16 The American Geologist Jau- is^o 
few miles in width. The sixth ward of Austin is typical of this 
subdivision of the Black Prairie. This is especially conspic- 
uous in Hill, Dallas and adjoining counties. It is usually flat, 
or possesses undulations of the diminutive character known 
as hog-wallows, caused by the shrinkage, cracking and erosion 
of the calcareous marls under the alternating conditions of 
extremes of moisture and drouth. For this subdivision which 
has not hitherto been differentiated, I propose the name of the 
Minor Black Waxy, or Eagle Ford prairies. 
This escarpment and its accompanying valley are like those 
in the middle districts in Britain running from Yorkshire 
towards Dorsetshire, between the Chalk range of the North 
Downs and the parallel ridge of the Lower Greensands, and 
described by Phillips^ as valleys of stratification, "which are 
chiefly alternations of clays and limestone resting successively 
upon each other and tilted up at an angle so that the several 
beds dip to the southeast," exactly as in the case of White 
Rock scarp, the Washita or Fort Worth limestone represent- 
ing the harder over and underlying limestones, and the Eagle 
Ford clays and the Lower Cross Timber sands the softer in- 
cluded layers. "The clay being formed of impalpable mud, has 
its surface particles loosened year by year under the influence 
of atmospheric agencies, etc. Thus, in time, the clay becomes 
hollowed out into a valley more or less deep and broad, while 
the limestone, which is less easily broken up by the frost and 
has few loose particles Avhich can be carried away by 
water and is only slowly dissolved by the carbonic acid, 
* * * * wastes less rapidly * * * * Qxxdi 
hence stands up as a terrace margining the valley hol- 
lowed out in the clay below it." And such is the origin of 
the White Rock scarp and the lower Cross Timber valley. It 
should also be remembered that such valleys and scarps of 
stratification are receding in the direction of the inclination of 
the rock sheets, and later a question will arise, how far has 
the White Rock scarp in the past traveled eastward across the 
state of Texas? In other words, what was the former extent 
of the white rock? Other conspicuous scarps of stratification 
in Texas are those of the Staked Plains and Grand Prairie 
to be mentioned later. 
The Balcones next to be described, upon the other hand, 
^Physical (ieography and Paheontology, Seeley, p. . 
