﻿I860.] 



FORBES BOLIVIA AND PERU. 



43 



side of the fault; but the metallic beds depicted in the section are 

 supposed by me to exist, for reasons which will subsequently be 

 explained. 



Crossing now over to the east side of the line of fault, we find an 

 immense development of the same fine-grained sandstones as those 

 noticed as composing the last bed met with on the surface to the west- 

 ward of the fault ; and in the lower part of this bed we find developed 

 a series of metalliferous beds differing considerably in their features 

 from the " vetas " (or veins, — more properly, beds) previously de- 

 scribed as seen and worked on the surface at the other side of the 

 fault. These, from their being of much less thickness, are called 

 by the miners " ramos," or branches ; and, for the sake of clearness, 

 only five of these are drawn in the section, whereas many more exist, 

 as known by the mining explorations in them : for example, in the 

 " Mina de Cimbani " there occur five principal or workable " ramos " 

 and nine lesser ones ; and possibly a still greater depth may bring 

 others to our knowledge. 



The strike of these " ramos " is tolerably constant, and only 

 affected by purely local circumstances ; but the dip was found to be 

 higher as we approached the fault : thus, in the Mina del Pozo the 

 following observations were taken : — 



Strike N. 25° W. Dip. 80° E. 

 „ N.20°W. „ 75° E. 

 „ N.35°W. „ 70° E. (Eamo de San Jose.) 

 „ N.35°W. „ 35° E, 

 „ N.25°W. „ 30° E. 



the angle decreasing with great rapidity as we get away from the 

 fault, showing that a sort of bend or curve had taken place in the 

 beds on settling down or coming to rest after the dislocation. 



A considerable amount of gypsum is found in the form of strings 

 or veins, also as small crystalline particles disseminated through 

 these and the beds of red sandstones of this whole series. 



These cupriferous beds are very extensively explored in Corocoro, 

 and produce a large portion of the supply of copper derived from 

 this district. The ore obtained from the " ramos " is very different 

 and in a much finer state of aggregation than that from the " vetas :" 

 this probably arises from the latter being situated in the midst of 

 much coarser and more porous or open beds of grit and conglomerates 

 of small pebbles. In both cases the ore is seldom continuous for 

 any great distance, but is found scattered through the metalliferous 

 sandstones, in irregular patches or spots of a white or greenish- 

 white colour, full of small grains of metallic copper : the colour of 

 these spots, forming a striking contrast with the deep red colour of 

 the rest of the bed, affords, at first sight, a sure indication of the 

 presence of the metal. This discoloration (for such it evidently is) 

 seems to indicate some chemical change having taken place, appa- 

 rently connected with the reduction of the copper to the metallic state, 

 and the formation of the sulphate of lime (gypsum) in these beds. 



An attentive study of this interesting formation has led me to 



