1 REPORT OF THE STATE GEOLOGIST. 



THE COMANCHE SERIES. 



The Lower Division of the great Cretaceous System, called the Co- 

 manche Series, is, like the Upper, principally a prairie country, and is 

 designated the Grand Prairie. Its eastern and southern boundary, as 

 it now appears above the surface, is the last that shows any marked 

 parallelism with the Gulf coast, and may mark approximately the an- 

 cient shore line of the sea in which the deposits already described were 

 laid down. It is also the last of the upper formations in which we find 

 the dip to the southeast prevailing. From Austin to the Rio Grande 

 this boundary is marked by the range of hills (Balcones) which rise 

 from one to two hundred feet above the undulating country at their 

 feet. Between the Brazos and Red River the line is the western edge 

 of the Lower Cross Timbers already described. The rocks of this se- 

 ries once covered the greater part of the area north and west of this 

 line, and even now it is of wide extent, although we have only what is 

 left after ages of erosion. This series consists of the Trinity sands, the 

 Fredericksburg limestones and chalks, the Washita limestones, chalky 

 in the east but more sandy towards the west, and the overlying clays. 

 All of these divisions are represented in regular descending order, but 

 to the west erosion has removed the softer upper strata in many places, 

 leaving the hard limestone which is the highest stratum of the Freder- 

 icksburg division as the capping stone of the table lands, plateaus, and 

 buttes, which owe their present existence to this protecting mantle. The 

 topography of the Grand Prairie Region is highly characteristic. In 

 places it stretches away in extended plateaus, which are comparatively 

 level and generally treeless, cut by a few streams, whose valleys are 

 often bounded by steep cliffs. At others long tongues or even buttes 

 are all that are now left to mark the decay which it has undergone. 



THE WASHITA DIVISION. 



The Washita, which is the upper division of the Comanche Series, 

 and therefore directly underlies the lowest beds of the Black Prairie 

 Series, is composed in its upper part of alternations of clays and im- 

 pure limestones, the fossils of which show clearly its littoral character.' 

 This part is called the Denison Beds. Below these come other clays, 

 which, like the Ponderosa Marls, yield a rich black soil, and also like 

 them are named from a fossil oyster — the Arietina (little ram's horn) 

 clays. The areal extent of these Arietina clays is not great in Central 



