THE LOWER OR COMANCHE SERIES. 125 



taceous gravels of that region. In some of these flints remarkable decompo- 

 sition is exhibited, the product being geode-like cavities lined with quartz 

 crystals and pulverulent material. In one instance an apparently unaltered 

 specimen picked up in situ, upon being broken open revealed a small cavity 

 filled with a liquid inclusion. Occasionally the flints, especially an opalescent 

 variety in Comanche County, possess nuclei in the shape of fossils, usually 

 the Requienia (Caprotina). 



The fact that 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, is very interesting, and especially since 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 them that the Indians made most of their flint im- 

 plements, and the ease of their lithologic identity will be of value to the an- 

 thropologist in tracing the extent of the intercourse and depredations of 

 former Indian tribes inhabiting this region. 



The decomposition of these flints and of the adjacent limestones has pro- 

 duced 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 porous, cavernous rock, usually of a deep red color from the hydra- 

 tion of the iron pyrites into limonite, composed of the siliceous pseudomorphs 

 of fossil Rudistes, Hippurites (rare), and other shells, the interstitial spaces 

 glittering with minute quartz crystals which line them. This red rock is 

 co-extensive with the areal outcrop of the Caprina limestone. 



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

 fault in the bluffs of the Colorado, occasional red decomposing spots occur in 

 the crumpled and faulted strata of the massive white chalky limestones. 

 Upon closer examination the apparently non-fossiliferous limestone is seen to 

 be undergoing decomposition into a dry pulverulent inflorescence, and as a 

 residuum there remains a dry red dust and exquisitely preserved calcite 

 pseudomorphs of many rare fossils, such as recently described by Roemer* 

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



The thorough investigation of these important and peculiar phenomena may 

 prove of great economic value, as traces of the following important economic 

 products have already been discovered by a few tests: Potash, salt, stron- 

 tianite, anhydrite, epsom salts, gypsum, and gold, but in quantities as yet 

 unknown. These inflorescences are coincident with the fault lines adjacent 

 to the ancient volcanic disturbance of Pilot Knob. The chalky deposit of the 



* Ueber einer durch die Heufigkeit Hippuriten Artiger Chamiden ausgezeichnet Eauna, 

 der oberturonen Kreide von Texas. Paleontologische Abhandlungen, vierter band, heft 4. 

 Berlin, 1888. 



f Bulletin No. 4, U. S. Geol. Survey, Washington, 1887. 



