Evolution of the Mountainous Texas Region. — IliU. 109 
level is an important question; if at the former there should be 
some record in the coastal formations now remaining; if at the 
latter there should be structural or topographic evidence through 
the central Texan region of a land barrier which separated the 
lacustral from the gulf waters. After careful study the writer has 
so far been unable to find corroborative evidence of the lacustral 
theory while there is abundant evidence of the identity in origin 
of certain coast deposits with those of the Great Plains region. 
With the close of the unstudied Mio-Pliocene epoch history 
again becomes clear, and during late Pliocene, Pleistocene and 
recent times there is a remarkable continuation of alternations of 
loading down and degradation of the coastal plain, and the initia- 
tion and life history of the present topographic forms and drain- 
age system. 
At the close of or during the Llano Estacado or Mio-Pliocene 
epoch occurred an event which has given a peculiar configuration 
to southwestern Texas. Under the weight of the increasing and 
successive accumulations of coastal sediments, a great fault line 
developed extending nearly across the state from Austin to Del 
Rio, the down-throw being to the east. The scarp line thus 
evolved became the shore line for the subsequent Appomattox sea; 
erosion attacked it with increased activity, but, owing to condi- 
tions of aridity, with relatively small effect. 
During the Appomattox epoch, which is located by its dis- 
coverer, Mr. McGee, in late Pliocene, the long degraded land of 
earlier Miocene and Pliocene again subsided; but the shore line 
as delineated on the map extended across only one-third of the 
state, passing via Texarkana, Austin and San Antonio, west of 
Eagle Pass to the Rio Grande. 
Before this epoch two river systems had been developed, and they 
remained distinct features of the landscape. The older system 
included those streams which at the culmination of the Mio- 
Pliocene subsidence were mountain streams debouching near the 
present Rocky mountain front, and which under apparently very arid 
conditions extended their mouths seaward with the receding waters, 
across the Great Plains region; the Canadian and Pecos rivers 
are examples of this system. The system, second in age, included 
those rivers originating in the uninterpreted interval between the 
close of the Great Plains epoch and the beginning of the Appo- 
mattoxv— the Colorado, Brazos, and Red belong here. 
