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Ostrea crenulimargo appears at the top of these sands, although not seen west of Hare Creek, and the Gryphaea breccia increases in thickness. This fossiliferous horizon continues through northeastern Wise County and southwestern Cooke County, being notably conspicuous at Blockers Creek.

In general the Paluxy sands thicken to the northwest, along the antidip, and thin with the dip to the south of east. The strike of the line of greatest thickness seems to be northeast and southwest from Decatur to the thirty-second parallel, and thence westward along the latter to the Llano Estacado. They are of greatest thickness along the western escarpment from Eastland County to Decatur, being 190 feet at Twin Mountain, Erath County, 120 feet at Weatherford, and 140 feet at Decatur, thinning to the southeast along the dip and to the south along the western border region (which is not coincident with the line of strike), finally playing out altogether as an arenaceous terrane in the latter direction.

An important stratigraphic feature of these sands is the fact that their embedded portion does not completely underlie the Black and Grand prairies, as do the Basement sands, but ultimately end coastward and south by changing into limestones and clays. The extent and limitations of this area of embed as determined from well drillings are more fully discussed elsewhere in this report.

Fossils.-The Paluxy sands are remarkably free from fossil remains other than logs of silicified wood, which occur in great abundance in Parker, Hood, Erath, Comanche, and Bosque counties. A few rare specimens of fish remains have been found, including the species of Pycnodontiadoe, described by Cope, which occur in these sands in the creek bed at Weatherford. In western Cooke and northwestern Wise counties the uppermost layers of the Paluxy sands contain the rare ostrean form, Ostrea crenulimargo Roemer, and in southern Indian Territory and along Red River Exogyra texana Roemer. The sands in these localities, however, are undoubtedly the homotaxial equivalents of the basal Fredericksburg division.

The chief exposures of the Basement sands are in the main belt of the Western Cross Timbers, at the base of the western border scarp of the Grand Prairie, extending from the Colorado River of Texas to Red River and eastward to Arkansas. (See Pls. LXVI, LXIX, LXVII.) Long, disconnected belts of these sands, from 10 to 30 miles in length, occur in the valleys of certain streams, like the Paluxy and Bosque, in the western portion of the main area of the Lampasas Cut Plain. West of the main belt along this border, within the Central "

Jour. Acad. Nat. Sci. Phila., Vol. IX, p. 443 et seq.