170 The American Geologist. March, 1892 
until the Cretaceous period, with the possible. exception of early 
Carboniferous times, the ancient Palaeozoic core of Texas has 
been subjected to denudation. Since the beginning of the Car- 
boniferous to the present, along the northern border of this area, 
there have been no disturbances of sufficient moment to affect the 
Carboniferous rocks, barring only the gentle tilting to which these 
have been subjected. In other parts of the area this is not true, 
for Dr. Comstock finds later disturbances, and the post-Carbonifer- 
ous age of the Burnet granite has been quite definitely settled. 
The point intended to be made here is, that the older rocks have 
not been subjected to extensive mountain building forces in post 
Carboniferous times, otherwise it would be difficult to understand 
the remarkably unbroken nature of the Carboniferous rocks which 
skirt its northern border. Post-Cretaceous disturbances to the 
southeast, such as that of the Burnet granite intrusion do not, of 
course, enter the problem under consideration. 
So, from Carboniferous to Cretaceous times the area was not 
only one of continuous denudation, but also one free from the 
rejuvenating effect of extensive elevation. How greatly denuded 
this area was in pre-Carboniferous times cannot be stated with 
absolute certainty, yet the valleys and promontories buried be- 
neath the Carboniferous are not sharp and deep in topographic 
outline, but, rather, rounded, as if of adolescent or mature 
forms. This seems to show that the area, though a mountainous 
area, was not at that time one of, striking relief. From that time 
to the beginning of the Cretaceous its relief must have been 
greatly reduced in the absence of distinct rejuvenation. Yet, as 
will be shown, it was at this time a land of considerable topo- 
graphic diversity. 
The lowermost Cretaceous bed is a hodge-podge of sand and 
conglomerate representing in large measure the old land, in part, 
if not entirely, worked over by the sea as it advanced upon the 
Paleozoic land, which was sinking beneath the ocean.  Strati- 
graphically this, the Trinity of Hill, is the lowest member of the 
nearly horizontal Cretaceous; but it may represent a wide period 
of time, for, in places it is found on the Paleozoic land at an ele- 
vation of 1,250 feet above sea level, while at other places it rests 
at an elevation of 1,900 feet. Thus this vertical difference of 
650 feet means approximately the time required for the land to 
sink 650 feet; and that there is this amount of difference of time 
