1893.] NATURAL SCIENCES OF PHILADELPHIA. 315 



settlers, in the absence of more striking elevations. West of the Pe- 

 cos the country is truly mountainous. 



(i) Drainage. — The drainage naturally falls into two classes, the 

 western, including the Eio Grande and Rio Pecos which receive 

 their water supply from the mountains of New Mexico, and whose 

 Texas tributaries are withered channels favored with water only at 

 rare intervals; and secondly, those of the east, including the Color- 

 ado and Brazos, whose headwaters are withered channels in the Llano 

 Estacado and the Permian plain, and whose lower tributaries, flow- 

 ing in a humid or sub-humid climate, are the source of constant sup- 



2. — General Climatic Conditions. 



Texas is humid in the east, excessively so near the Gulf Coast. 

 The climate in this region, for instance near Galveston, is semi- 

 tropical. Proceeding inland, with increasing elevation comes more 

 temperate conditions. Snow falls, and frost occurs in the greater 

 part of the State, this being due to the cold north winds, " the 

 northers" which are not uncommon during the winter. Even as 

 near the coast as Austin the decrease in rainfall is noticeable, and 

 it is necessary to travel inland from here but a hundred miles or so 

 to reach a subhumid belt. The town of Abilene lies in this sub- 

 humid region, and to the west of this the climate becomes rapidly 

 more arid until truly arid conditions are met with, and with this 

 change there is a change in the character of vegetation. The Trans- 

 Pecos region is arid, so is the Llano Estacado and the western 

 Permian. More than a third of the State is arid or subhumid. 



3. — General Geologic and Geographic Development. 



(a) Pre- Carboniferous Land. — This statement is necessarily 

 generalized and, with the present imperfect state of our knowledge 

 of the geology of large parts of Texas, it may in some details be 

 incomplete. I shall attempt nothing more than seems warranted 

 by the facts at present at hand; but these are sufficient for our 

 present purpose which is to trace the general development of the 

 Texas region. 



The oldest part of Texas is the Central Denuded Area, and this 

 has had a very complicated history. There is a Pre-Cambrian area, 

 referred by Mr. Walcott to lower Cambrian 3 and later to Algonkian* 



3 Am. J. Sci., XXVIII, 1884, pp. 431-33. 



4 Bull. U. S. Geol. Survey, No. 30, 1886, pp. 57-64. 



