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General Occurrence and Relations.

The Cretaceous rocks, except where locally veiled by alluvium, form the entire surface of the Black and Grand prairies, the Cross Timbers, Edwards Plateau, Stockton Plateau, and much of the interior portion of the Rio Grande Plain. (See P1. II, B.) They also form a lame part of the mountain masses of Trans-Pecos Texas. Remnants of these formations are scattered over the Central Province. The Tertiary formations of the plains and of the Fayette and Coast prairies are largely made up of their débris. The strata extend from Texas into Arkansas and continue east in the Atlantic Coastal Plain of the Eastern and Southern States and southward into Mexico. They formerly extended northwest and west into the Great Plains and Rocky Mountain regions.

The outcrops of the Cretaceous formations of the general Texas region, of which those of the Grand and Black prairie regions are a part, may be classified geographically into two general groups, viz:

• (1) Those of the main area, or central belt, which is a wide stretch across Texas from the Ouachita Mountains of southwestern Arkansas and Indian Territory south through central Texas to the Pecos and Rio Grande. This includes the Black and Grand prairies, the interior margin of the Rio Grande Plain, the Edwards and Stockton plateaus, the Callahan Divide, and the scarps of the plains northward to the head of Double Mountain Fork of Brazos River.
• (2) Those of the outlying areas in the Great Plains and the Cordilleran Province.

The present paper is concerned with only that portion of the main area which is north of the Colorado and within the Cross Timbers and the Grand and Black prairies of Texas and southern Indian Territory. In this Main Texas area a more complete sequence of the different beds of the Cretaceous section is exposed than elsewhere, and hence it may be regarded as a standard of comparison for the outlying provinces in New Mexico, Trans-Pecos Texas, Oklahoma, and Kansas, whose beds present variations therefrom.

Character and Origin of the Rocks.

The Cretaceous rocks are of various kinds, but are chiefly limestones and manly clays. They include sands of all degrees of fineness, from small conglomerate to very fine particles; clays, which are sometimes arenaceous, and again calcareous; and limestones, which are either great agglomerates of shell or fine chalky material organically extracted front the sea water. The rocks are largely of whitish or chalky colors, although there are exceptional beds composed of ferruginous sands and bituminous clays. "

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For a summary of the geology of these outlying areas, see Am. Jour. Sci., September, 1895, 3d series, Vol. L, pp.205-234, and July, 1897, Vol. IV, pp. 43-50.