chip. xv. Section up the Valley. 545 



discovered here. From the fusible nature and general 

 appearance of the finer-grained strata, they probably 

 owe their origin (like the allied beds of the Uspallata 

 range, and of the upper Patagonian Tertiary formations), 

 to gentle volcanic eruptions, and to the abrasion of 

 volcanic rocks. Comparing these beds with those in 

 the mining district of Arqueros, we see at both places 

 rocks easily fusible, of the same peculiar bright green 

 and red colours, containing calcareous matter, often 

 having a finely brecciated structure, often passing into 

 each other and often alternating together : hence I 

 cannot doubt that the onlv difference between them, 

 lies in the Arqueros beds having been more metamor- 

 phosed (in conformity with their more dislocated and 

 injected condition), and consequently in the calcareous 

 matter, oxide of iron and green colouring matter, having 

 been segregated under a more crystalline form. 



The strata are inclined, as before stated, from 20° to 

 80° eastward, towards an irregular north and south 

 chain of andesitic porphyry and of porphyritic green- 

 stone, where they are abruptly cut off. In the valley 

 of Coquimbo, near to the H. of Gualliguaca, similar 

 plutonic rocks are met with, apparently a southern 

 prolongation of the above chain ; and eastward of it we 

 have an escarpment of the porphyritic conglomerate, 

 with the strata inclined at a small angle eastward, 

 which makes the third escarpment, including that 

 nearest the coast. Proceeding up the valley we come 

 to another north and south line of granite, andesite, 

 and blackish porphyry, which seem to lie in an irregular 

 trough of the porphyritic conglomerate. Again, on the 

 south side of the R. Claro, there are some irregular 

 granitic hills, which have thrown off the strata of porphy- 

 ritic conglomerate to the NW. by W. ; but the stratifica- 

 tion here has been much disturbed. I did not proceed 



