﻿I860.] 



F0IU5ES — BOLIVIA AXD PEEf. 



27; 



ejected whilst in a pasty state, after the quartz had crystallized 

 and the temperature of the whole had become much lower than the 

 fusing-point of the entire rock itself. 



The trachydolerites differ from the trachytes only in having, in 

 addition to the felspar, quartz, and mica of the latter, crystals of dai'k- 

 grecn or black augite scattered through the mass, in which also the 

 quartz does not appear so predominant ; and the rock is frequently 

 considerably darker in colour. They form eruptive masses much 

 greater than the pure trachytes, and are seen largely developed on 

 [Section No. 1 at Batalla and Yunguyo. 



Dolcritic rocks I have only met with in situ on the eastern 

 declivity of the volcanic range of mountains between Tarata and the 

 River Mauri in Section No. 1 ; and in Section No. 2 they are seen as 

 abundant large blocks scattered over the slope of the volcano of 

 Chipicani. Prom their very compact structure, conchoidal fracture, 

 and dark bluish-grey or greenish-grey colour, they much resemble 

 basaltic rocks ; but their crystallization is so close-grained that I 

 could not distinguish whether olivine was present ; and, in fact, their 

 mineral composition is not recognizable by the naked eye, so that 

 their exact nature is open to inquiry : they do not appear to form 

 any great proportion of the mass of volcanic rocks here developed. 



The greater volcanic rocks, at least those which have broken 

 through as lava and remain in situ as a compact rock, are composed 

 of a crystalline felspathic lava, much more basic in chemical character 

 than the others (possibly if we except the dolerites), and which ap- 

 pear to be almost exclusively composed of one or more varieties of 

 felspar. They generally possess that peculiar parallel structure fre- 

 quently met with in all volcanic rocks, whether recent or more ancient, 

 and apparently clue to a cause similar to that of the stria? of fusion 

 visible in glass. This striated appearance is frequently rendered 

 more apparent from the different layers being of different shades of 

 colour, reddish or whitish grey, or, as at Pailumani, where these 

 rocks are very largely developed, of a dark grey colour, probably 

 from some admixture of augite in a non-crystalline condition and 

 intimately diffused through the felspathic mass so as not to be 

 visible to the naked eye. These rocks sometimes are also found to 

 contain a little mica or augite, in plates or crystals, but appear in 

 general to be free from quartz. They are developed on a large 

 scale between Hachecache and Tiquina, Section No. 1, and also at 

 Pailumani, Section No. 2, where they appear to form the entire mass 

 of the eruptive rock. 



In neither Section No. 1 nor No. 2 do we cut across any volcano at 

 present in activity, nor meet with any lava or scoria likely to have 

 been produced more recently; at but very short distances, however, 

 both to the north and south of these lines of section, volcanos were 

 observed in activity ; and during my residence in the country, the 

 " Misti " or Volcano of Ariquipa was in eruption. In the immediate 

 vicinity of Taeora various Solfataras might be seen in action ; and 

 their action on the Oolitic and Porphyry series was visible at great 

 distances, on account of the brilliant yellow, red, and brown colora- 



