﻿22 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Xov. 21, 



Yungas appear also to belong to diluvial accumulations of this same 

 geological age. 



The valley of La Paz, being entirely cut out of this great diluvial 

 formation by the action of the river which traverses it, is, as might 

 be expected, often exposed to considerable landslips during the 

 extremely heavy rains of the wet season : when residing there, I 

 witnessed such a landslip, which blocked up the valley and caused 

 it to be inundated to a considerable distance by the damming up 

 of the river. In many parts also the action of the rains and small 

 rivulets formed by them, cutting through these immense clay and 

 gravel strata, forms a most striking and picturesque landscape. 

 The slope of the valley of La Paz, for example, is seen cut up into 

 innumerable ravines of great depth, whilst pinnacles of more than 

 60 feet in height will be left standing in great numbers and of all 

 variety of form, frequently quite isolated, and, from their slender 

 proportions, often looking like needles or pillars formed artificially : 

 the sides of these show a very pretty section of the variegated clays 

 and gravels that previously had formed the beds from which these 

 had been carved out and left as standing mementos. 



At the same time the roads will be hollowed and traversed by 

 chasms, natural arches, and subterranean holes, of the strangest 

 form, too frequently proving dangerous to the rider passing over 

 them. These effects, in general only seen in miniature elsewhere, 

 present themselves on such an immense scale as to leave a very 

 decided impression on the observer. 



4. Volcanic Rods. — Although these rocks are occasionally more 

 recent than any of the deposits previously treated of, and are in places, 

 as from the volcano of Ariquipa, *kc, ejected at the present day, 

 still it is preferred to consider them in this sequence, from the epoch 

 so : -signed to them being one in which they appear to have attained 

 their maximum development, and in wliich they have produced such 

 grand changes in the configuration and level of this part of South 

 America. 



They are, as seen in .Section Xo. 2, contemporaneous with the great 

 diluvial formations at La Paz, and possibly may there represent an 

 early Tertiary period, from which time to the present they seem to 

 have been in more or less continuous activity, and to have presented 

 themselves with the same general characters and under very similar 

 circumstances. 



M. d'Orbigny* has classified these rocks as of two distinct ages, 

 known by their differing slightly one from another in their state 

 of aggregation and the presence of augite. An attentive study of 

 these volcanic deposits showed how clifHcult it was to draw any such 

 defined line of demarcation in rocks which, as before stated, possess 

 all main features in common ; and in fact seemed to show that M. 

 d'Orbigny's two classes are in reality (at least in many cases) one 

 and the same, presenting slight differences in mineral character on 



* M. Tissis iilso. Annates des Mines. 185G: "Recheirhes sur lc Systems dc 

 Soulevement dc l'Amerique du SucL" 



