In connection with the Gulf Tertiary deposits, which stretch entirely across the Coastal Plain of Texas, there is a narrow belt within which the outcrops of several distinct sandy formations are exposed. The shoreward margin of this belt averages about 100 miles from the present Gulf coast. The belt is sometimes less than 10 miles in width, very rarely widens to more than 20 miles, and reaches its broadest exposures of about 40 miles only on the Nueces and Rio Grande.
There are five of these sands. They are very similar in composition and appearance, fossils are comparatively rare in them, and it is often difficult to distinguish the one from the other, especially where the sands of one division overlap and are in direct contact with another.
These sands, in descending order, are:
Lapara (Dumble),2 carrying in the Nueces section vertebrates determined by Cope to be of . . .