CAUSES DETERMINING FOREST DISTRIBUTION. 11 



(9) The Stockton Plateau is simply the continuation beyond the 

 Pecos of the Cretaceous area of which the Edwards Plateau is a 

 province. This is not a timbered country at all, except in canyonsand 

 on some rough breaks, where mountain cedar occurs. The arid climate 

 and the compact adobe soils or rocky, debris-covered slopes are too 

 unfavorable for timber growth. The Stockton Plateau extends to the 

 eastern point of the Cordilleran Region. 



(10) The Cordilleran Region comprises a vast area of over 30,000 

 square miles (including the Stockton Plateau), being part of the South 

 Rocky Mountain Plateau. There are large tracts of high grass plains 

 and scattered mountain masses, with intervening undrained pockets 

 ("bolson flats"). The altitude in several instances is sufficient to 

 increase the rainfall to nearly 20 inches, where the normal for that 

 meridian is under 15. This and the fractured condition of the strata 

 render these mountains capable of sustaining rather heavy timber 

 of the Southern Rocky Mountains type. Especially noteworthy is 

 the timber growth in the Guadalupe, Davis, and Chisos mountains. 



CAUSES DETERMINING FOREST DISTRIBUTION IN TEXAS. 



The principal natural influences Avhich have determined the character 

 and extent of forests in Texas are rainfall, nature of the soil and rock, 

 temperature, sunlight, and winds. Of these the first two are by far 

 the most important. 



KAINFALL. 



The rainfall of Texas decreases progressively from east to west. A 

 map constructed to indicate the annual precipitation by 5-inch divisions 

 would show a series of zones extending in a general north and south 

 direction from the Sabine to the Pecos. Beyond the latter river the 

 elevated mountain masses probably bring up the annual mean of rain- 

 fall, but at the westernmost boundary this average scarcely reaches 

 10 inches. (See fig. 2.) The limits of the several rainfall zones are 

 approximately marked by the meridians of longitude. Thus, the 

 ninct3 T -fifth meridian about marks the western limit of rainfall exceed- 

 ing 50 inches; the ninety-sixth, of 15 inches; the ninety-seventh, of 10 

 inches; and so on to the one hundred and second meridian, where the 

 average annual rainfall has decreased to 15 inches. 



Corresponding in a general way with these zones of rainfall, there 

 is a series of zones of forests of different types. In the eastern region, 

 having a rainfall in excess of 15 inches, are found the swamp and 

 bayou forests of cypress, tupclo, water oak, swamp hickory, and other 

 water-loving species; in slightly better drained localities, the black- 

 gum, cottonwood, sycamore, beech, birch, and Spanish oak, and after 

 them red oak, white oaks, walnut, pecan, magnolia, holly, and the 



