340 METALLIFEROUS DEPOSITES. 



IX. Intendancy of Oaxaca. 



37. Oaxaca. 

 X. Intendancy ofPuebla. . 

 Several Mines. 

 XI. Intendancy of Vera Cruz. 

 Three Mines. 



XH. Old California. 

 One Mine. 



In the present state of the country the veins are 

 the most productive, and the minerals disposed in beds 

 or masses are very rare. The former are chiefly 

 in primitive or transition rocks, rarely in second- 

 ary deposites. In the old continent, granite, gneiss, 

 and mica-slate form the central ridges of the moun- 

 tain-chains ; but in the cordilleras of America these 

 rocks seldom appear externally, being covered by 

 masses of porphyry, greenstone, amygdaloid, basalt, 

 and other trap-formations. The coast of Acapulco 

 is composed of granite ; and as we ascend towards 

 the table-land of Mexico, we see it pierce the por- 

 phyry for the last time between Zumpango and So- 

 pilote. Farther to the east, in the province of Oa- 

 xaca, granite and gneiss are visible in the high plains 

 which are of great extent, traversed by veins of 

 gold. 



Tin has not yet been observed in the granites of 

 Mexico. In the mines of Comarya syenite contains 

 a seam of silver ; while the vein of Guanaxuato, the 

 richest in America, crosses a primitive clay-slate 

 passing into talc-slate. The porphyries of Mexico 

 are for the most part eminently rich in gold and 

 silver. They are all characterized by the presence 

 of hornblende and the absence of quartz. Common 

 felspar is of rare occurrence, but the glassy variety 

 is frequently observed in them. The rich gold mine 

 of Villalpando, near Guanaxuato, traverses a por- 

 phyry, of which the basis is allied to clinkstone, and 

 in which hornblende is extremely rare. The veins 

 of Zuriapan intersect porphyries, having a green- 

 stone basis, and contain a great variety of interest- 



