1893.] NATURAL SCIENCES OF PHILADELPHIA. 345 



times to an old topographic form, not unlike the hilly region of 

 southern New England. The Carboniferous beds were added to 

 this land, by elevation, first as a coastal strip, even before the end of 

 the Carboniferous. A gathering in of shore lines formed a great 

 interior sea, later a completely land-locked dead sea in which Per- 

 mian beds were deposited ; and from the close of the Permian to the 

 beginning of the Cretaceous there was a period of denudation dur- 

 ing which the younger Palaeozoic beds were reduced to base-level 

 and the older mountainous areas still farther degraded. A rapid 

 subsidence lowered the entire region beneath the Cretaceous sea, 

 then at the close of the Cretaceous the land was elevated, possibly 

 by a renewal of the mountain-building forces of the central area. 

 The Rocky Mountain uplift caused an uptilting, raising the land 

 still higher, and adding the Tertiary coastal strip to the Cretaceous. 

 A later uplift added the coastal prairies and a recent slight subsi- 

 dence has completed this record of change, and has given us the 

 Texas region. 



Drainage, established upon the part first raised, has stripped off 

 the Cretaceous covering from a portion of the buried Palaeozoic. 

 The consequent streams reduced this to an approximate base-level, 

 and by the more recent elevations these streams have been rejuve- 

 nated in the older region and have become extended streams in the 

 newer regions, and, in the latter, young consequent streams have 

 also been formed. The entire drainage is consequent, but the streams 

 of the central Palaeozoic are superimposed, and this has given rise to 

 several interesting peculiarities, among others the reaching of a 

 temporary base-level by the Colorado and its tributaries above the 

 Silurian barrier, the consequent degradation of its canon, and the 

 formation of extensive flood plains. The Colorado through this 

 cause has also lost some of its drainage area, and the tributaries, and 

 the Colorado itself, are adjusting themselves to the revealed structure 

 upon which they are superimposed. 



The climate of western Texas has also become more arid and 

 the effect of this is shown by the dessication of lake basins in the 

 Trans-Pecos region, the abandonment of streams in central west 

 Texas and in the growth of flood plains in the Tertiary forested 

 area. 



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