to Sĭlāo and the Hacienda of Chichimiquillas, there is a considerable descent, the rise afterwards, in passing the mountains of La Tlachiquera, is so great, that the plains of San Juan de los Llanos are supposed to be but little inferior in elevation to those of the Capital itself. They are called by the natives El Espinazo (the back-bone) of the Table-land; and throughout the whole extent of this dreary ridge, which stretches as far north as Saltillo, the stunted vegetation, and the scarcity of water, seem to designate it as one of the most elevated portions of the central plateau. There is a descent, indeed, into the valley of the Jărāl, and you afterwards rise but little between that place and San Luis Potosi, or between San Luis and the Cañada of Catorce; but the ascent to the town being at least 1,500 feet, the height of the plain would be only 6,200 feet above the level of the Gulf, if that of the Real itself be correctly computed. The question must be decided by a series of barometrical observations, taken both above and below; and I mention it upon this account as worthy of the attention of some future traveller.
Nov. 30.—We visited the Sŏcăbōn of La Purisima, the mines of El Serēno and Dolōres Trompeta, with the Socabon and mine of La Luz.
In order to reach these works you have to descend the North-eastern declivity of the range of mountains upon which the town stands, as far as the Potrero, a spacious platform, so little elevated above the level of the surrounding plains, that it is acces-
