The present paper, the first of two closely related articles on the geology of the Arequipa region in Peru, concerns itself with the tuff deposits of the region as a volcanologic phenomenon. Their origin, mode of occurrence, and character are described. The second paper will deal with climatic conditions during the Pleistocene as evidenced by their topographic effects upon the tuffs.

The tuff deposits are the result of a series of fragmental outbursts of rhyolitic lava, similar to that which occurred in the Valley of Ten Thousand Smokes in Alaska in 1912. The period of most of the outbreaks was apparently early Pleistocene.

In the Arequipa region the deposits are of great volume, but they are only part of a much wider field that extends to the northwest and southeast. Altogether they appear to be one of the greatest known examples of this form of eruption.

Two principal types and several minor varieties are present. The older of the two is perfectly white; the younger is salmon-colored. This difference does not appear to be much more than superficial.

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