1893.] NATURAL SCIENCES OF PHILADELPHIA. 323 



table-land one hundred to seven hundred feet above the sea level, 

 sloping gradually to the southeast and emptying its waters in the 

 same direction into the Gulf of Mexico. Since its elevation it has 

 undergone great erosion and is still being denuded at a tremendous 



do n 



rate. The strata are all composed of sands and clays, and succumb 

 very readily to the eroding action of atmospheric agencies. The 

 result is that all that is left of the once level surface of this table- 

 land are a few flat-topped hills and ridges. The highest points, 

 locally called mountains, are buttes or mesas having their summits 

 capped by an almost horizontal bed of iron ore or sandstone, and to 

 this covering they owe their existence. A large part of this area is 

 a heavily timbered region, and marks the southwestern terminus of 

 the great Atlantic timber belt, extending from the Arctic regions 

 continuously along the coast of the Atlantic Ocean and the Gulf of 

 Mexico until it finally disappears in the mesquite and cactus prai- 

 ries between the Colorado River and the Rio Grande. 



(b) Mature, Consequent and Extended Drainage. — The conditions 

 described above, under the Quaternary, in all probability are repe- 

 titions of similar conditions in the Tertiary area in earlier times. 

 This area is composed of horizontal and cross-bedded strata, now 

 partly consolidated clays and sands, which undoubtedly formed a 

 coastal strip at the close of the late Tertiary uplift. Streams which 

 had developed on the Cretaceous plains and had uncovered portions 

 of the buried Palaeozoic, extended across the coastal strip of Ter- 

 tiary strata, and between them young consequent streams began 

 their development. The Brazos, Trinity and Colorado were here, 

 as in the present coastal strip, extended rivers. The then young 

 consequent streams of the older plain, such as the Neches and Ange- 

 lina, are now extended across the Quaternary plain. 



The elevation of the old coastal strip was greater than that of the 

 more recent one for it now rises on its inner margin, in some places, 

 to an elevation of seven hundred feet above sea level and there is 

 evidence that it extended farther inland at one time. The topo- 

 graphic development of this region of Tertiary strata has proceeded 

 to the point of producing a rolling country of low and gently slop- 

 ing hills with broad valleys. In other words it has proceeded as 

 far as early maturity. It may seem at first thought that this is at 

 variance with the description of the Cretaceous area which follows. 

 Here the streams are topographically young, yet this land is vastly 

 older. This apparent anomaly is due to three causes: first, the fact 



