MEXICAN LOCALITIES. 17 



mercury were observed in the porphyry at considerable distances from the 

 vein.' At Durasno, between Tierra Nueva and San Luis do hi Paz, in tlio 

 State of Guanajuato, lie inspected a cinnabar deposit forming a layer" rest- 

 ing on porphyry. 



The cinnabar deposits in the mining district of Guadalcazar were dis- 

 covered in 1840. Though they are numerous they appear to be of no 

 great value, for in 1874 they were not yielding enough quicksilver to sup- 

 ply the demand in the state in which they lie.' This district forms the 

 subject of a paper by Mr. Ramirez,' from which the following notes are 

 taken. The country rock of the district is chiefly limestone, with a few 

 intercalated beds of shale. The rock is compact and usually of a bluish- 

 gray tint. No fossils are known to occur in it, nor does it stand in such 

 relations to other strata as to render a stratigraphical determination of its 

 ago practicable. It is supposed, however, to be Cretaceous both by Mr. 

 Ramirez and b}' Mr. V. d'Aoust.^ The region also contains granites and 

 porphyries; the latter inclose deposits of silver ores, but the quicksilver 

 ores are confined to the limestone in the district in which this metal has 

 been exploited. According to Noggerath, however, ciiniabar with pyrite 

 and galena is also found in granite in this region. 



Ores of quicksilver occur at numerous points along a belt nearly foi-ty 

 miles in length (sixty kilometers), which extends to the northwest of 

 Guadalcdzar. The deposits occur mainly as layers in the limestone, but 



'Essai politique sur le royanme de la Noiivelle Espague, p. 585. The vein is CiiUiscl the San Juan 

 de la Cliica. It traverses the mountain of the Calzones and extends to Chichindara. I have not been 

 able to find these localities oh the maps. 



«I shall nso this word to translate the term manto, which does not seem to correspond to any e,\- 

 pressiou recognized in Engli-sh or German mining technology and soonis also to bear a somewhat vari- 

 able meaning among Spanish-American miners. Humboldt (ibid., p. 584) defines nianto as "uno 

 couche horizontale," but horizoutality is certainly not a necessary attribute of mantos as the term is 

 used by Spanisb-Amerioan mining geologists. Rivero, in describing tbe deposits of Ilnancavelica, re- 

 peatedly uses the expression manto (5 capa, and capa is the term employed for a stratum of sediment- 

 ary rock. According to F. A. Moosta (Ueber das Vork. der Clilor-, Hrom- und lodverbinduugen, p. 2'>), 

 the Chilian miners use this word to describe any layer or sheet of mineral, irrespective of origin, so 

 that strata of sedimentary rock and veins crossing strata, as well as dikes, may all be called mantos. 

 Rivero, however, makes a sharp distinction between veins and mantos, and both ho and the Mexican 

 geologists seem to me to understand by manto either an ore-bearing stratum or a deposit resembling a 

 stratum, such as a bed-vein, irrespective of the question whether or not the ore deposition has accom- 

 panied sedimentation. No doubt the term is much more loosely used by miners. 



'Castillo, loc. cit. 



•• Anales del ministerio de fomento, Mexico, vol. 3, 1877, p. 339. 



^Comptes rendus Acad, .sci., Paris, vol. 83, 1876, p. 289. 

 MON xiir 2 



