36 RIO GRANDE COAL FIELDS OF TEXAS. [bull. 164. 
Cretaceous clays end and sandstones of the Carrizo Springs type are 
first initiated. 
Along the Nueces River.- — Here, again, there is indefiniteness, due to 
the similar lithologic character of the rocks and the dearth of fossils. 
A detailed section across the contact is given on p. 34 in the discussion 
of the Eagle Pass formation. The base of the Eocene is placed pro- 
visionally at the top of the Ostrea cortex ledge, because, (1) on the 
Frio River, 19 miles farther east, a ledge of these oysters is undoubt- 
edly the summit of the Cretaceous; (2) a short distance below the 
exposure of this ledge on the river, and stratigraphically above it, a few 
plant remains were found which Professor Knowlton considers prob- 
ably Eocene; (3) the strata above this oyster ledge on the Nueces are 
very similar to strata on the Frio River, known with certainty to be 
Eocene, or are lithologically identical with those strata. The proof 
that the writer has located exactly the Eocene-Cretaceous contact on 
the Nueces is not absolute, but the evidence in favor of the location is 
strong, and it is doubtful whether the contact will be determined with 
greater exactness, unless it subsequently becomes possible to make 
more extended use of the plant remains, which are few and imper- 
fectly preserved. 
Along the Frio River. — About 2 miles, in a straight line, below the 
Engelmann ranch and about a half mile above My rick's lower apiary, 
a definite clear-cut contact is revealed. The section exposed is described 
in detail on p. 51. Evidence of erosion exists in the pebbles of the 
basal Eocene, but this evidence is not very strong, because of the gen- 
eral lithologic constitution of the rocks (shallow-water sandstones and 
clays), and because local erosion unconformities or pebble beds might 
exist almost anywhere. The faunal break is as sharp as a knife's edge. 
As the Eocene rests on a ledge of the Cretaceous Ostrea cortex, to be 
sure some Eocene fossils are mixed with the oysters along the basal con- 
tact, but no Eocene species was found below that ledge, and no Creta- 
ceous species was found above the basal contact. These data prove 
absolutely that there must have been a break in the sequence of sedi- 
mentation long enough to permit a complete faunal revolution. . 
Although fragmentary, these data sustain the conclusion that there 
are in the Rio Grande region of Texas no transition beds between the 
Cretaceous and the Eocene. The respective faunas are absolutely dis- 
tinct, with as complete a break between them as is known anywhere. 
The difficulty in differentiating Cretaceous and Eocene rocks lies in the 
similar lithologic constitution of the two series. 
