M. S. Tarr — Superimposition of Drainage in Texas. 861 



before it cuts across it; and then the river flows northeast for 

 a mile before resuming its natural direction southeastward. 



Another hitherto unexplained phenomenon in connection 

 with the drainage of Central Texas is that the divide between 

 the Brazos and the Colorado rivers is close to the latter, being 

 in places only six miles distant, while it is fully seventy-five 

 miles from the former. All along the entire course the Colo- 

 rado has almost no drainage area on the east side. The reason 

 for this, so long a puzzle to me, now seems plain — it is the 

 result of accidents, brought about by superposition. The 

 accompanying diagram illustrates this peculiarity of drainage. 



It is probable that both the Brazos and the Colorado origi- 

 nated under practically the same conditions, that is, upon the 

 new Cretaceous land elevated above the sea during the great 

 Tertiary mountain uplift. Their course was plainly chosen 

 with reference to conditions appearing on the surface then 

 existing without regard to what lay below. After cutting 

 through the soft, nearly horizontal Cretaceous rocks, the Colo- 

 rado came upon the buried Paleozoic and encountered not only 

 the Carboniferous for a considerable distance, but also the 

 much harder Silurian with which it has long been struggling. 

 The Brazos, on the other hand, by the accidental choice of a 

 more easterly course avoided these difficulties. To be sure 

 this river in its middle and upper course is superimposed upon 

 the Carboniferous rocks, but the removal of these is a very 

 simple task compared to that the Colorado had imposed upon 

 it. The consequence of this difference between the two rivers 

 is that while the Colorado in Mills county flows at an elevation 



