112 The American Geologist. Augu^*t, 1892 
Lower Cretaceous floors to the Algonkian or Archean, which the 
Colorado is now eroding. 
During the Appomattox the mouth of the Colorado was at 
Austin ; during Columbia it reached base level at the same place, 
but continued a long estuary, like the Potomac, to the eastern 
coast; in post Columbian times it took a new channel from the 
Columbia to the present shore line; its old course is preserved as 
lakes and bayous; interior of that line it was twice superim- 
posed upon its old channel. 
Let us consider the drainage in relation to its task in denuding 
the land. In general, the divides of the Texas streams are flat- 
topped plateaus representing original deposition or consequent 
degradation plains over which head-water ramifications of the 
streams have not completely extended. In but one portion of one 
system does a state of completion exist, i. e. the condition in 
which the head-waters of opposing streams have reached across 
the plateau and etched away the original deposition level. This 
area is the northern half of the central region, or the area in 
which the Paleozoic and Red Bed rocks are now exposed, to which 
in the author's earliest paper upon Texas, the name Central De- 
nuded region was applied. Since the close of the Miocene Plains 
epoch, when the waters receded from the Rocky mountain front, 
it has remained an unsubmerged border-land, interior of Appo- 
mattox, Columbian, and present shore lines; degradation has 
been constantly at work upon it and erosion has cut through suc- 
cessive sheets of the Plains formation, the remnantal Upper and 
Lower Cretaceous and, in part, the Carboniferous to the alleged 
Archean. 
The Appomattox or Grand Prairie streams are approaching 
completion; and in a few places their headwaters have cut through 
the flat-top divides between them ; they are rapidly approaching 
maturity as shown by the butte and mesa type of divides. 
The streams of the Columbian epoch are adolescent, but have 
done much work in the softer deposits of the Eocene area. 
The streams of the present epoch on the emerged Columbian 
deposition plain are nascent or just beginning their life work. 
Extended portions of the older systems, i. e. , those portions 
of the Rio Grande, Pecos, Red, Canadian, Brazos, and Colorado, 
whose mouths extend oceanward with the receding coastal line 
without draining the area through which they flow, are still doing 
