22 The American Geologist. Jau. isoo 
ern edge. These have their origin in the springs on the east 
arising along the great fault line at the foot of the Baleonades. 
The Nueces, the San Marcos, Guadalupe, San Gabriel and 
Trinity belong to this class. The last two have completed 
their journey across the plain and now head in the Central 
region. 
Although the Grand Prairie is deeply scored by the tran- 
secting streams, and its western border fantastically carved 
by atmospheric erosion, it is nevertheless a continuous and 
uniform level plain, and a unique geographic unit. The plat- 
eau is treeless and contains many characteristic species which 
justify its separation into a floral province, intermixed with 
species from the arid region. After each season of rainfall 
its ordinary hue of dry-grass brown is succeeded by varied 
flowers of indescribable beauty in their changing colors. The 
soil is usually shallow, and is the residuum of the chalky sub- 
structure, which is of varjdng degrees of induration. Its pre- 
valent color is dark chocolate, which readily distinguishes it 
from other limestone soils in the state. Although differing in 
altitude, topography and structure from the Black Prairie 
region, this section has never been clearly differentiated. 
Owing to the shallowness of the soil and the different condi- 
tions of rainfall, but few small areas of the Grand Prairie are 
adapted to agriculture, while nearly every acre of the Black 
Prairie is utilized. The underlying structure of the Grand 
Prairie is that of the Comanche series," consisting of alterna- 
tions of chalky limestones and marls of varying degrees of 
induration and thickness. These rocks are so much harder 
than the upper Cretaceous sediments underlying the Black 
Prairie region that the region has been appropriately called 
the hard lime-rock region.** This name can not be retained, 
however, owing to the fact that the chalky rocks of the Grand 
Prairie are soft in comparison to the extensive areas of hard 
metamorphosed limestone in the older rock regions. The 
Grand Prairie, in view of these facts, may now be considered 
a plateau, with one exception, everywhere standing above the 
surrounding region. Its eastern edge suddenly bends beneath 
' See "Topography and Geology of the Cross Timbers of Texas," 
Am. Journal of Science, April, 1887, Am. Naturalist, Feb. 1887, Proc. 
Philosophical Society of Washington, Feb. 1887. 
** See "A Description of Future Texas," by Gov. O. M. Roberts, St. 
Louis, 1881. 
