﻿I860.] 



FORBES BOLIVIA AND PERU. 



61 



Iron-pyrites. 



Copper-pyrites. 



Tin-pyrites 



Sulphuret of Copper and Bismuth? 

 Jamesonite. 

 Plagionite. 

 Zinkenite. 

 Lonchidite. 

 Mispickel. 



Fahlerz. 



Silver-fahlerz. 



Zinc-fahlerz. 



Wolfram. 



Calc-spar. 



Carbonate of Iron. 



Sulphate of Lead. 

 Phosphate of Lead. 

 Arseniophosphate of Lead. 



of Lead. 



„ (niekeliferous). 

 Danaite. 



The above enumeration is doubtless far from complete ; but it is 

 quite sufficient to show at a glance bow strikingly the mineralogy of 

 these older strata differs in its general features from that of the more 

 recent rocks before described. 



I may here notice some rocks which occur on the immediate line 

 of the Bolivian coast, and which, for the present at least, I class along 

 with the other metamorphic Silurian strata of this part of the world, 

 not from being able to prove with certainty that they are of Silurian 

 age, but because, from their position and their relations to the 

 newer formations in contact with them, they appear to be only a 

 continuation of the beds which in Chile form the Silurian series of 

 the coast. 



They are so very much altered by the effects of the eruptions of 

 granite, porphyry, and diorite, which here break them up, that even 

 their sedimentary nature can hardly be recognized except at some 

 few localities. 



To the south, in the Desert of Ataeama, they appear as gneissic or 

 metamorphic schistose rocks, broken through in all directions by 

 granitic outbursts, the granite itself being precisely identical in 

 external appearance and mineralogical composition with the pre- 

 viously described auriferous granite of Silurian age in the Eastern 

 Andes. It is composed at Mexillones, for example, of white ortho- 

 clase, colourless quartz, and dark mica. Further southward the 

 sands arising from the disintegration of this granite have been proved 

 to be auriferous. 



At Cobija, the black rock which forms the rugged low cliffs and 

 detached rocks in the sea and along the shore appears also to belong 

 to this series, although in appearance it frequently resembles a com- 

 pact trappean rock or a black porphyry. On closer examination, I 

 am disposed to consider it as a clay-slate or other argillaceous or 

 calcareo-argillaceous rock, fused in situ by the action of the masses 

 of porphyry or diorite resting immediately upon it, or of the granite 

 which has upheaved and broken it up, and which, although not 

 itself visible at the Port of Cobija, is seen a little further to the 

 south. This rock varies in colour from a bluish-grey to grey or 

 bluish-black, and, like all altered rocks of this class, contains 

 green epidote frequently disseminated in it, or forming imbedded 

 geodes : geodes of quartz or calcedony also occur ; and when the 

 rock appears to have been completely fused I have noticed some dark- 



