330 PROCEEDINGS OF THE ACADEMY OF [1893. 



Whatever the former condition of this area, the drainage is at 

 present in a state of youth or early adolescence. Larger rivers, 

 such as the Colorado, flow in deep canons, sometimes with almost 

 vertical walls and with small flood plains, sometimes with none. 

 The river flow is rapid, and the channel is at times obstructed with 

 rapids. Into these larger valleys side streams empty through nar- 

 row canons with rapid slope, and the divides have not been 

 reduced to a state of tolerable permanency but often remain gently 

 rounded broad topped areas. This is particularly true of that part 

 of the prairie immediately to the west of the Balcones scarp, — 

 another suggestion of the recency of this fault. In the denuded 

 Paheozoic area other conditions exist, as I shall attempt to show 

 shortly ; in the lower lying Tertiary area to the east, the beds being 

 softer, the elevation less, and the streams overburdened, the topo- 

 graphy, as has been already stated, is less abrupt, less distinctly 

 young, and the streams, though rapid, are not carving their chan- 

 nels deeper but are building them higher in order to establish a pro- 

 file of equilibrium, a slope down which the sediment load can all be 

 carried. 



5. — Central Palaeozoic Denuded Area. 



(a) Topographic Description. — In this region there are three dis- 

 tinct classes of topographic relief, the result of erosion upon three 

 different classes of rock, the Silurian, Carboniferous and Cretaceous. 

 The Silurian, with its inherited rugged topography of pre-Cretaceous 

 age, has, since the removal of the Cretaceous, been exposed to quite 

 rapid denudation, because of its greater elevation. Consequently 

 the Silurian area is a region of marked topographic diversity, and 

 this feature tends to be preserved because of the great durability of 

 the rock. Not only is this varied outline brought about by eleva- 

 tion, inherited relief and the hardness of the rock, but also because 

 of the complicated stratigraphy which in a measure guides the ero- 

 sion. 



One can, in a measure, tell the difference between the Silurian 

 and Carboniferous by the topographic outline, for the hills of the 

 latter are more broadly flat-topped, though by no means as strik- 

 ingly so as the Cretaceous. The butte and mesa type of erosion so 

 characteristic of the Cretaceous in this region is so entirely absent 

 from the Carboniferous that generally no difficulty is experienced 

 in discriminating between Cretaceous and Carboniferous by the 

 topographic features alone. 



