212 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



shoreline, now submerged beneath a shallow sea, now a marshy land covered by 

 forests of the strange plants of the coal period. 



"Near the end of Pennsylvanian time there was another period of mountain 

 formation^ in west-central Arkansas and southern Oklahoma. The Cisco forma- 

 tion of north-central Texas was laid down after this period of mountain-building. 

 South and southwest of the Arbuckle and Wichita Mountains of southern Okla- 

 homa the Cisco sediments are red sandstones, conglomerates, and shales, showing 

 by their structures and vertebrate fossils that they were deposited as land sedi- 

 ments by rivers flowing southward and southwestward from the mountains on 

 a flattish plain much like the country along the shore of the present Gulf of Mexico. 

 Farther to the southwest the Cisco sediments become marine shales and lime- 

 stones, indicating rather clear sea-waters in that direction. 



"The beginning of Permian time in north-central Texas was a continuation, 

 without marked interruption, of the later Pennsylvanian. The earliest Permian 

 formation, the Wichita, is very like the Cisco, in the northeast river and shore- 

 line deposits of red color, and in the southwest marine limestones and clays. In 

 trans-Pecos Texas the lower Permian is mainly marine limestone with a smaller 

 amount of shale, and the sediments here have a thickness of about 8,000 feet. 

 Here again, although the exact relations between the Pennsylvanian and the 

 Permian are not yet known, it is probable that there was no great change in 

 conditions between the later Pennsylvanian and the earlier Permian. The 

 clearer and deeper sea, as before, lay to the westward. 



"There was a notable change in later Permian time. The upper Permian 

 sediments consist of red clays, and beds of limestone, frequently dolomitic, 

 gypsum, and rock salt. The gypsum and rock salt were deposited from the sub- 

 stances carried in solution by the sea-water upon the drying up of the sea. The 

 upper Permian basin of the Southwest centered somewhere beneath the region of 

 the Llano Estacado. It is very noteworthy that there are no coarse terrigenous 

 sediments in the upper Permian. The land-derived materials are mainly fine 

 clays. When sands occur, they are fine-grained. 



"We have in upper Permian time the condition of a nearly or quite land- 

 locked sea gradually shrinking through the drying-up of its waters. The sedi- 

 ments contributed to this sea were fine clays and sands derived partly from the 

 red sediments of the Wichita formation and partly from the maturely-weathered 

 residual soils formed during later Pennsylvanian and earlier Permian times. 

 Transportation of these flocculent clays and fine sands would not remove the 

 thin coating of iron oxide by attrition. Even if it did so remove it, the already 

 highly saline waters would not be likely to dissolve it. And even if they did dis- 

 solve the coating, the iron oxide would again be deposited before evaporation of 

 the sea-waters had reached a concentration high enough to deposit the gypsum 

 and salt. But it is most probable that the iron oxide coating was never removed 

 by attrition or solution. Fine sediments derived from residual soils are trans- 

 ported great distances by rivers of the present day without the removal of the 

 red coating. * * * 



"It is not necessary to assume that all red residual soils are formed from the 

 weathering of limestones. The Tertiary sediments of the southeast Texas Gulf 

 Coastal Plain are not limestones, yet their residual soils are red. * * * 



^ Taff, J. A., Preliminary Report on the Geology of the Arbuckle a!nd Wichita Mountains 

 in Indian Territory and Oklahoma, U. S. Geological Survey, Professional Paper No. 

 31. 1904- 



