166 
Tra^^sactions Texas Academy of Science. 
Hill, Eobert T. 
origin of the plains are Neozoic, the product of alternate subsidence and 
elevation- During these time-intervals the proeess of land degradation, 
sedimentation, and climate so varied as to produce the present differences 
in the- oomposition of formations, soils, drainage growth, and topography. 
Review of our limited knowledge of the topographic features at the close 
of Paleozoic time. Post-Paleozoic land in the form of an isthmus or a 
peninsular extended through the heart of Texas, and on its western shore 
the “Red Beds” were laid down. The Paleozoic rocks of Central Texas 
are exposed in two areas, separated by Mesozoic rocks which form the 
divide between the .(Brazos and Colorado rivers, dn the northern area the 
rocks a;re Carboniferouis-lMrinian ; in the southern they are exposed suc- 
cessively down to tlie Algonkian. The Archaean and Paleozoic formations 
were completely covered by Mesozoic deposits and now occur in a valley 
of erosion surrounded on every side by .horizontal scarps of the Cretaceous. 
Tlie north and south Paleozoic rocks are an iinportant factor in the evolu- 
tion of the topograpliy of the plains, as it partially outlined the existing 
surface features. It was a land barrier after the close of the Carbonifer- 
ous. Subsidence began with the J-^ower Ciretaeeous. During the invasion 
of the sea the Comanehe .Series .were laid .down. Oceanic waters covered 
all of Texas, West Indies, etc. At the close of the Comanche there was 
an uplift. Again there was a lo.adin.g of the coastal plain and the Upper 
Cretaceous was deposited. 
During the subsidence the sea advanced interiorly, and the Texan and 
Great Plains regions, including much of the. Rocky Mountain region, were 
buried beneath 2,000 feet of sediment. At the close of the Cretaceous 
another uplift took place, followed by the depoisition and subsidence of the 
Eocene, ‘‘It is not knO'Wn whethei .at the close of the Eocene ithere was any 
impoirtant event, such as the elevation or .subsidence of the coastal plain. 
iWe have knowledge of . only the basement history of the marine Eocene. 
Its upper contacts or gradation have never been studied or presented.” 
The oondition of the coastal region .during the Mio-Pliocene is obscure. 
An important question is raised concerning the Llano Estacado formation 
whether it was laid down at marine or laeustral base-level- The author 
has been unable to find oorroborative evidence of the laeustral theory. 
The evidence points to tlie identity in origin of certain coast deposits with 
those of the Great Plains. 
During the Llano Estacado or Mio-Pliocene epoch the great fault line 
extending from Austin to Del Rio was developed. Later, in the Appomattox 
epoch, the shore line crossed from Texarkana, via Austin and San Antoiiio, 
west of Eagle Pass to the Rio Grande.. Two river systems developed before 
this epoch remained; the Canadian and the Pecos represent one, the Color- 
ado, Brazos and the Red rivers the other. At the culmination of the 
Appomattox a third system was developed, represented by the San (Gabriel, 
Lampasas, Leon, Bosque, etc. The Columbian Epoch .of the Pleistocene. 
The upward movement of the land at its close. Review of the present con- 
dition of the river systems; their drainage and denudation. The part 
j)layed by the ancient Paleozoic floor. The plains of Texas are the product 
of .o.scillaiion, their topography of etching ‘‘by a series of consequent auto- 
genetic drainage systcius.” Tlie relation of denudation to the consolida- 
