52 Sketch of the Mine of Pasco. 



There is another class of formations, which is abundant in some 

 countries, namely, the volcanic, formed by a cause, opposed to 

 the preceding ; provided we are authorized to attribute the for- 

 mer to water in which its particles have been dissolved or sus- 

 pended, and the second we must of course attribute to subterra- 

 nean fire. The formations which compose the mineral chains of 

 Pasco and its environs, are worth studying ; they furnish interesting 

 data for geological investigation, confirm the observations made in 

 different countries upon the position of the strata, and render more 

 clear the invariable law, that only in the gres or sandstone of the 

 secondary is found the combustible called stone coal. As for my- 

 self, I have always observed it in Colombia and Peru, in forms 

 like those of St. Stephen, Wetin, Sarbruck, Newcastle, Liege, Sic. 

 It nevertheless confirms the important observation of Humboldt, that 

 in America the primitive soils do not contain the metallic repositories, 

 but they are contained in the transition and secondary. This is the 

 case with the greater part of the mineral districts of both republics 

 with which I am acquainted. But I will not confine myself to a de- 

 lineation of the relative existence of the strata, nor to a comparison 

 of them with others, but will only give a description of them, that 

 more learned geologists may draw the conclusions, which these ob- 

 servations enable them to do. The predominant rocks which com- 

 pose the ledge of Yauricocha, and extend many leagues both north 

 and south, are granite, blackish esqidto* sandstone, red porphyry, 

 blue limestone and conglomerate. The black esquito occupies the 

 lower part of the geological centre, which is, to appearance, in all the 

 space comprehended between Lake Quinlacocha and the mine of 

 Ayapota and the foundery, extending to the north and east. I have 

 traced it to tlie foot of the Pargas ledge : passing by the mines and 

 vent holes of the machine, it is seen again in the north, in the mines 

 near the church of Y auricocha ; and a proof that it passes it, is, 

 that it is found in the thirteenth lumhrera, made in the gravel of 

 Ayapota. The direction of this formation is from north to south, 

 inclining to the east; and from what I have observed in the excava- 

 tion of Quinlacocha, perforated in this rock, it undulates as much on 

 the surface as in the interior, and divides into strata. The grain of 



" We are not certain what rock is intended by the esquito, but from its associa- 

 tions, contents and characters, have concluded that it is argillaceous slate. 



