At the time of the visit of Humboldt in the early part of this century, it was estimated that three thousand distinct mines were in operation. The greatest of all the mines of Mexico are those of Guanajuato and Zacatecas. They were opened in 1558, upon the great vein known as Veta Madra. The great vein is chiefly in clay slate. It is of wonderful thickness, sometimes more than one hundred and fifty feet across, and is said to have been traced for about twelve miles. The vein is made up of half a dozen substances, the most important of which are native silver, sulphuret of silver, and red silver. Near the surface they are partly decomposed, but in their unchanged condition, farther down, they are known as “black ores.” The vein has been penetrated downward more than two thousand feet, and is found to be very rich at that depth. The mine of Valenciana, upon a rich portion of this vein, has averaged at times a product of two million dollars, or about one fifteenth part of the total product of all the mines of Mexico. At the present time no work is carried on in these mines. Operations were suspended some years ago by floods of water, and the unsettled state of the country, added to other disadvantages, prevented a renewal of work. Before any productive operations could be prosecuted, it would be necessary to erect powerful machinery; and to set it up and put it in operation would cost enormously; so great, in fact, would be the cost, as to deter any body of men, or any association of capitalists, from entering upon the enterprise. Until the present century the ores of the silver mines of Mexico have been worked by rude processes, very little in advance of those of the native Indians. Little or no mining machinery was used. The ores had to be transported out of the mines upon the backs of Indians, climbing up a series of long steps over slippery rocks, and working in a nude state. When the mines were troubled with water, rude pumps were set up, and in many cases there were no pumps, but the water was carried out on the backs of men. [Sidenote: HOW TO SECURE A PIG SKIN.]