^l^ R. T. HILL — THE COMANCHE SERIES. 



previous classification), while still further southward, where the open sea 

 continued during the pre-Dakota land epoch, it is very probable that there 

 is no break between these clays and the marly beds of the basal upper Cre- 

 taceous. At El Paso, however, the Denison beds are again represented by 

 arenaceous littoral beds, which suggests that there was a shore-line in the 

 vicinity. 



At Denison and throughout northern Texas these beds are unconformably 

 overlain by a magnificent development of the Dakota sands. I have failed 

 as yet in Texas to find a single species extending from one formation into 

 the other. In Kansas, how-ever, there are some apparent exceptions to this 

 rule, as has been shown by Cragin. 



Variation in Character of the Deposits. 



From a study of four parallel sections based upon actual measurements at 

 intervals of from 100 to 200 miles, extending from Indian territory south- 

 ward to the Rio Grande, the following deductions may be made : 



1. That these beds were laid down against the Ouachita mountain system 

 of Indian territory and over the whole preexisting area of Texas, except 

 the insular mountain areas of the Organ and Guadalupe mountains; 



2. That the more littoral terranes of the Trinity and Paluxy beds and the 

 Washita division increase in thickness and littoral character to the north- 

 ward and diminish to the southward ; 



3. That the deeper water or chalky terranes, such as the Comanche Peak, 

 'the Caprina limestone and the Glen Rose beds, thin out northward and enor- 

 mously increase in thickness southward, thus demonstrating that the pro- 

 found subsidence was to the southward, in which direction the open sea pre- 

 vailed, while oscillations of level are recorded only in the northern littoral 

 areas. 



Subsidence recorded in the Comanche Series. 



Reviewing the sections mentioned, the series resolves itself into strati- 

 graphic groups representing stages of subsidence, but of varying degree and 

 period. The topography of the pre-Trinity continent is not difficult to 

 interpret, a slight land barrier of Carboniferous and Silurian rocks in Texas 

 projecting southward, peninsula-like, from the Ouachita mountains and sepa- 

 rating lake from ocean. But little base-leveling was required to transform 

 this peninsula into an island or islands, smaller and smaller, until completely 

 covered by the Comanche Peak sediments. Beyond this barrier the ancient 

 lake bottom, whose inequalities had long since been overcome by the sedi- 

 mentation, stretched a comparatively unbroken plain to California, with a 

 few mountainous exceptions, like the old post-Silurian islands in the Organ 

 and Franklin ranges. 



