' THE MINERALS OP CHILE. 87 



Juan, and La HIguera, celebrated for tlieir copper mines. G-ranite hills project frequently above 

 the tertiary planes that extend to and rest on the first chain of granite rocks, which are low and 

 rounded. It is in these rocks, wherever seen, whether on the coast or projecting above the 

 tertiary planes, or, when still further east, projecting through secondary strata, that the copper 

 and gold are found. A good example of this is the Cerro del Cobre mountain, which elevates 

 itself at the bottom of the valley of Coiiiapo. This mountain is composed of an elevated mass 

 of porphyritic diorite, traversed by veins of iron and copper ores, containing considerable 

 quantities of magnetic iron and ferruginous oxide of copper, copper pyrites, &c. It forms a 

 species of granitic island in the midst of stratified porphyritic and other compact rocks, 

 more or less calcareous, and preserves all the characters of the coast rocks, even to the nature 

 of the veins that it contains. 



Further east, overlying the granite and dioritic rocks, are stratified porphyries; and here, 

 at a height of 2,250 feet above the level of the sea, as at Ladrillos, commence the indications of 

 silver, disseminated in extremely fine particles of chloro-bromide ; hut, on excavating, this 

 indication soon disappears, and it is not until we reach a more elevated point that silver is 

 found very abundantly, and where the stratification becomes more perfect. 



Above the stratified porjDhyries there are calcareous and schistose rocks, more or less disturbed 

 from their original position. 



What is here said of the geological structure of the country east of Copiapo is true of many 

 other parts of Chile, from the coast eastward. From these general views of the geology ot 

 Chile, I next pass to the consideration of the minerals collected by the expedition, accompanying 

 the mineralogical description of them with an account of the manner of their occurrence. 

 For the latter, I am also indebted to the geologists already made mention of. 



GOLD. 



Native Gold.- — The specimens of this metal were contained in quartz rock, exhibiting all the 

 usual characteristics of auriferous quartz. The gold contains silver, with but a trace of copper. 

 In Chile, this metal is found in veins as well as in the drift ; the whole granite of the country 

 is traversed by quartz containing more or less gold, associated with the peroxide of iron ; and, 

 at some depth from the surface, with iron pyrites ; sometimes with cupreous pyrites, arsenical 

 pyrites, blende, galena, and sulphuret of antimony. These veins, by their decomposition, fur- 

 nish auriferous deposits of considerable extent that are now Tvorked. 



Mention is made by M. Crosnier of a number of gold deposits, irregularly disseminated in 

 the midst of decomposed granite and red clay, whicli contains a large quantity of peroxide of 

 iron, and which appears not to have originated from the decomposition of regularly formed 

 veins. This fact is apparent in the neighborhood of Valparaiso. It is also stated that gold is 

 found in clay, more or less ferruginous, arising from the decomposition of the granite in the 

 most elevated portions of certain mountains, and consequently in a situation where it could not 

 have been carried by water. 



It is supposed that the gold came up with the mass 6f granite at the time of the elevation of 

 the latter, and not by subsequent injection of veins; and, in most instances, iron pyrites is 

 regarded as its original associate. This character of auriferous formation is, of course, the 

 exception, as, in most instances, the gold is traceable to regular veins, or to the decomposition 

 of these veins. Although gold seems to be quite generally distributed through Chile, but few 

 of the deposits remunerate exploration ; the most extensive are on the flanks of the Andes, 

 about 40 miles east of Chilian, where it exists to the depth of 35 feet in a very fine yellow clay, 

 mixed with black sand ; the yield of gold is not very great. 



COPPER. 



Native Copper. — This is very commonly found in all the copper mines of Chile. In one 

 specimen, from Andacollo, (Coquimbo,) it was found crystallized in modified octahedrons ; it is 



