Geographic Features of the Texas Region. — Hill. 77 
groups that compose this unclassified system, are the 
extensive flats, composed of the lacustral sediments of 
the post-Tertiary waters that once covered the region. 
The importance of these flats or basins has usually been over- 
looked. They probably constitute two-thirds of the whole 
region and in the study of their structure and origin will be 
found the key to the late history of the region. In fact it 
may ultimately be shown that the mountains of the trans- 
Pecos region are simply concealed beneath the lacustral sed- 
iments of the lakes which once converted them into islands 
and shore lines. 
Little has been added to the knowledge of these mountains 
since the report of the United States and Mexican Boundary 
survey, in which many fine illustrations and descriptions of 
their topography are given and wherein it is said that "the 
succession of basins form a prominent feature in the geography 
of North America, extending two-thirds its length and quite 
one-third its breadth. They belong to what has been appro- 
priately designated as the Basin system of North America.' " 
The chief bearing of these mountains upon the geography of 
the region, however, is that they were elevated through the 
great plains of the state, with the exception of the coast 
prairie and perhaps the Eo-Lignitic region, and are in a large 
part composed of the stratigraphic remains thereof, thus 
forming a natural limit to the Avhole of the Texas region under 
discussion. 
The possibilities which may result from future investigation 
in the mountain structure of the region are most inviting, as 
well as the temptation to yield to speculation concerning 
them. 
Within the triangle formed by the i)rojected axes of the 
Ouachita, and the trans-Pecos mountain systems and the 
Balcones fault, however, will be found the most important 
light that can be thrown upon the stratigraphic, orographic 
and topographic history of our southwestern country. Where 
the axes of the trans-Pecos and Ouachita systems meet in 
northwestern New Mexico is the great Rocky mountain eleva- 
tion, whicli brought all of the Texas plains to their present 
altitudes, and produced the present coastward inclination. 
' Report on the XJ. S. and Mexican Boundary survey, vol. i, p. 4(j. 
