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The Cretaceous formations are by far the most important in Texas, both in area and in economic value. Their composition produces certain features of value to man. Their texture and stratigraphic arrangement in the Black and Grand prairies is conducive to transmission or retention of underground waters and results in most extensive and prolific artesian-well systems. They supply the most valuable soil, building material, stone, lime, and cement of all the formations, and in one instance are the source of valuable oil fields. It is therefore necessary to give a thorough description of these rocks, including their composition, texture, thickness, arrangement, distribution, and classification. They have been briefly and technically described in previous ritings of the author and others, but will now be treated in greater detail, in order that they may be more readily recognized and that a knowledge of them may be of service to the public.

The facts concerning the structure have an important economic application, for they enable one to predict to a degree of certainty within a few feet the depth of any water-bearing stratum. This knowledge is of incalculable financial value to all communities within the vast region under discussion. To make plain these facts certain illustrations are presented, which include a geologic map, vertical sections, cross sections, photographic reproductions of the rocks, and illustrations of fossils characteristic of certain beds.

The geologic map (Pl. LXVI), which is on a scale of 10 miles to the inch, shows the area of the outcrop of each formation, so far as known to the writer. The underground continuation, or embed, of the more important rock sheets, as made known by the records of the artesian wells, is indicated on the artesian-well maps (Pls. LXVII, LX VIII, LXIX). The records of the wells have proved invaluable as aids in the analysis of the general structure and sequence of the rock sheets in regions where they are not exposed at the surface.

Numerous local vertical sections convey exact information concerning the detailed character of the formations in particular localities. The cross sections (see Pl. LXIX) extend across the Black and Grand prairies from their interior Paleozoic border to the Eocene formations on the east. They reveal the entire sequence of the Cretaceous formations, their variations, and the outcrop or depth of embed of any particular formation at any point they touch. These sections are shown on Pl. LXVII. "

See Am. Jour. Sci., April and October, 1887; Am. Geologist, January, February, 1890; Rept. Geol. Survey of Arkansas, Vol. 11,1888; Rept. Geologist of Texas, 1890; Bull. Geol. Soc. America, Vol. 1I,1891; ibid., Vol. V, 1894; Report on the Artesian and Underground Waters of Texas, 1892; Geology of portions of the Edwards Plateau and Rio Grande Plain, etc.: Eighteenth Ann. Rept. U. S. Geol. Survey, Pt. 11, 1898, pp. 193-322.