Page:Vol 3 History of Mexico by H H Bancroft.djvu/605

Rh little attention has been given to this science in a country whose interests are intimately connected with it. The more progressive spirit of the republicans has been so shackled by the unsettled condition of affairs, as greatly to retard exploration, and while certain districts have been the object of diligent investigations only too many remained unnoticed.

Proceeding from the sandy coast at Vera Cruz into the interior, it is not till one ascends the tierra templada that porphyries and limestone are encountered. Above these, volcanic rocks and lava extend over many parts of the plateau, intermingled with porphyries and pebbles under a layer of hardened clay. On the Pacific coast from Tehuantepec to Acapulco granitic rock prevails, but from the latter place toward Mexico porphyries again take their place, mixed with heavy formations of limestone similar to those on the eastern slope. In the volcanic region, almost in a direct line from east to west along the nineteenth parallel, lava and porous amygdaloids are most frequent, but beyond Querétaro they give way to porphyries. The latter indeed abound in the whole country, especially in the northern regions, yet south of Durango numerous veins of trachytic rock are found crossed in an easterly direction by beds of lava. . On the eastern slope of the Sierra Madre, in Durango, the porphyries sometimes overlap extensive beds of veryfine gritstone, and exhibit greater softness than those south of the capital, with a large admixture of mica. Quartz is the most common of gangues, and its outcrops in the plateau serve frequently as a guide to prospectors.

The general direction of metallic veins is from the north-west to the south-east, and this being especially the case with the richer kind it is always taken into consideration on filing a claim. The average breadth of the vein is six feet, except on the Veta Grande of Zacatecas, where it is from thirty to thirty-five feet, the maximum being even seventy-five feet. In some districts, as in Sonora and Chihuahua, the ore lies