1893.] NATURAL SCIENCES OF PHILADELPHIA. 331 



The Carboniferous has inherited from Pre-Cretaceous time a base- 

 levelled surface, and where the Cretaceous has only recently been 

 removed the Carboniferous hills are broad and flat-topped. All of 

 the Carboniferous, excepting that close to the larger streams, might 

 be said to have been recently uncovered ; but generally the surface 

 is more or less carved by recent erosion which has destroyed much 

 of the inherited topographic individuality. In San Saba county, 

 however, about midway between the Colorado and San Saba Rivers, 

 there is a long flat-topped divide extending for a distance of twelve 

 miles in an east and west direction, but in places somewhat destroyed 

 by the headwater erosion of some small creeks. This divide is 

 approximately the Pre-Carboniferous land surface, and that it has 

 very recently been uncovered is proved by the fact that these hill- 

 tops are all somewhat littered with pebbles of the Trinity (lower- 

 most Cretaceous) conglomerate — the last remnant of the Cretaceous 

 covering. The flat-topped divide is less than a mile in width. 

 For the conquest of this the small tributaries of Richland Creek are 

 combating with some small creeks tributary directly to the Colorado. 

 The latter have previously had the best in the contest and have 

 pushed the divide back to within three miles of Richland Creek. 



Away from the divide, and nearer to the center of strong erosion, 

 that is near the large streams, the Carboniferous is very much cut 

 up into sharp hills and deep valleys and this feature is now extend- 

 ing into the higher and more remote res-ions. Where denudation 

 of this strong type has been for some time in progress in the Car- 

 boniferous area, two distinct types of topography have arisen, one 

 in sections where the bed rock is of quite uniform character, the 

 other in regions of greater lithologic diversity. The great sandstone 

 area of San Saba county is a typical example of the former ; and 

 here sharply rising hills with deep valleys on the several sides have 

 resulted. In portions less exposed to denudation the surface is 

 gently undulating. 



Where the lithologic character is diverse, long hills running 

 northeast and southwest with steep bluffs on the southeast side 

 result. This is the direct result of the effect of strike and dip, and 

 furnishes an excellent example of monoclinal shifting of divides. 

 In Coleman County the topography is distinctly of this type because 

 here there are alternating beds of soft clays and hard limestones 

 with a gentle dip to the northwest. The hills are step-like, and in 

 travelling across the county one rises by a steep slope from one 



