108 CRETACEOUS ROCKS OF TEXAS. 



rine base level, which has not been completely scored by its still youthful 

 drainage system. The altitude of the plain is between 600 and 800 feet. 

 The surface of most of the Black Prairie region is a deep black clay soil, 

 which when wet becomes excessively tenacious, from which fact it is locally 

 called " black waxy." It in general is the residuum of the underlying clays, 

 and contains an excess of lime, which, acting upon the vegetation by com- 

 plicated chemical changes, causes the black color. It is exceedingly produc- 

 tive, and nearly every foot of its area is susceptible of a high state of culti- 

 vation, constituting one of the largest continuous agricultural regions in the 

 United States. Large crops of cotton, corn, and minor crops are annually 

 raised upon its fertile lands, and if there were facilities for proper transpor- 

 tation, it would soon be one of the leading districts of our country. 



The Black Prairie is subdivided longitudinally into four parallel strips of 

 country, differing slightly, and distinguishable only by slight differences in 

 topography and in the underlying rocks. The easternmost of these divisions 

 north of the Brazos River is distinguishable by the occurrence of sand in its 

 black soil, and occasionally purer belts of sand. Between the Brazos and 

 Colorado rivers, however, the sand is hardly perceptible. Immediately in- 

 terior of this is located the largest and most characteristic area, which is 

 marked by the stiffest of the black waxy calcareous clay soils. Upon dig- 

 ging throughout this area, the substructure is found to consist of a light blue 

 or yellow calcareous clay, called by the residents "soapstone" and " joint 

 clay," from its jointed and laminated structure. The surface, especially of 

 the high undrained divides, is also accompanied in many places by minute 

 depressions known as hog- wallows, which are produced by the drying, crack- 

 ing, and disintegrating character of these excessively calcareous clays in poorly 

 drained places. This, the main portion of the Black Prairie, constitutes fully 

 two-thirds of its total area. The cities of G-reenville, Terrell, Oorsicana, and 

 Kaufman are situated near the border of the sandy and black waxy strips. 

 Manor, Clarksville, Cooper, Taylor, and Temple are all situated in the main 

 black waxy belt. 



An outcrop of "white rock" or chalky country, forming a narrow strip av- 

 eraging two miles in width, from Red River to the Rio Grande, succeeds on 

 the west the main Black Waxy strip. This chalk region is marked by a to- 

 pography more rounded and deeper incised, but still void of the sharper lines 

 of stratification that characterize the Grand Prairie region. It is usually 

 treeless, but occasionally marked by clumps of handsome evergreens and 

 oaks. The western edge of this chalky region, as seen at Oak Cliffs, near 

 Dallas, at Sherman, Hillsboro, and other places, usually ends in an escarpment 

 overlooking a valley containing the minor Black Prairie and Lower Cross 

 Timber strips. It is upon this chalk that the most prosperous of the interior 



