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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 provisionally at the top of the Ostrea cortex ledge, because,

• (1) on the Frio River, 19 miles farther east, a ledge of these oysters is undoubtedly 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 probably 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 imperfectly preserved.

Along the Frio River.

-About 2 miles, in a straight line, below the Engelmann ranch and about a half mile above Myrick'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 general 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 contact, but no Eocene species was found below that ledge, and no Cretaceous species was found above the basal contact. These data prove absolutely that there must have been a break in the sequence of sedimentation 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 distinct, 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.