THE LOWER OR COMANCHE SERIES. 117 



which lies between the Cross Timbers. It is the so-called "mountain coun- 

 try" in western Williamson, Travis, Hays, Comal, and other counties of the 

 southwest, as it extends across the State immediately west of and parallel 

 to the Black Prairie region. South of Austin the edge of this plain, which 

 forms an eastwardly facing escarpment, is known as the Balcones. North 

 of the Brazos the Balcones scarp disappears and the narrow forest region of 

 the Lower Cross Timbers marks the eastern border of the Grand Prairie. 



The east and west railroads between Whitesboro and St. Jo, between Fort 

 Worth and Weatherford, Fort Worth and Decatur, Fort Worth and Gran- 

 bury, Waco and DeLeon, Belton and Brownwood, McNeil and Burnet, and 

 San Antonio and Kerrville, all cross the characteristic Grand Prairie. 



Upon closer study it will be readily seen that it differs in nearly every 

 physical feature from the latter region. In general it is more elevated, its 

 plateaus are nearly level instead of undulating, its valleys more precipitate, 

 and the valleys benched and terraced through the unequal resistance and 

 varying hardness of its alternate stratification. Its soils, except in valleys, 

 are generally shallow and rocky, while their color tends to yellow and choco- 

 late browns instead of black. The chief difference, however, is in the under- 

 lying rocks, which are the foundation of all the above features. These 

 compose a beautiful substructure, whose hundreds of feet of white chalk 

 and yellow magnesian layers of alternating degrees of hardness, gave to the 

 landscape a unique tone and topography not found elsewhere in America. 

 The western border of this region is carved into a rugged scarp, accompanied 

 by outliers of terraced buttes and mesas. 



At the base of this western margin in most places can be found a more or 

 less narrow sandy district of timbered country, like the Upper Cross Timbers 

 of Northern Texas, which is the areal outcrop of the lowest rocks composing 

 the series underlying the Grand Prairie. 



The rocks of the Grand Prairie region are as yet as little known in literature, 

 as its geography, because until recently they have always been confused with 

 those of the Black Prairie region or Upper Cretaceous series. It has been 

 shown,* however, that the Black Prairie is the uppermost of two distinct 

 Cretaceous formations of the United States, and that those underlying the 

 Grand Prairie region constitute a distinct and lower series of rocks, of even 

 greater thickness, to which I have given the name of Comanche series, out 

 of deference to the town of Comanche, where I first studied them. The 

 rocks of the Black and Grand prairies represent two distinct subsidences, 

 between which there was a land epoch of long duration. In general the Co- 



* The first announcement of this series was published by me in the American Journal of 

 Science, January, 1887, p. 75 r See Record of Science, for 1886, Smithsonian Report, 1887-8, 

 p. 220, 



