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the western border of the Grand Prairie and at widely disconnected intervals over the Central Province, as described later. This limestone stratum caps also the southern (Edwards) portion of the Plateau of the Plains, into which the Grand Prairie merges south of the Colorado.

The Central Province in its entirety includes the vast region between the Plateau of the Plains on the west and south and the Missouri, Ozark, and Grand Prairie countries on the east. It consists of two great divisions separated by the ranges of the Ouachita Mountain system-the northern or Kansan, and the southern or Texan. The northern area comprises various belts or prairie plains in Oklahoma, Indian Territory, and southern Kansas, closely allied in origin and nature and often continuous with those of the southern area. In this paper the latter only can be considered. This is bordered on all sides except the north by receding escarpments of erosion. On the west and south these are the eastern breaks of the Plateau of the Plains; on the east they are the western scarps of the Grand Prairie.

Callahan Divide.-'The highest relief features of the Central Province in Texas are the numerous flat-topped, circular, remnantal hills (mesa buttes) which are outliers of the Plateau of the Plains and of the Grand Prairie. (See Pl. IV.) These are capped by the bard Edwards limestone. Upon some of the most western of these are found gravel and other remnants of the Plateau of the Plains. The altitude of these mesas averages 500 feet above the principal drainways, and about 250 feet above the highest of the several intervening plains.

The remnantal mesas occur at widely separated intervals over that portion of the Central Province which is south of the northern head-water forks of the Brazos. Although widely distributed, they form less than 10 per cent of the total area of the Central Province. The principal group, which may be termed the Callahan Divide, occupies the watersheds of the Brazos and Colorado, lying approximately along the thirty-first parallel, and extends, like thickly set bridge piers, from the western border of the Grand Prairie west to the Plateau of the Plains, through Comanche, Brown, Eastland, Callahan, Coleman, Taylor, Runnels, and Mitchell counties. North and south of this line, separated from one another by great areas of lower-lying plain, there are many similar isolated remnants, such as Double Mountain, in Stonewall County, and Santa Anna Mountain, in Coleman County.

Collectively the summits, escarpments, and plateaus thus composed of the horizontal Edwards limestone represent a wide topographic level which once extended over nearly the entire Coastward Slope, from the mountain front to the eastern edge of the Grand Prairie and