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of most of the streams running eastward across the east half of the minor Black Prairie, artesian water can be obtained at from one hundred to three hundred feet. The source of this water is in the Lower Cross Timber sand. Many of the concretions and calcareous layers are probably suitable for making cement, but tests must be made. The clays may also prove of commercial value.

The medial and lower portions of these shales are at places bituminous, as at Austin, Fiskville, and other places, and frequently an appreciable amount of rock oil appears upon the surface of the waters obtained from them, but so far there have been no reasons to justify any expectations that this oil occurs in commercial quantities, the indications being rather against such a conclusion.

Immediately above and to the east of the Eagle Ford clays comes the white rock or Austin-Dallas chalk, which is the most conspicuous representative division of the whole Upper Cretaceous system. This occupies the narrow strip, as noted in the preceding topographic description, marking the western border of the main Black Prairie region, separating it from the minor Black Prairie. The outcrop of this chalk begins in the southwest corner of the State of Arkansas, and in the Indian Territory. It crosses Red River, the exposure continuing westward up the south side of the valley of that stream to the north of Sherman, from which place it deflects southward, passing near McKinney, Dallas, Waxahachie, Hillsboro, Waco, Belton, Austin, New Braunfels, San Antonio, and Spofford Junction, beyond which it bends northward, appearing in the disturbed mountains in the vicinity of El Paso and New Mexico. It is distinguished above all by its peculiar chalky substructure.

The rock of this formation is a massive, nearly pure, white chalk, usually "

The words "limestone" and "chalk" are used in these pages as follows: Limestone is employed generically for species of widely different origin and structure. Specifically they may be of five kinds: 1. Breccias composed of more or less comminuted and cemented shells of ancient ocean bottoms or shores. 2. Concretions or segregations formed by the segregation of lime in clays and sands after original deposition—rare in our rocks. 3. Chalks are composed of amorphous calcium carbonate, usually more or less foraminiferal, void of laminations, and of comparative deep sea (not abyssal) origin. These may be hardened by metamorphism into firm limestones; hence the term chalky limestone will imply chalky origin, and the term chalk present chalky condition. 4. Laminated, impure limestones, occurring as alternating beds in sands and clays, indicative of shallower origin than chalk. 5. Metamorphosed limestones, or any of the above which have undergone induration or secondary change. All laminated limestones thus far found in the Texas Cretaceous are in the Basal beds, and are more or less arenaceous or argillaceous, further proving their origin to have been in shallower water than those in which chalk is laid down.