R. T. Hill—Cross Timbers in Northern Texas. 297 
is common to the other areas, and which was due to the rising 
of the Rocky Mountain axes. The denudation resulting from 
the subaerial erosion has been very great, the whole of the geo- 
logic series, from the recent to far down into the Carboniferous, 
having been removed from its center or place of greatest denu- 
dation. In traveling in either direction from the center of this 
region, east or west, and sometimes south, the geologic forma- 
tions will be passed in ascending series, from the Carboniferous 
into the Tertiaries. The denudation is progressing now at a 
rate probably greater than in any time past, and the geologic 
features are being constantly modified by it. The Tertiary, if 
it ever existed there,* has already been removed from this vast 
area, and the Cretaceous is rapidly yielding—in places entirely 
gone. This fact being true, it is evident that the Cross Timbers 
cannot represent any post-Cretaceous sediments. 
Going from east to west along the line of the Texas Pacific 
Railway, as I have represented upon the accompanying profile 
and geologic section—from the area of the least denudation to 
that of the greatest—let us examine more closely into the rela- 
tions of the strata, and their connection with the present topo- 
graphic features. The second topographic area, alter passing 
west of the first, or Coast Plain (with which I shall not deal in 
this paper), is locally known as the ‘“ black waxy,” or “ white 
rock” prairie, two names derived respectively from the char- 
acter of the soil and of the underlying rock. It has such topo- 
graphic, geologic, and paleontologic resemblance to the ‘“ Rotten 
limestone ” of the other Gulf States, as well as direct geographic 
continuity, that there is no doubt but that it, as well as its 
immediately adjacent strata, is a continuation of the same 
formations from those States. The underlying rock (6)+ is a 
soft, almost structureless, slightly foraminiferous limestone 
rich in magnesium, which, on dry exposure, hardens and 
bleaches to a yellow-white, and on contact with moisture 
readily decomposes and crumbles. Natural sections, wherever 
exposed by the creeks or rivers, show that there is a gradual 
transition from rock to marl and marl to soil. In other words, 
the top of the rock, when acted upon by infiltrating moisture, 
is decomposed into the marls and soils. The uniformity of this 
under-surface decomposition over a large area, together with 
the tenacity of the soil, results in a level and well-defined 
prairie region. The eastern edge of this Rotten limestone, where 
it dips under the higher protecting strata, is much thicker than 
* The land divide which probably separated the brackish and fresh-water Ter- 
tiaries of the west from the purely marine Tertiaries of the Gulf region may have 
occupied this region at one time. ‘The marine Tertiaries end in the vicinity of 
Elmo, Kaufman County (a’). 
+ Letters in parentheses refer to the letters upon the profile and geologic 
section accompanying. 
