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to within 10 miles of Red River this feature is especially conspicuous in eastern Hill, eastern Johnson, western Dallas, western Collin, and western Grayson counties, to the Red River. This portion of the belt is 180 miles long and averages 10 miles in width.

South of the Brazos this belt is of varying width and is intermittent in character, gradually narrowing and dying out as a distinct topographic feature at the Colorado. The last typical appearance of this subdivision of the Black Prairie north of the Colorado is in the Sixth Ward of Austin. The Red River area begins in eastern Grayson County, and continues down the valley of Red River through Grayson, Fannin, and Lamar counties to the eastern edge of Red River County, where it ends. The southern margin of the Red River belt is approximately marked by the transcontinental branch of the Texas Pacific Railway- from east of Sherman, near Bells, to a few miles east of Paris, Lamar County. The southern border of this Red River belt is not marked by an escarpment of white rock as is the eastern border of the Main Texas belt.

The soil of the Eagle Ford Prairie, like that of the Taylor Prairie, is the residuum of manly clays, and in places is, if possible, even more productive and fertile than the latter. Along the western border of the belts this soil becomes slightly sandy as the substructure grades down into that of the Eastern Cross Timbers. Some of the richest agricultural lands in Texas, notably in Johnson County, are located upon this Eagle Ford division of the Black Prairie region.

The western border of the Main Texas area north of the Brazos and the northern border of the Red River area of the Black Prairie are terminated abruptly by a peculiar narrow ribbon of upland forest known as the Lower or Eastern Cross Timbers. This forest is largely composed of black-jack and post-oak trees, which grow in a deep, sandy soil. This is the most eastern of two similar belts which occur in the East-Central Province, a region of prairie plains, and which extend in a north-south direction, opposite to the prevalent courses of the streams.

The cause of these peculiar ribbons of upland timber between vast stretches of treeless prairie had long been a subject of inquiry before the writer, in 1887, showed that the forest growth was adapted to the geologic formations, the two belts of cross timbers being upon outcrops of certain arenaceous formations at the base of the Upper and Lower Cretaceous series of rocks respectively, the deep permeable "

See Geology and geography of the Cross Timbers of Texas: Am. Jour. Sci., 3d ser., Vol. XXXIII, April, 1887.