124 CRETACEOUS ROCKS OF TEXAS. 



The economic aspects of the Comanche Peak beds are mostly aesthetic. 

 Their weathering adds beauty to the landscape, and the remnants of ancient 

 marine life so abundantly preserved in it should afford abundant material for 

 instruction to those who are capable of appreciating the grander features of 

 the earth's history. The chalks, were they the only ones in our State, would 

 perhaps be of great economic value for cement making and agricultural uses, 

 and the remarks applied to the Austin chalk, on page 114, are equally appli- 

 cable to them. 



2C. THE CAPRINA CHALK AND CHALKY LIMESTONE SUBDIVISION. 



Without any observed stratigraphic break, the Comanche Peak chalk 

 gradates into 300 feet, more or less, of chalks and chalky limestones of vary- 

 ing degrees of consistency, from a pulverulent condition to firm limestones, 

 which seem to be a secondary condition of the chalk produced by superficial 

 hardening. These hard layers form the cap-rock of the buttes and mesas 

 or highlands of the extensive Grand Prairie region, and are the cause of 

 the flat topped topography of the so called mountains of Central Texas. 

 They usually occur at an altitude above 1000 feet along the margins of their 

 eastern outcrop, but at Austin they have been broken along the line of their 

 strike by a great fault, and as a result the eastern side has fallen down from 

 1500 to 750 feet above sea level, as exposed in the river bluffs between Aus- 

 tin and Mount Bonnell, on the Colorado, where the chalk has been more or 

 less hardened into firm limestones by the local metamophism accompanying 

 faulting. 



In these chalks and chalky limestones are well defined layers of exquisite 

 flint nodules, occupying apparently persistent horizons in localities. These 

 flint nodules are oval and kidney-shaped, ranging in size from that of a 

 walnut to about two feet in diameter. Exteriorly they are chalky white, 

 resembling in general character the flint nodules of the English chalk cliffs. 

 Interiorly they are of various shades of color, from light opalescent to black, 

 sometimes showing a banded structure. These flint nodules are beautifully 

 displayed in situ in the Deep Eddy canyon of the Colorado, above Austin, 

 where they can be seen occupying three distinct belts in the white chalky 

 limestones. 



Where these chalky limestones form the mesas of rapidly weathering 

 plateaus, such as the remnants of the Grand Prairie west and southwest of 

 Austin, the flints are left in great quantities as a residuum (the softer chalks 

 being more readily decomposed into soils and washed away), and they cover 

 large areas of country. They have also been transported eastward in past 

 geologic times by marine and river action, and are distributed over large 

 areas along the margin of the Black Prairie region as a part of the Post-Cre- 



