﻿I860.] 



FORBES BOLIVIA AND riCBU. 



25 



loiirless felspar, hexagonal brown or black mica, and black or green- 

 black crystals of augite, along with black magnetic oxide of iron, always 

 found as a black magnetic sand when a magnet is drawn along the 

 surface. In passing over these plains the traveller's attention is at- 

 tracted, and his eyes dazzled and wearied, by the glittering specks 

 arising from the reflection of the sun's rays from the numerous small 

 quartz -crystals strewed along the surface. The solid volcanic rock 

 is only occasionally met with ; but hillocks are frequently seen which, 

 judging from their surface at least, are entirely composed of larger 

 masses or fragments of trachytic and trachydoleritic rocks. I do not 

 consider these tracts as representing spaces occupied by the actual 

 protrusion of volcanic matter, but in some cases regard them as only 

 covered by sheets of such trachyte or trachydoleritic lava poured 

 out from longitudinal dykes or fissures such as before described ; and 

 in other instances I even suppose them to be, in part at least, composed 

 of volcanic ashes, tuff, or debris, spread over the surface by the action 

 of water*. 



As far as I could observe, the volcanic rocks do not anywhere 

 appear breaking through the Silurian rocks; but in the north of 

 Bolivia they are seen on the Map (PI. I.) as cutting through the 

 Devonian series near Hachecache and the Lake of Titicaca, a di- 

 stance of more than 200 miles from the coast in a direct line : this I 

 believe may be considered as the most inland point at which volcanic 

 phenomena make their apj>earance on the western side of the high 

 Andes. These rocks are all in situ, and are true trachydoleritic and 

 felspathic lavas which have broken through the strata, part of which 

 are in consequence greatly altered. These lavas are further charac- 

 terized by the peculiar parallel arrangement of their mineral con- 

 stituents, which give that ribboned appearance due to the stria? of 

 fusion, such as are frequently seen in more recent lavas. 



Professor Philippi having allowed me to examine a series of rock- 

 specimens which he had procured during his travels in the desert of 

 Atacama, I found that the volcanic rocks from Punta Negra, Tilopozo, 

 Toconado, Sorras, Atacama- Alta, &c. were all trachytes or trachytic 

 tuffs, and precisely identical in mineral composition and character 

 with those from the more northern part of Bolivia which I have 

 more specially examined ; and from his notes, which he also kindly 

 placed at my disposal, I find that from San Bartolo to Chanaral 

 Bajo, a distance of about 250 miles, we have, as in Sections Nos. 1 

 and 2, a ridge formed by an almost continuous series of lateral 

 outbreaks of such lava cutting through and flowing over the dioritic 

 rocks and the porphyries, shales, &c. of the Oolitic series, which, as at 

 Chaco and other places, contained in abundance Liassic Posidonice, 

 Ammonites, &c. 



The large lateral ovex'flow of lava, from 25 to 30 miles long and 

 several miles in breadth, extending from San Bartolo to San Pedro 



* The plain at Santiago de Machaca, Section No. 2, contains much volcanic 

 alluvium, as described, and seems rather to have been formed by such aqueous 

 action. I did not find sufficient evidence for colouring it as a sheet of trachytic 

 lava, as M. PiEiis has done. 



