found anywhere in 

 the neighbourhood of La Paz, I was for a long time greatly puzzled 

 to account for the occurrence of this very peculiar bed in the midst 

 of diluvial strata. The general inclination of the beds themselves, 

 dipping to the south, indicated that they had been drifted from the 

 north or north-east, and they appeared to become narrower towards 

 Lake Titicaca, near which I found the large volcanic outburst of 

 trachytic and trachydoleritic rocks shown in Section No. 1, and from 

 which doubtless the tuff forming these beds had emanated, and had 

 been carried down by aqueous action, and deposited as a sedimentary 

 bed in the series of clays, gravel, &c, forming this groat thickness 

 of drift. 



The total thickness of these beds below La Paz must certainly 

 exceed 2000 feet, and probably reaches 2500 feet, being certainly 

 one of the most finely developed examples of this class of deposit, 

 both as to magnitude and superficial area. I am unable to assign 

 any correct limits to this formation, which appears to extend from 

 north to south through the entire length of Bolivia ; to the north, 

 or towards the Lake of Titicaca, it appears to diminish in thickness, 

 and may possibly wedge out entirely. The beds seem to have a 

 general, but slight, dip to the southward. 



I may here mention that in a small pool of water at a place called 

 the Tambo 'de Perez, about half-way between La Paz and the Lake 

 of Titicaca, I found numbers of a small fresh-water bivalve, which 

 Professor Philippi, of the University of Santiago in Chile, kindly 

 examined for me, and pronounces to be the Cyclas Chilensis 

 (D'Orbigny), found first near Conception in the south of Chile, 

 where it is common, according to Dr. Philippi, both in Yaldivia 

 and Puerto Montt. In these localities this shell is found at but a 

 small elevation above the sea-level, in the coldest inhabited part of 

 Chile ; whereas in Bolivia, as above stated, we find it under the tro- 

 pics, but at an elevation of about 14,000 feet above the sea ; so that 

 we may here regard this excess in elevation above the sea-level as 

 equivalent to the difference of about 40 degrees in latitude. 



Amongst the clay-beds of this diluvial formation, near La Paz, as* 

 also at the foot of Illimani and near Poto-Poto in the valley of the- 



c 2 



