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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



mass of some hundred feet of intruded augitic porphyry, very cha- 

 racteristic, and different from all the other porphyries met with in 

 these parts, from its containing black augitic crystals along with 

 crystals of white felspar, in a brown, black, or grey felspathic base. 

 Above these are seen shales similar to those at the base, of a red 

 colour, and as if calcined*; these are succeeded by a black porphyry, 

 on which rest altered shales, in the midst of which a tbin bed of 

 grey limestone is seen, with very indistinct traces of organic remains ; 

 above this a red porphyry, with white felspar- crystals and black 

 specks of augite, succeeded by a second series of shales, with an 

 intercalated bed of limestone similar to the first; above these shales 

 a second red porphyry, then a third bed of shales, and, lastly, great 

 beds of red porphyry and some pebbly porphyry-conglomerates, 

 which contain agates and nodules of calc-spar, the latter frequently 

 covered by a coating of a green mineral. I do not at present ven- 

 ture to pronounce any definite opinion on the true position and age 

 of these beds, but only think that the evidence of their being Carbo- 

 niferous is not sufficiently strong to be conclusive, especially when 

 it is considered that we have no strata of that age anywhere deve- 

 loped along the coast of the Pacific. 



Before concluding these remarks on the porphyries, I may also 

 notice the occurrence of eruptive porphyries and some stratified por- 

 phyry-tuffs in the midst of the Silurian formation further inland. 

 These are seen to the north breaking through the ridge which 

 separates the valley of Illabaya from that of Sorata, a ridge which is 

 in itself so sharp and steep as to make it appear very surprising to 

 find it broken through hy erupted porphyry, which has left the top 

 of the ridge as a peak of somewhat hardened clay-slate : the porphyry 

 is of a red colour, with white crystals of felspar. 



Similar eruptions of porphyry occur near Oruro, breaking through 

 the Silurian rocks. The latter are eminently stanniferous, from which 

 circumstance, at the point of contact of the porphyry with the Silurian 

 slates at one locality, the tin-ore or oxide of tin was found fused (M. 

 Kroeber informs me) by the heat of the porphyry to a true white-tin 

 enamel, such as is commonly made artificially. 



In the Cerro de Potosi, celebrated for the richness of its silver- 

 mines, and situated still further to the south, such porphyries are 

 again found developed ; and further south of this probably they run 

 into the porphyries of the Desert of Atacama, which are, as before 

 mentioned, contemporaneous with the Upper Oolitic beds (these emi- 

 nently fossiliferous). 



Drawing a line through these three points, which are coloured on 

 the accompanying Map (PI. I.), we find they are in one and the 

 same direction, and have a general bearing of nearly north-west and 

 south-east. 



7. Permian or Triassic Formation. — The rocks now about to be 

 treated of and considered as representing in Bolivia either the Per- 

 mian or Triassic formation of Europe, are seen cut through in their 



* On the top of these is a bed of saline and recent accumulation, often very 

 calcareous, and about from 1 to 2 feet in thickness. 



