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This division of the Cretaceous System consists chiefly of limestone and sandstone, of which the former is much the most abundant and varies much in character. It presents every degree of compactness, from soft chalk to hard subcrystalline limestone. It is, however, generally compact, of a dull earthy appearance, and varies in color from pure white through shades of light gray, yellow, blue, and brown. In many localities it contains a good deal of silex, or sometimes passes into pure silicious sandstone; in others it exhibits a more or less argillaceous character, though beds of clay are rarely observed in it.

The strata are sometimes cherty, and often, especially in the southern extension of the formation, they are crowded with light gray, blue, and dark nodules of flint from one to six inches in diameter. These are sometimes arranged in nearly parallel bands, but more frequently scattered promiscuously through the strata, now and then occurring so abundantly as to form a kind of conglomerate. As we travel north, the Cretaceous strata assume a much more arenaceous character, so that near the headwaters of Red River they are composed almost exclusively of silicious sandstone. But generally the sandstone alternates with the limestone in bands of from twenty to one hundred feet in thickness. It is usually highly ferruginous, and often contains spherical concretions of iron. Its color varies from light gray to deep red and brown. In some localities it is quite hard, durable, and possesses all the requisites of a good building material, but it is often soft and yields readily to the action of the weather.

This division of the Cretaceous System has heretofore been regarded as occupying a comparatively limited portion of the region under consideration. From our own observations, however, taken in connexion with those of other explorers, we are convinced that its real importance in this respect has been greatly underrated, and that instead of being confined merely to isolated and distant points, it really constitutes the prevailing formation over a very large portion of the Plains.

Commencing in the southeast, these rocks occur at a number of points along the route traveled by your Expedition between Victoria and San Antonio. They do not, however, present here a vertical thickness of more than twenty or thirty feet, and are confined to limited areas.

In the vicinity of San Antonio the Upper Cretaceous strata occur in