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downs of England, and in others to the hills of France than to other regions. So far as the United States is concerned, this country is unique, without analogy or counterpart. In topographic, economic, and cultural aspects it is a distinct geographic region.
BELTS OF COUNTRY.
Not all the Cretaceous prairies are alike in physiographic, geologic, vegetal, and cultural features, but they are divisible into several distinct and strongly contrasting types of country arranged in irregular parallel belts. These belts of country are due to the varying composition and degrees of hardness of the Cretaceous strata upon which they are developed and which are minutely described on later pages.
The belts of prairie extend in an approximately north-south direction for 2.50 miles, from the Colorado to the vicinity of Red River. Here, in southern Indian Territory, and in Grayson and Fannin counties, Texas, they change in direction from north-south to due east-west. This radical change takes place at the point where the prairies cross a fault line, elsewhere described as the Red River fault zone, which extends in a northwest southeast direction from the vicinity of Marietta, Indian Territory, to near Cooper, in Delta County (see geologic map, Pl. LXVI, A). The Black and Grand prairies, with the belts of Cross Timbers, are separated by this fault line into two great subdistricts, which may be termed the Main Texas and the Red River areas, and the ends of the two subdivisions meet along this line like the bands of molding at the corner of a picture frame.
Collectively the Cretaceous prairies consist of gently tilted dip plains, those of the Main Texas area comprising a portion of the continental slope from the Rocky Mountains to the sea, which has an average gradient of about 40 feet to the mile. This gradient varies in places, as will be shown. The plains of the group vary in altitude from 1,500 feet above sea level at the western edge to about 400 feet on the eastern edge. The dip plains of the Red River area incline southward from the Ouachita Mountains. Because of the difference in slope, strike, and direction between these two subareas, the writer frequently throughout this paper uses the general term "coastward" instead of east in discussing the directions of dip and slope. When the wide extent of the prairies as a whole is considered, the general slope is remarkably uniform-more so than in any other large area in our country ; but even in this slope there are certain important variations, which are elsewhere discussed.
From the profiles on Pl. LXVII it will be seen that a slight change in rate of slope apparently takes place in the Main Texas area along a north-south line from Red River to the Colorado via Bowie, Decatur, and Weatherford, the rate of slope increasing east of the line. This