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Rh that the volcanoes just named lie on the same great vent of the earth's crust, and approximately on the nineteenth parallel of north latitude (vide Cosmos, vol. v, p. 377, et seq.).

The most important geological event in Mexico since the Spanish Conquest is the elevation of the volcano of Jorullo, which took place in the year 1759 (see Section IV, in Part Second, for a long account of it). The description of Jorullo explains, in a general way, the manner in which volcanic mountains are formed.

There are still many parts of the Mexican Republic where the hammer of the geologist has not yet sounded, because scientists have thus far confined their observations chiefly to the vicinity of the metallic deposits and the volcanoes. A great variety of minerals and precious stones is found in Mexico. Señor Busto states that the number of mineral species is three hundred and sixty-five, the majority of which occur as ores.

We have not sufficient space in this volume to give the complete list, but among the gems of the country we may mention the ruby, diamond, opal, topaz, emerald, garnet, agate, carnelian, and tecali, or so-called Mexican onyx, which is a variegated calcite.

For further information on the geology of Mexico, the reader is referred to Humboldt's Cosmos and New Spain; Busto's Estadistica de la RepilUica Mexicana; Burkart's Aufenthalt und Reisen in Mexico in den Jahiren 1825-1834 ; Wislizenus, Memoir of a Tour to Northern Mexico; and various articles in the American Journal of Science and Arts, in the Annales des Mines, in Poggendorff's Annalen, and several other French and German scientific periodicals.