112 CRETACEOUS ROCKS OF TEXAS. 



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 mak- 

 ing cement, but tests must be made. The clays may also prove of commer- 

 cial 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. 



NO. 3. THE WHITE EOCK, OE AUSTIN-DALLAS CHALK. 



Immediately above and to the east of the Eagle Ford clays comes the 

 white rock or Austin-Dallas chalk, which is the most conspicuous representa- 

 tive division of the whole Upper Cretaceous system. This occupies the nar- 

 row 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 south- 

 ward, 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 lam- 

 inations, 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, oc- 

 curring 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. 



