2 
Transactions Texas Academy of Science. 
[ 108 ] 
ROCKS. 
The sedimentary rocks of the region comprise deposits of Cretaceous 
age overlain by a volcanic complex, probably Tertiary, but possibly con- 
tinuing on into Quaternary time, and these are cut by eruptives and 
interbedded with them. 
CRETACEOUS. 
We found representatives of all the members of the Cretaceous system 
of the West Texas section from the Fredericksburg to the Exogyra Pon- 
derosa marls, inclusive, and, although these beds present some differences 
from those of western areas as well as from their more eastern exten- 
sions, a sufficient number of characteristic fossils were obtained to fully 
identify everything above the Fredericksburg. The uncertainty as to the 
exact division between the Fredericksburg and the Washita that we have 
found elsewhere in West Texas and Mexico exists here also, but I have 
placed the line, provisionally, at that point where definite Washita forms 
prevail. At this point there is here a decided difference in the two lime- 
stones also, so that it makes a well-marked horizon. 
Fredericksburg. 
Deposits of the Fredericksburg limestone are well exposed some four 
or five miles east of Alamo de Caesario, where they form a line of hills 
which trend nearly north and south. The rocks are dark-colored, massive 
limestones with beds of Gryphsea. Towards the summit of the hills, 
beds with different forms of Caprina, etc., are found. The limestones 
here have also the siliceous character which marks them at other places. 
This line o'f hills is the axis of a fold involving both lower and upper 
Cretaceous sediments. The dip of the beds on the west of the hills is 
toward the west, while on the east the inclination is eastward. 
In one of the canyons in this range of hills we found materials under- 
lying the Fredericksburg rocks which we supposed to belong to the 
Comanche Peak group, but a sufficient number of fossils were not found 
for full identification. 
Good exposures of the Fredericksburg are numerous in the region lying 
south and southeast of this and an escarpment running north and south 
for several miles is formed almost entirely of these rocks. 
At the California mine these heavy bedded and massive, darkrcolored 
limestones contain large numbers of a small gryphsea with a short beak 
similar to that observed in rocks of the same age in Coahuila. They 
also have the cavernous weathering of the Fredericksburg, and locally 
contain much chert. In general appearance and weathering they are 
entirely different from the overlying limestone containing Washita fos- 
sils. They are more highly metamorphosed and are strongly rugose, 
their exposed surfaces having an extreme roughness which is not found in 
