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Collectively these features cover an area about 275 miles in length and having an average width of 65 miles. They extend south from the Ouachita Mountains of Indian Territory to the Colorado River of Texas. (See Pl. X.) There are also a few inliers of the Cretaceous prairies within the Tertiary area of the southwest corner of Arkansas and northwest Texas, and some outliers over the Central Province, as will be further explained.

These prairies are bordered on the north by the east-west line of the Ouachita Mountains of Indian Territory; on the east approximately by the western border of the Atlantic Timber Belt; on the west by the Central Province; and on the south, where they merge into the Rio Grande Plain and Edwards Plateau, arbitrarily by the Colorado River. These borders are sharply defined on all sides, but least regularly on the west.

On the east the Cretaceous prairies are overlapped by the low forest-covered plain of the East-Texas Timber Belt. Although there is no topographic change at their border, the forest country is different from the Cretaceous prairie in all other natural aspects. It is the "

Geologically speaking the Black and Grand prairies are the northern area of the great belts of Cretaceous strata which extend across the State from the Ouachita "Mountains north of Red River to the east front of the Cordilleras of western Texas and northern "Mexico, across the East-Central, Southern, and a portion of the Plateau provinces. The broader areas thus underlain are, in general, distinguished by the calcareous (exceptionally arenaceous) soils, the white or chalk-colored character of the outcropping rocks, and certain peculiarities of vegetation. The whole is almost severed into two great north and south bodies by the Colorado River, which cuts across its most narrow portion. These two are still further differentiated by climatic, structural, and hypsometric differences. Of these two principal subdivisions of the Cretaceous prairies, the one lying south of the Colorado is the larger, embracing about two-thirds the total area. This includes two belts of country-the most eastern of which is the interior margin of the lower-lying Rio Grande Plain and the southern end of the Plateau of the Plains, known east and west of the Pecos as the Edwards and Stockton plateaus. This subdivision, situated largely in the semiarid region, is relatively less adapted to agriculture and a dense population except along its eastern margin. In general it is a pastoral country. The subdivision north of the Colorado includes the Black and Grand prairies with their accompanying belts of cross timbers. This is better situated climatically for agricultural pursuits and presents more extensive bodies of arable land. In a previous paper (Geology of portions of the Edwards Plateau and Rio Grande Plain, etc.: Eighteenth Ann. Rept. U. S. Geol. Survey, Part II, 1898, pp. 193-322) -Mr. Vaughan and the author have discussed the Cretaceous prairies south of the Colorado. The present paper will be restricted to those lying north of that stream.