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84 of Popocatepetl to procure sulphur for the manufacture of gunpowder. The abundance of hot springs on the Mexican plateau has already been mentioned (see p. 83).

The volcanoes are perhaps the most interesting features in the geology of Mexico to travelers. There are four active volcanoes in the Republic; but no eruption has occurred in any of them during the present century. Earth-

quakes are common in the vicinity, however, and soJfataras, fumaroles emitting hot aqueous vapor, and adjoining warm springs, indicate that these volcanoes are still in a semi-active state.

Beginning on the western coast, the Mexican volcanoes are: Colima, Jorullo, Popocatepetl, Iztaccihuatl, Orizaba, and Tuxtla. The heights of these mountams are given in Part Second.

Humboldt, who was the first scientific observer to make an extensive geological reconnaissance of Mexico, has