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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nor. 21, 



Paver Chuquiaguillo, I found interstratified a bed of carbonaceous 

 matter, approaching to lignite. In many parts this appeared as if 

 wholly composed of carbonized marine or marsh plants, resembling 

 rushes, reeds, and alga? ; but I likewise found one or two pieces of 

 unmistakeable lignite or carbonized wood. At all these places it is, 

 probably, that it is but one and the same bed which appears at Poto 

 Poto ; this bed does not attain a thickness of more than from 6 

 inches to a foot. At the foot of Illimani it is, however, of much 

 greater thickness. 



A chemical examination, which I made of several of the clays from 

 these strata near La Paz, showed that they contained but a mere 

 trace of lime, as might be expected, knowing that the Silurian rocks 

 of the high Andes, from which they appear to have been derived, 

 contain but traces of limestone. 



As the Silurian origin might indicate, this formation is everywhere 

 eminently auriferous, and has been both since, and probably even 

 before, the time of the Incas very largely explored for gold. The 

 great quantities of gold found in Peru at the time of the Spanish con- 

 quest, had in greater part, if not wholly, been derived from these 

 diluvial accumulations. 



The rabbit-like burrows made by the Indian gold-workers into 

 the more auriferous beds are everywhere visible along the sides of 

 those valleys where a supply of water was not too distant to prevent 

 these workers from transporting the auriferous earth for the purpose 

 of working it : and frequently later explorations have disclosed the 

 mummies or skeletons of unfortunate Indians, who have perished in 

 these narrow and tortuous holes, from the falling in of the superin- 

 cumbent earth, and been buried along with their mining implements. 



This system of working is now entirely abandoned, and the mode 

 of operating is very different at present, and can easily be understood 

 from the annexed sketch (fig. 1), which may be supposed to repre- 

 sent a general view or sketch-section of the operations carried on at 

 the gold-washings of Chuquiaguillo, near La Paz, belonging to Mr. 

 Saienz of that city. 



As will be seen in this sketch, the valley is, in the first instance, 

 completely closed up, and the course of the river stopped, by a rude 

 wall or dam of stones and earth, provided with sluices, and having 

 a portion of the wall seen to the right hand somewhat lower, in order 

 to carry off any overflow of water which otherwise might disturb the 

 workings. A longitudinal excavation is then made close up to the 

 one side of the valley, and of such breadth as can be conveniently 

 carried on by the number of hands at disposal ; and, in making this, 

 the large boulders and stones, too heavy to be carried off by the rush 

 of water, are piled up to one side, whilst the earth, gravel, and clay 

 are merely loosened and flushed off by the water turned on from the 

 sluices, allowing the force of the stream to cany them down the river. 

 On arriving at the several successive auriferous beds, which are 

 known from previous trials, and which are denoted by the dark 

 bands running horizontally across the excavation, as seen in the sketch, 

 more care is taken, but the whole of the auriferous earth is likewise 



