THE UPPER OR BLACK PRAIRIE SERIES. 113 



free from grit, and easily carved with a pocket knife. Under the microscope 

 it exhibits a few calcite crystals, particles of amorphous calcite, and innu- 

 merable shells of foraminiferse. The air-dried indurated surfaces are white, 

 but the saturated subterranean mass has a bluish-white color. The rock 

 weathers in large conchoidal flakes, with an earthy fracture. 



In composition it varies from 85 to 94 per cent of calcium carbonate, the 

 residue consisting of magnesia, silica, and a small percentage of ferric oxide, 

 as can be seen from the following analyses of unselected specimens: 



Eocky 

 Texas. Comfort. 



Calcium carbonate 82 . 512 88 .48 



Silica and insoluble silicates 11. 451 9.77 



Ferric oxide and alumina 3 . 648 1 . 25 



Magnesia 1 . 189 trace. 



The thickness of this chalk is about 500 feet. So far as observed in Texas 

 it averages the same thickness at Austin, Sherman, and Dallas. It is of great 

 uniformity throughout its extent, but there are a few local differences in 

 hardness, which are sometimes due to surface induration and to igneous action, 

 having been converted into marble at Pilot Knob, south of Austin. 



In the vicinity of Austin the soft and chalky structure is somewhat de- 

 stroyed by the volcanic disturbances of the vicinity, such as the co-deposition 

 of volcanic ash, and excessive jointing and faulting, but it maintains its pure 

 chalky aspect elsewhere. 



A great portion of the former extent of this chalk has been destroyed by 

 erosion, and its western border in Central Texas is now receding eastward 

 under the influence of excessive atmospheric decomposition and denudation. 

 From Austin to San Antonio it is more stable, but west of the latter place 

 erosion again becomes great. That the whole group may once have extended 

 far to the west, and perhaps entirely across the State, is not at all improb- 

 able. 



It so resembles some of the beds of the underlying Comanche and of the 

 overlying Upper Cretaceous that until recently they have not been differen- 

 tiated. Upon close examination, however, it is noticeable that the Lower 

 Cretaceous beds are distinctly stratified and very much harder and generally 

 more or less crystallized from pressure, solution, and redeposition of the 

 caibonate of lime in the chalk. The topography of the white rock beds is 

 also of a milder type than that of the Comanche series, and is recognizable 

 even at a distance. Above all, it is distinguished by its softness and by its 

 entirely different fossil remains. They are also distinguished from the other 

 chalky beds of the Upper Cretaceous by their greater firmness, different 

 fossils, and by their higher percentage of calcium carbonate. With the 

 exception of the White Cliff chalk of Arkansas, the other beds of the Upper 



