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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



ness of quartz-rock in Bolivia before having visited that country, I 

 afterwards paid considerable attention to this point ; but in the di- 

 stricts traversed by me, although containing a vast area composed of 

 more or less compact sandstones and impure siliceous beds, with in- 

 terlaminated partings of blue, olive, or brownish-red shales or slates, 

 I did not meet with any very extraordinary thickness of them. 



A superficial observer, particularly if passing rapidly over the 

 ground, might, in several places, easily be deceived into the belief 

 that such a thickness really occurred, from finding the strata dip- 

 ping to one side over a great distance, as, for example, between 

 Hachecache and Tiquina, where the beds passed over might for this 

 reason appear to form part of an immensely thick series. As shown 

 in Section No. 1, these beds are in reality contorted and doubled up 

 into an almost innumerable series of extremely sharp, small folds ; 

 and in this case I counted no less than 23 such folds in the short 

 distance between Hancoamaya and the commencement of the Car- 

 boniferous series, owing to which the appearance of a very general 

 dip to the eastward was presented. 



In the annexed map (PL I.) the Devonian series is coloured toge- 

 ther with the Silurian with one tint, from my being unable to draw 

 so definite a line of separation as is found in M. D'Orbigny's map : 

 their mode of occurrence is so well illustrated in Sections Nos. 1 and 

 2, that a description would be superfluous. 



The strata themselves consist of white sandy beds more or less 

 compact, yellowish impure sandstones and grits, and, as is seen in 

 Section No. 1, at Hachecache, quartzite-like rocks, showing them- 

 selves both to the east and west of that place, and easily recognized 

 in section from their rugged and shattered appearance, due to their 

 having been too rigid to bend along with the other beds ; interstra- 

 tified with these are blue, olive-green, or reddish-brown shales, and 

 beds of blue clay-slates. 



Sir Roderick Murchison, some years back, in his ' Siluria,' when 

 reviewing the Devonian formation of Bolivia as described by M. 

 D'Orbigny, expressed himself thus : — " In the absence of sufficient 

 proof, doubts may be entertained whether these sandstones and 

 quartz-rocks of the Andes may be of Upper Silurian rather than of 

 Devonian age;" and my own researches have tended to make me 

 adopt this opinion : at the same time, however, I think it probable 

 that there is in Bolivia a true Devonian system at the base of the Car- 

 boniferous strata, consisting of white, yellow, and brown-red sand- 

 stones, with intercalated shale-partings, which collectively do not 

 attain any very great thickness nor occupy any very extensive super- 

 ficial area*. 



* These beds probably are of Upper Devonian age. I have not examined any 

 part of the country which lies on the eastern slope of the main chain of the 

 Andes and is coloured by M. D'Orbigny as Devonian. As will be seen from the 

 comparative sections appended to this memoir (PI. III.), M. Pissis does not en- 

 tertain the same opinion, but in his section he represents these beds as being 

 lower in position than the whole of the very thick strata wliich I am now about 

 to describe as representing the Silurian epoch. 



