NATURAL DIVISIONS OF THE STATE. 9 



Prairie. The inner boundary runs approximately from Seguin (an 

 arbitrary western limit) through Elgin, Cameron, Corsicana, and 

 Sulphur Springs to the Red River. The open soil structure and heavy 

 rainfall— especially eastward— make this a natural forest area, and in 

 fact it is covered by the southwestward continuation of the Atlantic 

 Forest Belt. 



(4) The Rio Grande Plain, of over 20,000 square miles, in a narrow 

 sense would include only the, Rio Grande embay ment/' ; but as here 

 employed includes the western portions of the three preceding divi- 

 sions, from about 97° 30' to the Rio Grande, Its interior boundary is 

 the escarpment of the Great Plains Region, whose southern province 

 is the Edwards Plateau. The soils of the Rio Grande Plain are largely 



a Hill, Physical Geography of the Texas Region, p. 2. 



