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Below the Cretaceous rocks which constitute the surface of the Edwards Plateau and Rio Grande Plain lies a foundation of the older and different rocks of the Paleozoic system. These are exposed by erosion only along the foot of the northern scarp of the plateau in Burnet, Blanco, Gillespie, and Mason counties. So far as known, no drill has as yet penetrated them beneath the Rio Grande Plain, and as they are so deeply buried they need little consideration in the presemt discussiom. In the Edwards Plateau, however, the well drill has reached the Paleozoic rocks beneath the Cretaceous in several places, notably at Kerrville and at Morris ranch, in Gillespie County. From data derived from these drillings we are led to believe that the horizontal Cretaceous beds of the Edwards Plateau lie upon an uneven floor of the older rocks. The probable relation of these Paleozoic rocks to the overlying Cretaceous is shown in the figure of the wells at Kerrville (fig. 68, p. 270). One hundred feet of Carboniferous rocks form the base of the section exposed at the month of Hickory Creek on the Colorado River (see p. 220).

The Cretaceous formations of Texas are by far the most important in the State, in both areal extent and economic value. They are seamade rocks, and rest unconformably upon the older Paleozoic rocks, which formed the ocean floor when their deposition began. They constitute almost the entire surface of the Edwards Plateau and much of that of the Rio Grande Plain. Their southern margin is covered by the Eocene overlap, and they are overlain in places by superficial deposits of gravel of Neocene and Pleistocene age. They contain all the artesian water described in this paper, and for this reason it is important that their sequence and occurrence be well understood. They likewise supply the most valuable building material stone, lime, and cement-and some of them. contain oil and gas.

They are mostly limestones and clays, sometimes containing slight admixtures of very fine silica, but there are also great beds of sand and sandy mixtures, occurring principally at the base, middle, and top of the group. The limestones are of many kinds, predominantly light colored, more or less chalky in texture and composition, and rarely of the hard, ringing, close textured character, such as is usually met in American Paleozoic limestones. Some of them are very porous, even cavernous, in texture; others are more massive; some have slight admixtures of magnesia, clay, or silica; others are almost pure carbonate of lime. In some instances they are soft, often marly; in others, quite indurated. The clays are likewise very chalky, so that on exposure to air they readily crumble and weather into soils. The many