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These lie to the eastward and immediately above the Lower Cross Timber sands, and are the foundation of the minor Black Prairie streak.

Beneath the scarp of the white rock (Austin-Dallas chalk) at Dallas, and extending westward through the Mountain Creek country to the Lower Cross Timbers, can be seen typical localities of this division, the thickness of which I estimate at 400 feet. These clays in their medial portion are dark blue and shaly, highly laminated, and occasionally accompanied by gigantic nodular septaroae, locally called turtles. The uppermost beds gradually become more calcareous, gradating rather sharply into the chalk. There are also occasional bands of thin impure limestones, which are readily distinguishable from all other Upper Cretaceous limestone by their firmness and lamination. Fossil remains of marine animals are also found in these clays, including many beautifully preserved species, the delicate color and nacre of the shells being as fresh as when the animals inhabited them.

At Austin these beds occur in less thickness, and at one place—where Tenth Street crosses Shoal Creek—they are entirely missing, the chalk resting upon the Shoal Creek limestone. The northwestern part of the city is underlaid by these clays, which are here more calcareous and accompanied by beds of laminated limestone. South of the river, along the International Railroad, they are finely displayed in Bouldin Creek, with the characteristic blue color on fresh exposure. They also appear at San Antonio, near the cement works there, and at New Braunfels, and other intermediate points. North of Waco they increase in extent and thickness, forming extensive black waxy areas in Hill, Johnson, Ellis, Dallas, Collin, Lamar, Fannin, and Grayson counties, west of the white rock scarp.

The chief economic value of the minor Black Prairie will ever be its magnificent black calcareous clayey soil, while some of the chief geological considerations are the ascertainment of means to make this soil more easily handled and less tenacious by devising suitable mixtures, the discovery of road-making material, and the increase of water for domestic and agricultural purposes. Owing to its clay foundation the soil now retains for plant use treble the quantity of moisture of some of its adjacent sandy districts, but surface and flowing water is scarce. Fortunately, however, this district is also within the Central Texas artesian well area, and an abundant supply of water can always be obtained at a depth of less than 1500 feet, as has been proved in the course of our investigations. When this fact is fully appreciated the region will be one of the most prosperous in Texas. In the valleys "

Oysters, fish teeth, chambered shells (Scaphites, Hoplites, and Acanthoceras), and Inocerami, are most abundant.