294 Geologic Stori/ of the Colorado River. — Hill. 
clear that it is safe to predict that after the knowledge of this 
region recovers from the misinterpretation and speculation of 
theorists, it will be the hight of impropriety to speak of the 
American Cretaceous as a single formation. 
The two formations, the upper or Meek and Hayden's section, 
and the lower (the author's Comanche series) respectively, were 
separated by a land epoch probably as long as either of them. 
Each records every phase of a profound marine subsidence ; 
each records the transition from littoral sands to shales and 
deeper chalk, making long periods. 
The lower Cretaceous rests directly upon the metamor- 
phosed limestones of the Carboniferous at Burnet. Their 
stratigraphic characteristics are well exposed in the slope and 
canons of the Colorado valley from Burnet to Austin. From 
the summit of Post mountain at Burnet its scarp can be 
traced northwest and southeast for 60 miles, while the river 
upon leaving the Carboniferous east of Smithwick enters this 
formation and follows it for 50 miles to Austin through ver- 
tical canons often cutting to a depth of 750 feet. 
In ascending sequence the basal, littoral. Trinity beds of this 
formation are soon followed by deeper limestones which were 
once pure chalks ; and these in turn are succeeded by the Exo- 
gyra arietina clays and these by a uniform lime stratum, here- 
tofore unmentioned, 100 feet thick, with an undescribed fauna 
to which I have given the name Vola limestone from the char- 
acteristic and beautiful fossil resembling the Vola quinque- 
oostata Lamarck, or Janira Heuriansiana d' Orb. 
The base of this formation — the Trinity beds — has a fauna 
predominately Wealden, and the top, some 1,200 feet above, 
contains fossils distinctly Turonian or middle Cretaceous. 
That this lower Cretaceous marine epoch was closed by eleva- 
tion is shown by a series of vertical faulting near its eastern 
edge (which must not be confused with disturbances of a later 
date) and by the unequally eroded surface of the deep sea 
limestone upon which the littoral beds of the unconformable 
and entirely different basal beds of the upper Cretaceous form- 
ation are deposited. This unconformity has been traced across 
Texas north-east and south-west for 600 miles. The lower 
Cretaceous sediments represent a profound subsidence until 
lately not considered as a factor in American Cretaceous his- 
tory. This subsidence deposited its deep sea sediments over 
