12 GEOLOGY. 



Sta. Maria on the south. The waters of these lakes having no outlet, are strongly impregnated 

 with saline suhstances, so as to he unfit for drinking. 



The adjoining mountains on our route are of igneous character, heing composed of vesicular 

 and amygdaloid trap, forming more or less continuous ridges, ranging north and south. 



Associated with the fact of running v/ater in this region, we see the country characterized 

 by an unwonted appearance of fertility and verdure, not alone confined to the immediate 

 borders of the stream,' but extending over the hills and plains adjoining. 



Our route, after crossing the Sta. Maria river, takes a course S. 70° W., (mag.,) passing 

 over country characterized as above, bounded by mountain ridges of less height above the 

 general surface than those before passed. The greatest development of mountain range lies to 

 the west. The various rock exposures exhibit most abundantly forms of amygdaloid trap ; more 

 rarely we meet with local exposures of limestone strata, or variable metamorphic products. 



Conspicuously in view in our direct course are the mountains in which the silver mines of 

 Corralitas are located, consisting of an assemblage of rounded and peaked summits of various 

 heights, rising from 500 to 1,500 feet above the adjoining plain. 



These mountains occupy an area of about 5 miles in length from north to south, and 2 to 3 

 miles in width. They rise isolated in the midst of a broad alluvial plain, sloping gradually on 

 the east and west towards the respective valleys of the Sta. Maria and Corralitas rivers. A wide 

 intervening depression also separates them from higher mountain ranges north and south. The 

 latter mountains present a marked contrast in their precipitous sides and exposed rock of a 

 basaltic character to the uniform smooth outline of the mineral-producing mountains. In these 

 latter, indeed, the geological formation is everywhere concealed from view by a variable deposite 

 of earth and gravel, thickly covered with a growth of grass. It is this fact which has probably 

 given to these mines their Spanish appellation of '^ Minus del mineral de la Escondida," or 

 hidden mines. 



The mines at present worked occupy the most northern point of the mountains, though mine- 

 ral indications and abandoned excavations are common over the exposed face of the whole 

 mountain range. The various excavations bring to view a very uniform character of formation, 

 first passing through a variable layer composed of angular fragments of rock, imbedded in a 

 dry brown earthy medium. The superficial rock exhibits a siliceous limestone of very close 

 compact texture and dark blue color ; to this succeeds the true silver-bearing rock, being a form 

 of subcrystalline limestone showing the action of internal heat, of a much softer texture than the 

 preceding, and of a whitish gray color, (specimen rock, No. 99.) 



In this latter rock are exposed the veins of argentiferous galena, frequently extending into 

 the upper siliceous rock, but acquiring its greatest thickness and richness in this lower forma- 

 tion. 



The veins of mineral penetrate this rock in the' form of variable sheets, dipping regularly at 

 an angle of 45° to the northwest. 



Further details in reference to the character and working of these mines, with such reliable 

 mining statistics as could be procured, will be found under a separate head. 



From the mines, by a gradual and continuous descent over a wide grassy plain, scattered 

 with low mezquite bushes, we have in view at the lowest depression the valley of the San Miguel 

 or Corralitas river, and the towns of Baranca and Corralitas. This plain, though usually dry, 



