SOUTHERN CONTINENTAL HISTORY. 519 



The Trinity division records in its basal grits the beginning of the great 

 Comanche subsidence, and the disappearance of the wonderful Pernio-Jurassic 

 seas of the red bed epoch. The Trinity sands were soon followed by a 

 brackisli fauna of the Pleurocera beds, which gradually, as the ocean bottom 

 deepened, became sublittoral and marine in character, as shown in the 

 chalkier alternating beds, w^hich indicate a long period of moderate depth. 

 What happened at the close of the Trinity is somewhat more problematic. 

 The Paluxy sands indicate the recurrence of shallower conditions. Toward 

 the south these beds become less and less arenaceous and more argillaceous, 

 foraminiferse (Nodosarci) and plant remains (Equisetwin) having been found 

 associated in them at Del Rio on the Rio Grande. To this southern argil- 

 laceous continuation I have previously applied the name Exogyra arletina 

 beds. 



The Comanche Peak division is, par excellence, the deep-water deposit of 

 the series, as attested both by its sediments and by its fossils. The Paluxy 

 sands no doubt represent the beginning of its subsidence, which is further 

 recorded by the succession of the marine Exogyra texana clays and the 

 Comanche Peak chalks, which covered all of the Texan and Mexican and 

 no doubt a large part of the South American area, during an epoch perhaps 

 longer than that in which thousands of feet of littoral sediments would have 

 been deposited. 



The Washita division, composed principally of laminated calcareous clays 

 (marls), often alternating with impure chalky limestones, with its compara- 

 tively deep-water fauna, indicates a shallower condition than the Comanche 

 Peak epoch. This shallowing was the forerunner of the sublittoral condi- 

 tions that followed the Denison beds. As in the Comanche Peak division, 

 the limestone and chalky characters of the Fort Worth beds increase south- 

 ward until (as at Del Rio) they become pure chalks. 



The Denison beds are preeminently, in their northern portion, a near- 

 shore and shallow-water marine deposit, as illustrated in the character of 

 sediments, in their assortment, and in their gradual lithologic change from 

 argillaceous to a ferruginated arenaceous character, and in the presence of a 

 fauna of littoral species mixed with lignite and other land debris. 



Stratigraphic Value of the Terranes. 



Having defined the units of the Comanche series so that they may be 

 intelligently discussed, I now propose to present a few general deductions 

 therefrom : 



1. Each of these divisions presents a complete and distinct stratigraphic 

 and paleontologic aspect, and they should no longer be discussed as a single 

 geologic unit. In addition to the broad lithologic differences I have enume- 



LXXVI— Bull. Gkol. Soc. Am., Vol. 2, 1890. 



