Ot 



26 PROCEEDINGS OF THE ACADEMY OF [1893. 



sediment supply. The slight submergence of the coast line has 

 merely increased the tendency to build flood plains. 



(d) Origin of Forests. — The rolling Tertiary area is a forested 

 area unlike the coastal prairies on the one side and the Cretaceous 

 plain on the other. I am unable to say definitely why this is so. 

 The soil is more sandy and consequently more porous than that of 

 the Cretaceous and this in all probability is the reason why forests 

 exist here. 2 " The coastal strip is entirely too wet for forest growth. 



(e) Rio Grande Enibayment. — I have entirely omitted the con- 

 sideration of the Rio Grande embayment for the reason that I have 

 no personal knowledge respecting it, and indeed little is known 

 about it. It is evident that the lower Rio Grande was estuarine in 

 Tertiary times, but of the extent and nature of this I am unable to 

 speak. 



4 — The Cretaceous Grand Prairie. 



(a) Topographic Description. — Professor R. T. Hill has so fre- 

 quently described this region 21 from various standpoints that I feel 

 called upon to make in large measure merely a brief summary of 

 his results. This great Cretaceous plateau is in reality made up of 

 two plateaus separated by a quite continuous scarp, the Balcones 

 scarp of Hill, in part. That portion of the plateau which lies to the 

 east of this escarpment is called by him the Black Prairie region, 

 while the name Grand Prairie is assigned to that part which lies to 

 the west. The Black Prairie is a gently undulating plateau with 

 varying width, merging to the east into the forested area, to the 

 west ending abruptly at the base of the scarp. The soil is black 

 and sticky when wet, whence its name, the Black Waxy Prairie. 



The scarp is in part a scarp of erosion, but a fault scarp in the 

 vicinity of Austin, and to the southwest as far as the Rio Grande. 

 It forms a marked topographic feature, a rather abrupt wall rising 

 to a height of two or three hundred feet. The fact that the fault is 

 so plainly indicated in the topography seems to point to a recent 

 origin, though this is advanced only tentatively. A number of large 

 springs occur along the fault line. 



20 Penrose states (First Ann. Rept., Texas Geol. Survey, 1880, p. 9) that the 

 early settlers found this region much less densely timbered than at present. This 

 was pmicularly the case on the uplands. He suggests as an explanation of this 

 the habit which the Indians had of burning the tall grass in their search for 

 game. 



21 See particularly, Am. Geol., Vol. V, 1890, pp. 9-29- 



