REPORT OF THE STATE GEOLOGIST. xlvii 



and gyps am, and thus the beds contain within themselves the requisite 

 elements for constantly renewing their fertility. 



THE AUSTIN CHALK. 



The Austin Chalk, which underlies the Ponderosa Marls, marks the 

 middle portion of the Upper Cretaceous Series, during which its per- 

 sistent chalky strata were deposited. While the area of its present 

 exposure in Texas is only a comparatively narrow band, stretching 

 from Ked River to the Rio Grande, it was doubtless originally of 

 great extent, and it carries on it to-day such cities as Dallas, Waco, 

 Austin, and San Antonio. The rock is a comparatively pure white to 

 bluish-white chalk of various degrees of induration and containing 

 numerous foraminifera and casts of fossils. 



The purity of much of the rock of this formation adapts it particu- 

 larly for the manufacture of lime, great quantities of which are now 

 produced from it, and also for the manufacture of Portland cement by 

 combination with the clays adjacent to it. It may in time be used, 

 where necessary, on lands deficient in lime, unless the greensand marls 

 which are nearer to them are found to contain a sufficient quantity 

 of that element in addition to its other ingredients of value. These 

 rocks are also suitable in many instances for building material. 



THE EAGLE FORD SHALES. 



The Eagle Ford Shales which form the Minor Black Prairie west of 

 the Austin Chalk are somewhat similar to the Ponderosa Marls, but are 

 dark blue and shaly in the middle of the deposit, becoming more cal- 

 careous towards the top, and contain many large septaria and well-pre- 

 served fossils. Like the Ponderosa Marls, too, its principal economic 

 value is its soil. The area of this deposit, as exposed in Texas, is 

 comparatively small, but it is nevertheless a region of grand possibili- 

 ties in agricultural and horticultural development. In Trans-Pecos 

 Texas the Eagle Ford Shales with characteristic fossils are found in 

 the northern part of the Eagle Mountains. A short distance north of 

 the large springs they may be seen in a small detached area lying 

 above strata of the Upper Carboniferous, from which they are sepa- 

 rated only by a seam of intrusive porphyry, without any of the older 

 Cretaceous rocks appearing. To the west of this, and beyond the 

 small creek on which the Eagle coal mine is situated, there is a much 

 larger deposit of the shales, cut in places by dikes containing pitch- 



