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cities of Texas are located, including Paris, Sherman, McKinney, Dallas, Waxahachie, Waco, Austin, New Braunfels, and San Antonio, all of which are dependent upon the agricultural products of the adjacent black prairies.

West of the "white rock " or chalky division, and generally at a slightly lower altitude, occupying a valley across the State, is a second narrow strip of black clayey land, of a nature similar to that of the main Black Waxy area, and likewise accompanied by hog-wallows. This is the country east of Denton and Whitesboro, in the Mountain Creek district of Dallas County, and along the line of the Missouri Pacific Railway, from Alvarado to Waco. The Sixth Ward of Austin is located upon these clays, and to them it is indebted for its characteristic black mud.

The Lower Cross Timbers—a narrow belt of forest country extending from the Red to the Brazos rivers—represent the westernmost strip of the Black Prairie region, and belong to it geologically, as will presently be shown.

The substructure of the Black Prairie region is epitomized in the vertical section given beyond. The eastern margin is the outcrop of the Upper Arenaceous or Glauconitic division, No. 5 of our section. The main Black Prairie division, the surface of the marine clays, called the Ponderosa marls, No. 4. The white rock division is the outcrop of the Austin-Dallas chalk, aggregating about six hundred feet in thickness, No. 3. The minor Black Prairie, No. 2 of our section, is also composed of clays like those of the main division, and hence the similarity of the topography. Collectively with the Lower Cross Timbers, No. 1, these rock sheets, between which there is no stratigraphic break, represent the sediment deposited in the oceanic waters during a long continued subsidence, geologically known as the Upper Cretaceous period, for which collectively we have chosen the name of Black Prairie series. This Upper Cretaceous series has five conspicuous stratigraphic and lithologic divisions, which approximately correspond with the topographic divisions of the Black Prairie above mentioned. These will now be described in ascending order, beginning with the lowest beds of the series.

The continuation of the Upper Series has been well studied in the Northwestern States by the late Prof. F. B. Meek, the geologist who has contributed the most that is known concerning the Cretaceous formations of that country. His descriptions are found in a volume entitled "A Report of the Invertrebrate Cretaceous and Tertiary Fossils of the Upper Missouri Country." By F. B. Meek, Washington, 1876. The series of Texas, while varying in many specific details from the section therein described, is so generically allied that it is evident those variations are merely local differences in the same great subsidence, and that nothing but long and arduous labor, yet to be performed, will reveal their exact affinities.