R. T. Hill—Cross Timbers in Mesihepn Texas. 801 
much greater resistance to the constantly attacking forces of 
erosion than the others. As a result of this fact it forms a 
table land for many miles, and is the highest portion of the 
prairie. The huge ammonites* which are characteristic of this 
horizon, project everywhere through the thin soil. ‘This is the 
third character of prairie, a prairie resulting from resistance. 
The western edge of this prairie projects as an escarpment 
above the next topographic feature that we here describe. The 
ragged edge is well marked, and from it we look down upon 
the Upper Cross: Timbers to the westward. Sometimes we can 
see the flat topped buttes,t left by the constantly receding line, 
as remnants of the formation’s former extent. These buttes 
sometimes appear like islands rising from the depression occu- 
pied by the Cross Timbers, but usually they are situated on 
the edge of the adjacent prairie region. They are generally 
capped by the same peculiar stratum of limestone,{ resting 
upon looser strata that quickly wash away when the cap suc- 
cumbs. Sometimes they appear as mere hillocks, representing 
remnants of the buttes from which this protecting stratum has 
been eroded. ‘These buttes are the most prominent features of 
this area of greatest denudation, and are distributed over it 
usually upon and around the edge of the Carboniferous expos- 
ures. From the top of these buttes and the edge of Grand 
Prairie, the Cross Timbers look like the waters of a long and 
narrow lake viewed from an adjacent highland, and in some 
cases, the opposite shore, as in Comanche county, may be recog- 
nized. But the resemblance to a lake is only superficial. It 
is a case of pseudomorphism, if I may be allowed to use that 
expression, wherein the forces of sub-aerial erosion have imitated 
those of wind and wave. The essential features of shore topo- 
graphy and lacustrine sedimentation are absent. The marks 
of erosion are plainly evident. 
On descending the escarpment at the western edge of the 
Grand Prairie, we reach the sandy soil of the Upper Cross Tim- 
bers. A geologic section will show that the surface soil is 
detritus of the underlying strata (4, g), which dip to the east, 
under the adjacent limestone prairie. This series of sandy - 
Strata varies in structure as we descend them. The upper 
strata abound in Dinosaurian bones and teeth, the lower in 
Lepidodendrons and Calamites. They mark the contact of the 
basal Mesozoic and the Carboniferous. 
The sands of the eastern half of the Upper Cross Timbers are 
purely siliceous, fine-grained, and utterly free from any cement- 
* A. vespertinus Morton, 1834; A. Texana Roemer. 
+ Caprina Limestone of Shumard (e?). 
¢ Comanche Peak Group of Shumard (e°). 
