Evolution of the Mountainous Texas Region. — Ilill. Ill 
developed the latest and newest drainage system of the region, 
a fringe of small streams flowing directly into the gulf of Mex- 
ico, readily traced in the present Coastal Prairie region. 
At the close of the Columbia there was another upward move- 
ment of the continent. The streams of systems 1, 2, 3 and 4 were 
again accelerated by an increased gradient and the prolongation 
of the coastal ends renew^ed. 
It may be well to review the present condition of the systems. 
Those of oldest or Miocene origin, for instance the Canadian 
and Pecos, in their youth drained the limited Rocky mountain 
and Plateau region, attained base level near the present front of 
the Rocky mountains, still receive their greatest drainage from 
the original receiving area and practically flow long distances 
without receiving any of the later developed drainage. In early 
Pliocene time their base level receded eastward with the shore line. 
AVith all the oscillations of elevations, invasions, and reced- 
, ences of the sea shore, these streams have continued to carry 
their loads of mountain sediments to the sea, until to-day they 
are merely long viaducts receiving most of their supph' at their 
heads and draining little of the country through which they 
pass. 
At some yet undetermined time in historj^ the Rio Grande, 
which was a great interior basin stream, cut its way across the 
continued axis of the Rocky mountain system and joined the 
Pecos; the writer inclines to think this was earlier than the Pleis- 
tocene and that through the water gap of the Rio Grande the 
great lakes of the basin region had a partial outlet to the sea. 
The rivers of the second system — the Red, Brazos, Colorado — 
have since their origin in the obscure epoch between Miocene and 
Pliocene time been at work upon the coastal plain developed 
at the close of the obscure Staked Plains or Mio-Pliocene epoch; 
their drainage with each succeeding event has degraded larger and 
larger areas. The coastwax'd extension of these streams follow- 
ing the receding shore lines is also shown in the medial portions 
of their courses. In Appomattox time they received none of the 
new lateral drainage which flowed down the slopes coastward and 
parallel to them. With each oscillation of baee level their work 
has been retarded or accelerated until tlu^y have stripped vast 
regions in the active head-water areas and cut far below the level 
of the newer systems through Miocene, Eocene, Upper and 
