Caprina Limestone Beds. 99 



the flints are left in great quantities as a residuum of the softer chalks 

 (which are 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 dis- 

 tributed over large areas along the margin of the Black Prairie region as 

 a part of the Post-Cretaceous gravels of that region. In some of these 

 flints remarkable decomposition is exhibited, the products being geode- 

 like cavities lined with quartz-crystals and pulverulent substance. 



These are the only flint horizons, so far, at least, as is known to the 

 writer, in the whole of the immense Cretaceous deposits of the United 

 States. They occur about the middle of the Lower Cretaceous series 

 instead of at the top of the Upper series, as in England. It was from 

 these flints that the ancient and modern Indians made their flint imple- 

 ments, and the ease of their lithologic identity will be of value to the 

 anthropologist in tracing the extent of the intercourse and depredations 

 of former Indian tribes inhabiting this region. Occasionally the flints, 

 especially an opalescent variety in Comanche county, possess nuclei in 

 the shape of fossils, usuall}'^ Requlenla. 



The decomposition of these flints and of tlie adjacent limestones has 

 produced some peculiar and unique effects in the rocks and landscape of 

 the region, the silica replacing the calcium carbonate and leaving as a 

 remnant a peculiar j^orous cavernous rock, usually of a deep-red color 

 from the hydration of the iron pyrites into limonite, composed of the 

 siliceous pseudomorphs of fossil Rudistes and other shells, the interstitial 

 spaces glittering with minute quartz crystals whicli line them. 



Immediately west of Austin, along the downthrow of the great Bonnell 

 fault in the bluft's of the Colorado, another peculiar transformation takes 

 place in the Caprina limestone. Occasional red decomposing spots occur 

 in the massive white chalky limestones. Upon closer examination the 

 apparently non-fossiliferous limestone is seen to be undergoing decompo- 

 sition into a dry pulverulent inflorescence, and as a residuum there re- 

 mains a dry red dust containing exquisitely preserved calcite pseudo- 

 morphs of many rare fossils, such as recently described by Roemer and 

 "White, the occurrence of which I have located in this horizon. 



Traces of tlie following economic i^roducts liave been discovered : Pot- 

 ash, salt, strontianite, anhydrite, epsom salts, gypsum, and gold, but in 

 quantities as yet unknown. 



At Austin a fixult of about 750 feet downthrow has broken this lime- 

 stone division into two different areas, and hitherto confused its measure- 

 ment. 



The limestones are more resistant to erosion than the over 

 and under lying strata, and hence form the summit scarps and 

 mesas of the peculiar buttes and divides in Hood, Comanche, 

 Hamilton, Bosque, Coryell, Lampasas, and other counties of 

 the Grand Prairie regions of Central Texas. It also occurs as 

 the surface of extensive jDrairie regions in western Williamson 

 county. It also caps the summit of the Jehosaphat plateau in 



