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the thickness which we have accordingly given it in the general vertical section; nevertheless it is proper to state that at no point on Red River were we able to see the base of the formation, so that even this enormous thickness may prove to be an underestimate.

PHYSICAL FEATURES.

The principal distinguishing features of the district occupied by this formation are a gently undulating surface, low ridges often with precipitous sides, and abrupt truncated conical hills. The surface of the country is usually of a deep red color, and the hills and ridges, being composed chiefly of clay and sandstone in alternating and nearly horizontal bands, exhibit a remarkably striated appearance. Owing to the general softness of the strata composing this formation, the beds of the principal rivers are generally broad and deeply sunk below the general level of the country, and the clay mixing with their waters in the form of sediment tinges them their characteristic red hue.

This division of the Cretaceous System, like the preceding, exhibits evidence of denudation on a grand scale. The different hills and ridges, which form such a striking feature in the otherwise monotonous scenery, are the remains of once continuous strata which have been gradually eroded. Many of these hills are four or five hundred feet high. They are, as already remarked, usually isolated and rise abruptly from the surface of the prairie in the form of truncated cones, their summits being usually smooth, flat, and covered with short grass. The accompanying section (No. V) exhibits the geological character of these hills as they occur along the Big Witchita River.

In addition to the above phenomena, there are others of a less general character that serve occasionally to relieve the dull uniformity of the region occupied by this formation. The sandstone and clay, being of unequal hardness, weather unevenly, and the former is sometimes seen projecting in a series of shelves several feet from the face of the bluffs. At other points we observe detached layers of sandstone ten or fifteen feet in diameter supported several feet from the ground on slender pedestals of clay, as exhibited in the following section [No. VI] taken from near the source of the Little Witchita River.

Caverns and deep pits are of frequent occurrence in the gypsum, the former sometimes extending many feet. The pits are usually quite smooth and funnel-shaped.

Dip.-These strata exhibit little evidence of violent disturbance, and are usually found dipping at very slight angles E. S. E. In a few instances they are undulated and unheaved, but these are mere local exceptions.