106 The American Geologist. August. 1893 
of Paleozoic time; a period durin<^ wliicli our continental outlines 
underwent their most radical changes and, for this region at least, 
the evolution of modern topography began. The eastern half or 
Appalachian outline of the continent was approximately defined 
east of the UTth meridian. Another but as yet obscurely de- 
fined land or islands existed in the region of the western Cor- 
dilleras (the Rocky mountain, Great Basin and Sierra regions). 
Between these main continental areas was the region which has 
since been evolved into the Great Plains, and is supposed to have 
been a vast enclosed basin extending far southward from the 
southwest corner of Appalachia. Through the heart of Texas 
was a long narrow isthmus or peninsula of post-Paleozoic land 
against whose western shore were laid down the vast "Red beds" 
formation of Permian, Triassic, and probably early Jurassic time. 
The rock structure of this strip of land was of Carboniferous and 
earlier age; it generall}' dipped westward or in a direction diamet- 
rically opposite to the great series of Neozoic sediments later 
deposited, from earl}' Cretaceous to the present, which from their 
general dip to the gulf maj' be termed the coastward incline. 
The Paleozoic rocks of central Texas, although continuous in 
extent, are exposed as two distinct areas separated and covered 
by a remnantal mass of Mesozoic rocks, which now occupy the 
divide of the Brazos and Colorado rivers, and once continued en- 
tirely over them. The northern area, so far as exposed, consists 
of Carboniferous-Permian rocks. In the southern or Burnet area 
erosion has cut deeper into them, exposing successively lower 
rocks down to the Algonkian. There has been no permanent 
land area of Archean or Paleozoic rocks in this region since Meso- 
zoic time, for they were completel}' covered b\' subsequent sheets 
of deposition, and now occur in a valle}' of erosion surrounded 
on every side by the horizontal scarps of the Cretaceous, down 
which the drainage flows to the centre of the alleged Archean 
land. Thus are exposed the successive older rocks — Carbonifer- 
ous, Silurian, Upper Cambrian, — and in the lowest drainage valley 
the Algonkian is reached. If this had been a persistent land area 
it would not have been covered by Cretaceous sediments, and the 
drainage would flow outward from it instead of downward into it. 
This north and south strip of Paleozoic rocks, which is now 
becoming exposed across Texas, is an important factor in the 
evolution of the plains topography, for it has been influential in 
partially outlining the present surface features. 
