XXVI.
Geology.
This chapter may be read in connection with that on mines. Much has been written by Europeans and Americans on the geology of Mexico. But, interesting as the subject is, we have only space for an outline of the formation and physical structure of the country. A large part of Mexico is overlaid by igneous rocks, which consist chiefly of trachyte, feldspar-porphyry, and amygdaloidal basalt.
In the Sierra Madre, the metamorphic rocks, such as granite, gneiss, and clay-slate, are common. The great argentiferous veins frequently occur in the latter rock, although sometimes in porphyry, e. g., at Real del Monte, or in talcose slate, e. g. , some mines at Guanajuato.
Limestone is found at Tasco and Orizaba. It is extensively quarried at the latter town. The same rock constitutes the greater part of the eastern branch of the Cordillera between San Luis Potosí and Monterey. According to Dr. Wislizenus, the limestone at Saltillo belongs to the Silurian age. The lower part of the tierra caliente consists mostly of alluvial soil, although in a few places rocky ridges extend to the coast, e. g. , at Acapulco.
We have referred to the localities of the ores of the principal metals and of coal in the chapter on mines. Deposits of nitre, kaolin, common salt, and Glauber's salt, or sulphate of soda, are abundant on the table-land. Petroleum occurs plenteously in the States of Vera Cruz, Puebla, Tabasco, and Oaxaca. Sulphur is found at the volcanoes, especially those of Popocatepetl and Orizaba. A large supply is now obtained from the former.
It is said that Cortes's warriors descended into the crater
