Geographic Features of Texas. — Hill. 21 
reached, as at the valley of the Colorado near Austin. The 
entire length of this scarp with its principal meanderings 
across the state of Texas can be little less than 1,000 miles. 
Accompanying the scarp are innumerable circular flat-topped 
outliers of the main plateau which have been completely 
separated from it by this fantastic atmospheric erosion, and 
which fringe the margin throughout its extent. These are 
typical "buttes," the level mesas or tops of which are capped 
with the identical stratum and geological horizon which sur- 
mounts the main plateau of the Grand Prairie. In symmetry 
of proportion and horizontality of the composing strata ; 
and in clearness of every detail of structure, there are no 
grander or more unique examples of atmospheric erosion in 
our country. Often these buttes are forty to one hundred 
miles from the main area of the Grand Prairie, and are inval- 
uable landmarks in tracing the history of its degradation. In 
a previous paper I gave the central palseozoic area the name of 
the Butte or denuded region, from the distribution of these 
features over it. Among the most characteristic and typical 
of these buttes are Comanche peak. Hood county, Johnson's 
peak, Round mountain, Santa Annamountain, Church moun- 
tain, Castle mountains, "Pilot Knob," Williamson county, 
the Two Star mountain in Hamilton and Comanche counties, 
Post mountain, Burnett county.*^ 
The altitude of the Grand Prairie gradually increases from 
1,000 feet at its eastern edge to 2,000 feet along its westermost 
border, where it is covered by the Staked Plains formation. 
There is little or no disturbance throughout its area. 
The major rivers have cut deeply through the Grand 
Prairie and their valleys present the same atmospheric 
terracing of the western border.^ In places these river valleys 
assume the aspect of vertical canons, as in the Colorado, 
Pecos, Rio Grande and Red rivers. The depth of these val- 
leys below the level of the plain increases southwestward from 
200 to 700 feet. Another set of rivers are wearing their way 
by backward erosion across the Grand Prairie from its east- 
* In a previous list, by slip of pen, the writer included Packsaddle 
mountain, Burnett county, in this category. This is entirely a distinct 
type of geologic and geographic structure. See Walcott, Am. Jour. 
Sci., 1885. 
' The drainage of the entire region will be discussed in the second 
part of this paper. 
