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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



Still further westward, at Pontezuelo, out of reach of the imme- 

 diate action of the fault, the beds were found as follows (commencing 

 from the westward) : — 



N.35°W. Dip.50°S.W. Pine red sandstone beds. Found to 



affect the magnetic needle. 



N.35°W. „ 45°S.W. Coarser red sandstones, about 150 ft. 



thick. 



N.35°W. „ 45°S.W. Fine laminated and thick beds of red 



sandstone. 



N.40°W. „ 45°S.W. Fine grey sandstones. 



N. 45° W. „ 40° S.W. Coarse grits and fine conglomerates, 



consisting of white quartz-pebbles, 

 hardened grey, black, and greenish 

 slates, and fragments of red sand- 

 stone. 



Starting from the westward, over a series of fine-grained red 

 sandstones, we come upon some coarser and more gritty strata, in 

 which are imbedded several seams containing copper, visible on the 

 surface by the green colour acquired by oxidation (they are not 

 worked, being considered too poor) ; pebbly conglomerates are then 

 passed over, some of which are also impregnated with copper ; and 

 we then arrive at the Yeta de Buen Pastor, a fine-grained sandstone, 

 impregnated not only with copper, but also with native silver, dis- 

 seminated in fine metallic grains through the mass of sandstone. 

 As the silver is of more value than the copper associated with it, 

 this bed is worked exclusively as an argentiferous exploration. 

 The succeeding strata are still coarse grits and fine conglomerates ; 

 and we come upon the Veta de Pejo, or Veta Copacabana, which 

 also differs essentially from all the others from being rich in copper 

 in a mineralized state of combination with arsenic, sulphur, <fec. 

 The ore from this mine being very dark in colour, from the presence 

 of much arseniate of copper, this stratum is frequently termed the 

 " Veta Negra," or black vein. Still lower in the same class of beds, 

 the Veta Remacoia, or main seam of copper, is encountered and 

 found to produce native copper, disseminated irregularly through a 

 coarse grit, in grains, irregular lumps, or plates, sometimes of very 

 considerable size. This seam is considered to have been the most 

 anciently worked deposit of Corocoro, as it had been extensively 

 worked by the Indians before the Spanish conquest ; at present it 

 is regarded as nearly exhausted, notwithstanding its extent of several 

 miles, over which it has been explored. It is probable that by 

 " exhaustion" is only meant a miner's mode of expressing that the 

 depth of the workings and difficulty of keeping them free from water 

 does not equal the value of the produce. 



Below this metallic bed we find some gritty strata, and then have 

 a characteristic bed of fine-grained crumbly red sandstone of im- 

 mense thickness, the upper edge of which is seen on the surface 

 close to the line of fault. Nothing is now known of the strata, 

 metallic or otherwise, which may exist in depth on this (western) 



