98 THE MINERALS OF CHILE. 



rocks, more or less argillaceous; some of the calcareous rocks are dolomitic, while others are 

 without man;nesia. The stratification is regular, and almost iiorizontal. The argillaceous 

 matter in tlie rocks are of two kinds — a wliite clay, and another comjjosed ol' a silicate of alumina 

 and iron. 



This locality has been thoroughly examined by M. Domeyko, and he finds no organic remains 

 in those ])arts of the mountain where the metal veins are found. The same geologist has, how- 

 ever, been informed that an ammonite was found in the rock of Reventon Colorado, at 

 some distance beneath the surface. In other parts of this mountain organic remains are abun- 

 dant in the calcareous rocks, especially the Turritella Andii and Terebratulaj. 



From the summit of the Chanarcillo mountain to the lowest workings of the mines is a little 

 less than 1,000 feet, and in that space there can be distinguished something like three distinct 

 divisions in the formation of the rocks. 



The plane at the summit of the mountain is composed of a dolomitic rock, having in some 

 places a thickness of 100 feet; it consists of about one-third clay. The rock is split in all 

 directions, and the surface of the fissures covered with small crystals of calcareous spar. In 

 some places it is so much split that it looks more like a mass of broken rocks piled together, 

 the interstices being filled with an earthy matter, as pulverulent as chalk, and composed of 

 one-third carbonate of lime and two-thirds clay. It is in these fissures of the upper layer that 

 very considerable masses of chloro-bromide of silver have been found. 



The second division of the rocks diifers but little in character from the last, being an argil- 

 laceous limestone; it is, however, more regular, and not so much fissured; at the same time the 

 metalliferous veins traversing it are much poorer. The thickness of this division is over 320 

 feet; and here commences the third division, where the limestone contains less clay and but a 

 little trace of magnesia. The color of the rock is a bluish gray, mottled with yellow ; of a 

 compact structure, and conchoidal fracture. This rock contains the principal wealth of the 

 Chaiiarcillo mines, and in it seems to be the principal deposit of chloro-bromide of silver; the 

 thickness of this bed is estimated at nearly 400 feet. Below this again lies another bed, 

 where the calcareous rock is again more argillaceous, and the veins poorer. In this portion of 

 the mountain porphyritic rocks are found at the lowest depths to which the workings have 

 gone. 



Numerous metalliferous veins traverse this mountain in every direction. The materials con- 

 stituting these veins (and mixed with which the silver ores are found) are the carbonates of 

 lime, iron, and magnesia ; zinc and manganese, and the sulphate of baryta, which, however, 

 exists in less quantity in these mines than in those in other parts of Chile. The metalliferous 

 portions of these veins are composed principally of chloro-bromide of silver, mixed with native 

 silver, and a small portion of sulphuret and sulpho-arseniuret of silver. The chloro-bromide does 

 not show itself in equal abundance at all dej)ths of the productive calcareous bed, already 

 mentioned: it is, particularly in the upper, one or two hundred feet; below this depth the 

 gangue becomes less and less calcareous, and the mineral changes its nature. At first it is the 

 pure chloride, or little mixed with sulphuret ; then the jiroportion of sulphur, antimony, native 

 arsenic, and ruby silver commence to increase ; so that, at 300 feet depth, hardly a trace of 

 chloro-bromide is found, the silver being associated with sulphur, arsenic, and antimony. 



These are the general features of these famous silver mines, and, as here described, some 

 idea can doubtless be formed of their geological character. Although the general character of 

 the mines resembles those just described, still the minerals and the containing rock frequently 

 differ ; thus, in the San Antonio mine, in the valley of Potrero Grande, the rock of the country 

 is porphyry, regularly stratified, and the gangue rock of the veins a dark, ashy gray, argillaceous 

 rock, of an earthy fracture. It is oftener found impregnated with calcareous and pearl spars, 

 which form veins and nodules in the midst of the gangue. The iron found in these veins is in 

 the form of protoxide, while that at Chaiiarcillo is in the form of hydrated jjeroxide. Again, 

 the mines of this latter locality abound in chloride and chloro-bromide of silver, while on the 



