OF AT AC AM A AND COQUIMBO. 261 



C Considerable masses of the chloro-bromide of silver are found in the crevices, hollows, and 

 cavities in the rock of this stratum. In general, the external aspect of the rock, its innumera 

 ble crevices and dislocations, its cavities filled with broken pieces of the same, all denote that 

 this portion of the hill has experienced in situ reiterated shocks posterior to its formation, and 

 independently of the movement upheaving the rest. We will call this part of the hill the 

 plateau stratum ; miners are in the habit of designating the lower portions of the same forma 

 tion by the name of manto. 



&quot; Below this stratum follow others of divers thicknesses, composing that part of the mountain 

 called by the miners mesa-piedra (literally stone table), and which they consider entirely sterile, 

 because the greater number of the veins traversing it become poor or entirely sterile. The 

 rocks constituting this stage differ little in their composition from those above ; they are gene 

 rally more argillaceous and compact ; their fractures following old fissures, so that it is difficult 

 to obtain a new one. Moreover, the surfaces of fractures ordinarily present dendritic designs, 

 and the rock exhales a strong argillaceous odor when breathed upon. 



&quot;A sample of rock from this mesa-piedra taken from the Valenciana, gave fifty-eight per 

 centum of argil not acted on, and six per centum of similar substance affected by acids. 



&quot; The strata of this part of the mountain are more regular than those above, and have neither 

 the clefts nor the cavities mentioned as belonging to the preceding. The entire stratum of the 

 mesa-piedra is about one hundred metres thick. 



&quot; About one hundred and thirty metres below the surface of the plateau begin strata called 

 by the miners mantos pint adores, or strata that enrich veins. The most common rock is a 

 calcareous clay containing about forty per centum of residuum not acted on by acids and con 

 taining only traces of magnesia. Its color is a bluish-grey, spotted with yellow ; its structure 

 compact, and fracture conchoidal, splintering in some parts. In general it much resembles the 

 calcareous rocks most widely spread in the muscle-chalk formation of Europe. 



&quot; The stage which comprises all these strata, or mantospintadores, encloses the principal wealth 

 of the Chaiiarcillo mines, and the true lines of the chloro-bromide silver ores. It descends to 

 thirty or forty metres below the surface of the mine of San Jose, situated near the base of the 

 mountain. The two spiral hills (collines en limacon) are also in the same vicinity, and I think 

 I am not much deceived in estimaing the thickness of this stage at one hundred and twenty 

 metres, which is divided into strata of divers thicknesses, though always composed of the same 

 rock. Indeed, this undergoes very slight modifications of structure and color, and its strata are 

 ordinarily thick, though sometimes separated from one another by an extremely thin layer of 

 yellowish argil. 



&quot; Beneath this stage, and about two hundred and forty metres below the surface of the plateau, 

 a second mesa-piedra is reached similar to the one above, causing metals to disappear from the 

 veins in the same manner as the other. It is composed of rocks which are harder, more argil 

 laceous, and more compact than those of the preceding stage, and to me is apparently not less 

 thick. Up to the present moment (1846) it has only been observed in the mine of San Jose ; 

 and consequently we cannot decide whether it renders all the veins in that part of the mountain 

 sterile, or only those of the mine cited. 



&quot; In the last, at the bottom of the deepest excavations, we also find a porphyroidal rock simi 

 lar to that seen on the road from Ingenio to Chanarcillo. This rock effervesces with acids ; we 

 perceive within it incomplete feldspathic crystals disseminated amid a greyish paste, and its 

 crevices are spread over with a red argil. It is found to be composed of 



Carbonate of lime 0.076 



Carbonato of magnesia 0.034 



Part acted on by acids 0.316, holding 0.08 of silica soluble in potassium. 



Part not acted on 0.572 = 0.998 



cc I should add that the argillaceous rocks, those containing a small proportion of carbonate 

 of lime, sometimes have a schistoidal structure, as we see on the western slope of the mountain 

 near Bolaco Nuevo; and secondly, that on the northwest side, towards the mountains touching 



