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laterals and sublaterals are dry except in times of rainfall, when they carry off the surface water.

The stream valleys of the through-flowing rivers and of the main trunks of the prairie rivers in their lower portion are accompanied by old alluvial formations, often of considerable width and extent, as more fully discussed in the geologic part of this paper.

In addition to the types of streams mentioned, the Indian Territory extension of the Red River subdivision of the Cretaceous prairies is crossed from north to south by a number of deep and copious laterals of Red River, which originate in the Ouachita Mountains, including the Little, Kiamitia, Boggy, Blue, and Washita rivers. Furthermore, Red River itself, instead of flowing across the strike of the different belts, as do the Brazos and the Colorado, first follows the Red River fault zone to the east edge of Grayson County and then practically follows the strike of the Eastern Cross Timber belt until it leaves the region.

The various belts of Cretaceous prairies under discussion, both of the Main Texas and of the Red River subdivision, may be broadly grouped into two major types of country of about equal area, differing from each other in certain physical, cultural, and geologic conditions, known as the Black and Grand prairies, each of which is accompanied at its interior border by a belt of forested country, known as the Eastern and Western cross timbers, respectively.

The Black Prairie Country embraces all the area between Red River on the north, the Eastern Cross Timbers, the East-Texas Timber Belt, and the Colorado. The Missouri, Kansas and Texas Railway from Denison to Austin marks approximately the western edge of the Black Prairie belts. The Eastern Cross Timbers clearly define this border as far south as the Brazos, but between the latter river and the Colorado the line of demarcation is geologic rather than topographic. The broadest portion of the Black Prairie is to the north, in the counties between Trinity and Red rivers. Southward the width of the Black Prairie is restricted to its narrowest limits, about 20 miles, along the Colorado River, between Austin and Elgin.

The Black Prairie owes its name to the deep regolith of black calcareous clay soils which covers it. When wet these assume an excessively plastic and tenacious character, which is locally called "black waxy." These soils are the residua of the underlying marls and chalks, or local surficial deposits derived from them, and hence are rich in lime. Complicated chemical changes, probably due to humic acid