70 The American Geologist. Feb. i89o 
raced basins as they cross the Black Prairie and Eo-Lignitic 
regions, and in places these extend westward to the eastern 
edge of the Grand Prairie. These terraces, at Austin,*^ upon 
the western edge of the Black Prairie, extend two and one- 
half miles inland from the present flood-plain, and attain an 
elevation of two hundred and fifty feet. They are especially 
common in the Eo-Lignitic region where the ancient drainage 
has nearly destroyed the continuity of the plain. Bones of 
the mastodon and elephant are found in most of these de- 
posits. 
THE LAKE BASINS OF THE TRANS-PECOS MOUNTAIN REGION. 
Other important valleys of erosion and sedimentation are 
the recently drained lake-basins of the Trans-Pecos mountain 
region. These are extensive flats, as they are locally termed, 
which occupy nearly all of the intervening space between the 
peculiar and apparently unsystematic mountain groups west 
of the Pecos. "Eagle flat" near the junction of the Southern 
and Texas Pacific railways, is a fair illustration of this struc 
true Here the lake structure is very apparent, and so well 
preserved that it is not difficult to imagine that the waters 
have only been recently drained. In other places in Texas 
and in Mexico these basins are still occupied by saline lakes. 
The present valley of the Rio Grande east of Ysleta is cut out 
through one of these basins. More will be said of these basins 
later. 
MOUNTAINOUS AND DISTURBED REGIONS. 
Having classified the plains and valleys of the Texas region 
let us turn to the mountains or other records of disturbance 
and elevation. The whole region is included in the apex of 
an angle between two great and important mountain systems, 
upon the north and southwest respectively, while within the 
heart of the state is a little known area which has once been 
the scene of much disturbance. In addition to the^e the great 
fracture of the Balcones, has extended across the state from 
one great mountain system to the other. In these systems 
are to be found the history of the limitation and elevation of 
the plains, and other important bearings upon their origin. 
The Ouachita Mountains. 
South of the Arkansas and Canadian rivers and north of 
« See this journal, May, 1889. 
