THE UPPER OR BLACK PRAIRIE SERIES. Ill 



NO. 2. THE EAGLE FOED CLAY SHALES. 



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 septarise, locally called turtles. The uppermost beds gradually be- 

 come more calcareous, gradating rather sharply into the chalk. There are 

 also occasional bands of thin impure limestones, which are readily distinguish- 

 able from all other Upper Cretaceous limestone by their firmness and lami- 

 nation. Fossil remains of marine animals are also found in these clays, in- 

 cluding 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 rest- 

 ing 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 

 Eailroad, 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 mag- 

 nificent black calcareous clayey soil, while some of the chief geological con- 

 siderations 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 appre- 

 ciated the region will be one of the most prosperous in Texas. In the valleys 



* Oysters, fish teeth, chambered shells (Scaphites, Hbplites, and Acanthoceras), and Inoce- 

 rami, are most abundant. 



