NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 49 



layers. The heavy sandstones are responsible for a change in the topography 

 west of a line approximately marked by the Seymour- Vernon road. At 

 Craddock's ranch, on the south side of the river, the lower hills east of the 

 road give place to sharp buttes and deep, steep-sided valleys. The same 

 topography is found a little farther west on the north side. The heavy 

 sandstones at Craddock's ranch are plainly due to the deposits of a strong 

 stream, or streams, from the west, as the cross-section of the bed of such a 

 stream is clearly shown at one place (plate 8, fig. i). Above this series of 

 sandstones, clays, and shales is a very thin calcareous layer, and in places 

 paper-thin red shales, with some insect tracks. This seems to be the highest 

 traceable layer of the Clear Fork. The sandy, easily eroded layers above 

 form a flat or rolling surface as far west as Sagerton, where the Double 

 Mountain beds appear. 



SUMMARY OF CLIMATIC CONDITIONS. 



From the above-assembled evidence it is apparent that the Wichita and 

 Clear Fork beds represent the accumulations of sediment on a wide, flat 

 delta or coastal plain, crossed by numerous streams, dotted by inclosed or 

 partly inclosed pools, subject, especially in Clear Fork time, to oscillations 

 of level slight in themselves, but sufficient on such a flat surface to produce 

 wide migrations of the strand-line. The surface was well exposed to the 

 air, and well above the level of ground-water, except during the infrequent 

 periods of submergence, and there were no great areas of stagnant water. 



The climate was semiarid in general, but there were changes of short 

 period, apparently due to repeated incursions of the "Albany" sea, with a 

 quick response in the increased abundance of vegetable growth due to the 

 increased humidity. 



The source of the material forming the beds of Texas and Oklahoma 

 which carry the vertebrate fossils was undoubtedly the Wichita Mountains 

 and adjacent elevated masses, and we can not doubt that they were much 

 larger in Permo-Carboniferous time than now. 



The blue clays, white clays, and light-colored sandstones associated with 

 the Hmestones of the Clear Fork are similar to the red deposits in all respects 

 but the content of ferric oxide. The source of the material remained 

 unchanged, and there could have been no diminution in the available iron. 

 There is little evidence (a few beds of conglomerate) of any reworking of 

 the once-deposited material. The iron-free beds, or those with ferrous iron, 

 owe their condition to the removal or the reduction of the ferric iron derived 

 from the original rocks. The only available agent for this action would be 

 the carbon derived from the decay of a considerable amount of vegetable 

 matter. As has been repeatedly shown, carbon thus derived would exhaust 

 the available free oxygen, and even extract oxygen from oxides already 

 formed. The carbon dioxide thus formed would convert the iron into the 

 soluble carbonate, which would easily be leached out of the beds. 



