THE UPPER OR BLACK PRAIRIE SERIES. 107 



belt of country, outcrop in long narrow belts, sub-parallel to the present ocean 

 outline. Thus it is that as one proceeds inland from the coast he constantly 

 crosses successively lower and lower sheets of these formations. The oldest, 

 or lowest, in a geological sense, of these outcrops, forms the Upper Cross 

 Timbers, those above these make the Grand Prairie, the next sheet forms 

 the Lower Cross Timbers, the next the Black Prairie, etc. Each of these 

 weathers into a characteristic soil, which in its turn is adapted to a peculiar 

 agriculture. Each has its own water conditions and other features of eco- 

 nomic value. Some of these rock sheets, like the Upper Cross Timber 

 country, may be comparatively unfertile in the region of outcrop, yet they 

 may serve to carry the rain which falls upon the thirsty sands far beneath 

 the adjacent country, where by artesian borings it becomes an invaluable 

 source of water supply for a distant and more fertile region. 



The Cretaceous country of Texas, as a whole, like the system of rocks of 

 which the surface is composed, is separable into two great divisions, each of 

 which in turn is subdivided into a number of subdivisions. These two re- 

 gions are known as the Black Prairie and Grand (or Fort Worth) Prairie 

 regions, each of which includes in its western border, north of the Brazos, 

 an elongated strip of timber known as the Lower and Upper Cross Timbers, 

 respectively. 



I. THE UPPER, OR BLACK PRAIRIE, SERIES. 



The Black Prairie Region. — This occupies an elongated area extending 

 the length of the State from Red River to the Rio Grande. The eastern 

 border of the Black Prairie is approximately the southwestern termination 

 of the great Atlantic Timber Belt. The Missouri Pacific and the International 

 railroads from Denison to San Antonio approximately mark the western 

 edge. A little south of the centre, along the Colorado River, from Austin 

 eastward to the Travis County line near Webberville, the Black Prairie is 

 restricted to its narrowest limits. Westward the Black Prairie is succeeded 

 by a region with some superficial resemblance to it, which, on closer study, 

 is found to differ in all essential points. This is the Grand, or Fort Worth, 

 Prairie, or "hard lime rock region," described on page 116. The so-called 

 mountains west of Austin are the remains of the Grand Prairie. In general, 

 the Black Prairie region consists of a level plain, imperceptibly sloping to the 

 southeast, varied only by gentle undulations and deep drainage valleys, un- 

 marked by precipitate canyons. It is transected at intervals by the larger 

 streams, whose deep-cut valleys, together with their side streams, make in- 

 dentations into the plain, but not sufficient to destroy the characteristic flat- 

 ness of its wide divides — remnants of the original plain, or topographic ma- 



