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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 economic 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 regions 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.

The Black Prarie 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, unmarked by precipitate canyons. It is transected at intervals by the larger streams, whose deep-cut valleys, together with their side streams, make indentations into the plain, but not sufficient to destroy the characteristic flatness of its wide divides—remnants of the original plain, or topographic marine