332 PROCEEDINGS OF THE ACADEMY OF [1893. 



plain to another, the face of the slope being clay, littered with 

 bowlders dropped down from the overlying limestone stratum which 

 forms the floor of the plain above. If these beds were horizontal 

 there would be here a butte and mesa topography instead of a step- 

 topography. 



Almost completely surrounding this area is the irregular Creta- 

 ceous escarpment with its buttes and mesas, and, upon the Carboni- 

 ferous itself, often many miles from this scarp, are degraded rem- 

 nants of Cretaceous in isolated buttes, or small mesas, or in degraded 

 patches the last remnants of former buttes. These buttes are cap- 

 ped by a hard limestone and once this is cut through and destroyed 

 the complete destruction of the Cretaceous is easy, since the major 

 part of the beds beneath are soft and cpiite incoherent. These 

 buttes and mesas are here because they are divides between streams, 

 and, as at the divide the development of the stream is least rapid, so 

 here at the divides the removal of the Cretaceous covering has been 

 least complete. 



(b) Peculiarities of Creek Erosion. — The stream valleys of the 

 central Texas region furnish interesting peculiarities, the result, in 

 large measure, of the peculiar rainfall conditions. During the 

 winter which I spent there, from November to February inclusive, 

 there was not a single rain that raised the volume of the stream per- 

 ceptibly; but in the spring and summer the country is liable to 

 excessive rains and the streams to excessive floods. For the greater 

 part of the year little erosive work is done but occasional floods do 

 much more in a few days than is done during the entire year in a 

 rainy country. The violence of this erosion is everywhere shown in 

 the creeks both by the character of the bars and other deposits and, 

 in the larger streams, by the presence of rafted logs well up in the 

 tops of the pecan trees. On the Colorado I have seen large logs 

 lodged- in the trees fully fifty feet above the low water surface of the 

 river. 



Creek erosion under these circumstances differs in some minor 

 respects in different parts of the region, particularly in the sandstone 

 and limestone regions. In the Silurian, for instance, where the 

 rocks are chiefly marble, the creek valleys are deep, precipitous and 

 picturesque, with bluffs and isolated columns of limestone, and, over 

 all, a beautiful mat of moss and ferns. Some of these valleys are 

 fully seventy-five feet deep, yet, during the greater part of the year 

 no water flows in these creeks, or, at the best, only a small, clear 

 stream slowly trickles down. The water stands in pools and slowly 



