yav<,han.] RELATION OF CRETACEOUS TO EOCENE. 35 
tusks. The causes that have brought about the conditions favorable 
for their growth are not known to the writer. 
Eagle Pass formation. — These beds have been shown to possess a 
total thickness of over 4,000 feet along the Rio Grande. They thin 
very rapidly to the north, so that the total thickness exposed 'along 
the Nueces River, in the Uvalde quadrangle, is only 100 or 200 feet. 
How much of the formation is buried by the Eocene overlap can not 
at present be determined, but the decrease in thickness is enormous, 
as is shown by the fact that the thickness below the lowest Ostrea 
cortex horizon, near Eagle Pass, is some 1,700 feet, while below the 
same horizon on the Nueces it is probably not more than LOO feet. 
These beds are known to be thicker farther to the north than they 
are along the Nueces River, but how much they are influenced by the 
Eocene overlap is unknown. These facts reveal an extremely inter- 
esting problem — the effect of the Eocene overlap, in regard to which 
much more data are needed before the problem can be satisfactorily 
solved. 
RELATION OF THE CRETACEOUS TO THE EOCENE. 
In order to avoid repetition the details of the basal Eocene will not 
be described here. They are discussed further on, in the description 
of the Eocene sections. 
Here general statements only will be made. 
Bio Grande section. — As has been pointed out by White, Penrose, 
and Duinble, there is no sharp lithologic line between the Creta- 
ceous and the Eocene. The contact between the two series has not 
been discovered. The principal result of the writer's work on the Rio 
Grande was in proving the existence of Eocene fossils some 3 or 4 
miles above the Webb-Maverick county line, 6 or 7 miles above 
where Penrose and Dumble first found such fossils. The fossils 
obtained here are typical Lower Eocene, and there has been discovered 
in this region no greater indication of the commingling of faunas than 
has been found in Alabama or other States, where it has been dis- 
tinctly shown that an erosion interval occurred between the two series. 1 
Because of these faunal relations, it is the writer's opinion that the 
Cretaceous and Eocene series here were separated by an erosion inter- 
val, as is known to be the case elsewhere, and it is quite probable that 
more extended investigations will subsequently establish the exact 
contact, though this may not be possible because of the limited num- 
ber of fossils obtainable and the lack of good exposures. 
Between Eagle Pass and Carrizo Springs. — The probable contact 
between the Eagle Pass formation and the Eocene has already been 
described (see p. 30). No actual contact was observed. The probable 
contact is 9 miles east of the Robert Thomson ranch, where the Upper 
1 Harris: Bull. Am. Pal., No. 4, 1896, p. 28. 
