46 RIO GRANDE COAL FIELDS OF TEXAS. [bull. 164. 
A quarter of a mile below (i. e., Webb bluff) is a bluff 50 feet high of indurated 
sandy clay, containing mica and ferruginous scales between the strata. Dip 1° 
south. One mile and a half below are seen similar deposits, but with no fossils, and 
containing numerous gray calcareous concretions, with veins of brown crystalline 
calcite. Two miles beyond this, on the Mexican side, is a bluff a quarter of a mile 
long and 75 feet high, of interlaminated gray sands and chocolate clays, with sulphur 
and gypsum in places, and occasional ferruginous spots. Hard gray clay-ironstones 
with leaf impressions are also found. The sand beds are from 1 to 5 feet thick, and 
the clay is in thin laminae. Dip undulating from 1 to 5° southeast. The mica and 
black specks in the sand, the laminae of chocolate clay, the presence of sulphur and 
gypsum crystals, all show a strong resemblance to the Tertiary of east Texas. From 
here to Hardin Ferry, and thence to the mouth of Cavezeras River, are seen similar 
strata, frequently causing rapids where they cross the Rio Grande. In one place the 
indurated bluffs encroach on the river until it narrows down to 30 yards. Here the 
waters have cut a deep channel and rush through at a great velocity. Frequently 
interbedded glassy ferruginous layers, 1 or 2 inches thick, are found in the sandstone. 
Three miles below "the Hardin" is a bluff 60 feet high composed of friable sand- 
stones, the harder and softer layers blending into each other and occasionally show- 
ing ferruginous patches. Dip 1° south. For 19 miles below this point we pass over 
identically similar strata, frequently containing calcareous concretions 1 to 3 inches 
in diameter. These contain seams of crystalline calcite and are of a gray color, 
weathering brown or red in concentric layers. 
Similar strata are seen from here to the San Tomas coal mines. These are sit- 
uated on the Texas side of the river and at the mouth of San Tomas Creek, about 25 
miles by river above Laredo. 
For 3 miles below this are seen indurated greenish clays with leaf impressions, 
broken stems, and specks of lignite. Occasionally seams of chocolate clay and cal- 
careous nodules are found. As usual, the bluffs are capped with pebbles or sand, and 
dip 2° southeast. Fifteen miles above Laredo is a bluff reaching a maximum height 
of 40 feet and about 1 mile long. It is composed of interbedded coarse sand, with 
calcareous nodules, and sandy clay with gypsum and sulphur. The sand grains are 
red, yellow, white, and gray, and the whole bluff has a greenish appearance, spotted 
in places by ferruginous matter. Many similar outcrops are seen for 7 miles below, 
and as the dip is often horizontal, or nearly so, the exposures show simply different 
parts of the same bed. Eight miles above Laredo is a bluff about 80 feet high and 
half a mile long, composed of semi-indurated buff sands, with an undulating dip. 
Similar exposures are seen down the river to Laredo, and in fact that town is built 
partly on the same beds, which are here succeeded by those of the next division of 
the Eocene. 
This section appears to be quite different from that observed a few 
miles inland from the river. The very sharply defined Carrizo sandstone 
appears to be absent, at least its presence was not noted. 
RECONNAISSANCE BETWEEN EAGLE PASS AND CARRIZO SPRINGS AND SOUTHWEST OF CAR- 
RIZO SPRINGS. 
The change from the Upper Cretaceous clays to what is probably 
the basal Eocene has already been described on page 30. 
About 15 miles from Carrizo Springs the sandstone is rather fine 
grained, of a brown ocher color, and the grains are distinctly crystal- 
line. 
