THE UPPER OR BLACK PRAIRIE SERIES. 109 



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 in- 

 debted 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. 



G-EOLOG-IC SUBSTRUCTURE OF THE BLACK PRAIRIE REGION. 



The substructure of the Black Prairie region is epitomized in the vertical 

 section given beyond. The eastern margin is the outcrop of the Upper Are- 

 naceous or Glauconitic division, No. 5 of our section. The main Black Prai- 

 rie 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, aggregat- 

 ing 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 strati- 

 graphic 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 se- 

 ries. 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 vol- 

 ume 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 vary- 

 ing 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. 



