R. S. Tarr — Topographic Feature of Texas. 809 



origin have cut valleys in places of structural weakness. This 

 te well Bbown in the case of the Waldrip coal beds, a Beries of 



:lv eroded strata, which, for a distance of forty miles, arc 

 marked by the presence of small streams ami valleys. Similar 

 instance- are numerous. 



In the erosion, accomplished under such complicated circum- 

 stances of accident and geological peculiarity a series of distinct 

 topographic outlines has been evolved. Tin 1 Silurian, a region 

 of hard rocks, complicated by innumerable folds and probably 

 also retaining a well defined topographic outline evolved in 

 pro-Cretaceous times, is now a hilly country. A portion of the 

 Lower Carboniferous is involved in this region of strong relief 

 but practically the border of the Silurian marks the beginning 

 of the highlands. Once partially rid of its Cretaceous cover, 

 this area would necessarily become a region of strong erosion 

 and this explains the almost complete removal of the Creta- 

 ceous from that surface. 



The Carboniferous on the other hand is a region of soft rock 

 and gentle dip. and this becomes rapidly degraded. The Car- 

 boniferous surface, as shown by the position of the Lower 

 Cretaceous upon it, was when the present drainage systems 

 reached it, considerably below the Silurian ; and using this 

 -a me datum plane, it is evident that very little of the Carbon- 

 iferous has been since removed. One chief reason for this is 

 that on reaching the Paleozoic the down-cutting of the Col- 

 orado has been indefinitely delayed by the hard Silurian and 

 other rocks which it is now engaged in removing. The San 

 Saba river, for instance, for a distance of fifteen miles above its 

 mouth, liows in a flood plain because of the impossibility of erod- 

 ing deeper until after the Colorado has removed the barriers 

 below. The small streams tributary to the San Saba are sim- 

 ilarly affected, and the same is true of other tributaries of the 

 Colorado, as for instance the Pecan Bayou. The chief erosion 

 that is being done in the Carboniferous of the Colorado river 

 is in the headwaters of the small streams, and the material thus 

 removed is deposited lower down in flood plains to be removed 

 when erosion at that point can proceed. A local base-levelling 

 is thus in process relative to the dammed-back Colorado. This 

 process is aided by the peculiarity of rainfall. During the 



ater part of the year the small creeks either contain no 

 water at all or so little that no work of erosion is done. 

 During some months tumultuous torrents rush down the stream 

 valleys covering the previously dry bed with water twenty 

 feet deep and thirty feet wide. These torrents subside very 

 quickly, but during their brief existence do an immense work 

 of erosion. The Colorado river has a wide channel for or- 

 dinary periods of flow ; but the high-water channel is two or 



