Geographic Features of Texas. — Hill. 15 
other prairies. The soil is very black and sticky when wet 
and has the tenacity of wax, from which fact is derived its 
name. It is the residuum of the thousand feet of chalky clays 
(marls, in the English sense). The black color, which is 
superficial, is caused by the reaction between the excess of 
lime and the roots and debris of the surface vegetation. The 
average altitude of this prairie is from 500 to 800 feet through- 
out its extent and constitutes a uniform bench or plain slop- 
ing gently to the southeast. As above stated there is no per- 
ceptible scarp or other topographic line of demarcation 
between its eastern border and the timbered region, except the 
cessation of timber growth, but its western border is every- 
where most conspicuous from strong escarpments. The 
nature and origin of these scarps, however, are radically differ- 
ent north and south of the center of the state. From the Red 
river to the Colorado the western border is marked by the 
scarp of white chalky rock (the westernmost outcrop of the 
Austin Dallas chalk, Niobrara) surmounting blue clay 
shales (Eagle Ford shales). This escarpment is continuous 
except where cut by rivers, from Austin to Denison, 200 miles 
above the depression occupied by the Cross Timbers to the 
west. In common with every other inequality of the earth's 
surface in Texas, this scarp is locally called "mountains."^ 
The chalk is likewise known as "white rock," and hence I pro- 
pose for it the name of the White Rock scarp. It can be dis- 
tinguished even upon ordinary maps by the small fringe work 
of minor streams which drain its summit to the eastward, and 
the streams which are deflected along the strike of its base. 
The chalk or white rock forming the summit of this scarp is 
the immediate geologic antecedent of the marly clays under- 
lying the main black waxy area, and I classify it as a sub- 
division of the black prairie region. It marks the western 
border of that region throughout its extent, but seldom has an 
areal outcrop of more than a mile or two. Its topography is 
slightly different from the main prairie, in that it is a little 
more undulating and usually covered with a sparse growth of 
handsome live oaks. In general appearance it is always con- 
trasted with the Downs of England by those who have seen 
both regions. Immediately beneath the chalk there is another 
horizon of clays (the Eagle Ford shales) which, especially 
north of the Colorado, makes another black waxy strip of a 
