chap. xv. Coquimbo , Arqueros. 539 



very singular nature of the almost vertical strata com- 

 posing it. These consist chiefly of a finer or coarser 

 granular mixture, not very compact, of white carbonate 

 of lime, of protoxide of iron and of yellowish garnets 

 (ascertained by Prof. Miller), each grain being an 

 almost perfect crystal. Some of the varieties consist 

 exclusively of granules of the calcareous spar; and 

 some contain grains of copper ore, and, I believe, of 

 quartz. These strata alternate with a bluish, compact, 

 fusible, feldspathic rock. Much of the above granular 

 mixture has, also, a pseudo-brecciated structure, in 

 which fragments are obscurely arranged in planes 

 parallel to those of the stratification, and are con- 

 spicuous on the weathered surfaces. The fragments 

 are angular or rounded, small or large, and consist of 

 bluish or reddish compact feldspathic matter, in which 

 a few acicular crystals of feldspar can sometimes be 

 seen. The fragments often blend at their edges into 

 the surrounding granular mass, and seem due to a kind 

 of concretionary action. 



These singular rocks are traversed by many copper 

 veins, and appear to rest conformably on a granular 

 mixture (in parts as fine-grained as a sandstone) of 

 quartz, mica, hornblende, and feldspar ; and this on 

 fine-grained, common gneiss ; and this on a laminated 

 mass, composed of pinkish orthitic feldspar, including 

 a few specks of hornblende ; and lastly, this on granite, 

 which together with andesitic rocks, form the surround- 

 ing district. 



Coquimbo : Mining District of Arqueros. — At 

 Coquimbo the porphyritic conglomerate formation ap- 

 proaches nearer to the Pacific than in any other part 

 of Chile visited by me, being separated from the coast 

 by a tract only a few miles broad of the usual plutonic 

 rocks, with the addition of a porphyry having a red 

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