12 The American Geologist. Jau. is'jo 
which covers with its mantle of a continuous flora the whole 
of the Atlantic slope (except the Coast Prairie) and the 
Appalachian region. This penetrates the northeastern portion 
of the state and continues southward across it toward the Rio 
Grande,but becomes less conspicuous, and almost obsolete south 
of the Colorado river, where the climatic conditions are more 
arid. Its western border terminates abruptly'', as if there were, 
though there is not, some great topographic barrier, as a lake 
or a desert. To the ordinary observer there is no reason why 
the forest should end so abruptly, but to the geologist it is 
readily explicable when he perceives the radical change in 
structure and composition of the underlying formations, the 
western border of the forest coinciding almost exactly with 
the western border of the previous soils of the arenaceous, 
non-calcareous. post-Cretaceous formations, and that of the 
compact, super-calcareous marls of the Cretaceous. The post- 
Tertiary subsidence has reduced the parting of the Upper 
Cretaceous and Tertiary formations to a common level. Al- 
most concealed by this forest covered area of northeastern 
Texas, is a most interesting topography. In riding over it, 
with the view obscured by the dense timber, it at first glance 
appears to be a succession of rounded hills, but an occasional 
flat-topped divide is reached, which, upon comparison with 
others, proves that the whole country is the remnant of a 
greatly degraded but still distinguishable plain and that the 
inequalities are those of the drainage slopes. These drainage 
basins, owing to the readiness with which the unconsolidated 
structure yields to erosion, now occupy a far greater area than 
the remnants of the ancient plain in which they are carved. 
The present level of the rather sluggish streams is from 100 
to 200 feet beneath the divides, and very little above tide 
water. Their flood-plains are wide and somewhat unstable, 
a few feet above these are the inevitable accompaniments of 
all the major streams of the southern planting region, known 
as second bottoms, often a mile or more in width, while still 
above and beyond these, marking the edges of the valley, may 
be one or more benches, which are usually inconspicuous, 
because of the unstable condition of the vmconsolidated struc- 
ture and the resemblance between the transported terrace 
material and that of the underlying beds. The flat-topped 
divides and wide valleys characterize the whole extent of the 
