﻿I860.] 



FORBES BOLIVIA AND PERU. 



no doubt as to the extensive development of this formation, which, 

 throughout its whole extent, is strikingly uniform in all other 

 characters. 



The thickness of these beds is very considerable ; but it would be 

 too hazardous to venture any estimate of its magnitude without 

 more decided data than are at present at my disposal. The area 

 occupied by these rocks, as seen in the accompanying Map, is likewise 

 very great ; and we find that nearly the whole of the coast-line is 

 formed exclusively of the above-mentioned rocks, which here form 

 the " Cordilleras de la Costa," whilst in Chile they constitute the 

 back range, or " Cordillera de los Andes," a nomenclature which has 

 caused some confusion, and has been the main cause of the inaccuracy 

 with which the mountain- chains of this part of South America has 

 been delineated by geographers. 



If we except the small strip of land at Mexillones and the in- 

 cluded dioritic eruptions, we find that the whole coast-line of Bolivia, 

 and as far north as Arica in Peru, is formed of these rocks. At 

 Cobija, in Section No. 3, the Upper Oolitic beds and porphyries 

 strike about N. 20° W. (magnetic), and dip at an angle of 30° east- 

 ward, from having been tilted up by the dioritic eruption seen in 

 the section, which has rendered metalliferous, and also considerably 

 altered the nature of, the rocks themselves near to the point of con- 

 tact. I noticed a vein here containing grey and yellow copper- 

 pyrites, with a little atacamite, carbonate of copper, &c, showing 

 itself on surface, bearing N. 60° E., with a dip of 12° S.E., and cutting- 

 through both the dioritic and porphyritic rocks and shales ; the latter 

 were bleached, and at several points converted into " Tofo," or a 

 species of clay sometimes of a pure white colour. These clays, as 

 previously noticed, have evidently been produced by the action of 

 acid gases accompanying the dioritic eruptions on the felspathic base 

 of the porphyry-tuffs, &c, and which, by removing the lime and 

 iron contained in the same, leave behind a more or less pure silicate 

 of alumina, in the form of a white clay, or " Tofo," as such are here 

 termed ; at the same time the volatilization of compounds of iron has 

 coloured the surrounding rocks with various shades of yellow, red, 

 and brown. 



High up the sides of the mountains in this section, a copper-mine, 

 called the Manto de Ossa, is being worked on a considerable scale ; 

 and in this mine the ore does not occur as a vein or lode, but as a 

 regular bed, in amongst the other strata. The cupriferous stratum 

 itself has evidently been originally a bed of porphyry- conglomerate, 

 or breccia, in which the interstices between the pebbles, or, rather, 

 fragments composing the bed, have been filled up with metallic sul- 

 phurets, most probably infiltrated or injected from some neighbouring- 

 vein. 



Further north, at Gatica and Tocopilla, numerous veins of copper 

 are worked in these rocks ; and at the former place they appeared to 

 have a general run of about north-east. The Mina del Toldo, which 

 I examined, showed for many miles a constant strike of N. 80° E., 

 dipping about 85° to the west. The metallic compounds in these 



VOL. XVII. PART I. D 



