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It is, however, too early to theorize on this subject, as the data are as yet very few and scattered. The great stretch of country between the Brazos and the Sabine will doubtless clear up many mysteries.
BASAL OR WILLS POINT CLAYS.
At the base of the Tertiary and immediately overlying the eroded surface of the uppermost Cretaceous strata in East Texas is a great bed of stratified clay, which, on account of its position as the lowermost bed of the Eocene in this region, has been provisionally called the Basal Clays. These underlie a stretch of interspersed prairie and timber land, the country being composed mostly of prairie, with occasional belts and groves of timber. This timber is all hard wood, consisting mostly of post oak, blackjack, and hickory. The belt is sometimes over ten miles wide, and runs between the western edge of the timber and the Central Texas prairies, from the northern part of the State to the Colorado River and beyond. The stratification of these beds is very characteristic, and is very different from the massive structure of the underlying Upper Cretaceous "Ponderosa Marls," but on a weathered surface, where the stratification is not seen, the clays of the two formations are not easily distinguished. They consist of a stiff laminated clay, yellow, gray, blue, or bluish-green in color, frequently interbedded with seams and laminæ of sand, containing many concretionary masses of gray non-fossiliferous limestone, the latter much cut up by veins of brown crystalline calcite, and varying in size from a few inches to six feet in diameter. They are generally of a flat elliptical shape, and of a gray color. Large quantities of gypsum are also found in places in the clay. On Burnet Creek, one mile east of Wills Point, gypsum crystals five to six inches long are frequently found. One of the most constant characteristics of the clay is the presence in it of soft small white calcareous concretions one-tenth of an inch to two inches in diameter, and often having the cauliflower-like form of some of the geyserite of the Yellowstone Geyser basins. These are found very plentifully, and often collect in large quantities in creek beds. No lignite beds have been seen as yet in these clays. Such deposits are found well developed at Wills Point, in Van Zandt County. Going east from this place, they are traceable for two and a half miles, when they finally dip under the overlying sandy strata. West of Wills Point similar strata are seen until we reach Rocky Cedar Creek, a distance of five miles. Here is seen a deposit of shell limestone, composed almost entirely of"
The Basal Clays are probably largely derived from the destruction of the underlying Cretaceous strata.
These clay beds probably represent the Eo-lignitic of Heilprin's Eocene section, the base of Hilgard's "Northern Lignitic" in his Mississippi section, and the Arkadelphia Shales at the base of Hill's "Camden Series" in Arkansas.