46 THE PERMO-CARBONIFEROUS RED BEDS OF 



of the sea-level, or the land-level, need have been only very slight to have 

 produced a widespread transgression, and a reverse motion, equally slight, 

 would have excluded the waters. That the effect upon the climate was 

 equally short Hved is indicated by the fact that, while the underlying clays 

 are devoid of iron, the limestone above is spotted by ferric oxide and weathers 

 to a rusty brown. 



The condition of the limestone is rather puzzling. From general consid- 

 erations it is supposed that the area of northern Texas, at least as far west 

 as the center of the Staked Plains, was a broad, fiat area of aggradation 

 crossed by large streams, a delta of the large size postulated by Barrell in 

 his discussion of the Paleozoic deposits of the Mississippi Valley, and that 

 this was normally more or less arid, its deposits freely exposed to the oxidizing 

 action of the air, and devoid of much vegetation. The advance of the 

 "Albany" sea inducing a greater humidity and plant growth on the land 

 ahead of it and a consequent increase in available CO, would have resulted 

 in the deoxidization and leaching out of the iron, resulting in the deposition 

 of the light-colored sandy clays. The further advance of the sea would have 

 continued the humid conditions, and we would find the light-colored clays 

 farther to the west and north, and the limestones which replaced them would 

 be free from ferric iron. Either of two explanations may be offered for the 

 presence of the ferric iron in the limestone: (i) The iron, leached from the 

 clays, leaving them white, may have been carried out to where limestone was 

 forming and there deposited as ferric iron; (2) the sea, whose western limits 

 we do not know, may have extended so far toward the land-masses of igneous 

 rocks that there was little of the coastal plain left, and there was no place 

 for the iron derived from the rocks to be deposited and then leached out 

 again before reaching the sea. 



The first of these explanations seems the more probable of the two. 

 From all indications, the land, either to the north or the west of where the 

 alternations of white clay and limestone appear, was so remote that it is 

 improbable that the limestone forming sea could have approached near to 

 them, and if, as supposed, there was an increase in the humidity due to the 

 encroaching sea, the degradation of the rocks would not have resulted in 

 the formation of ferric oxide, for Russell has shown that this would occur 

 only when there is a considerable annual period of drought or relative dry- 

 ness in the region subject to degradation. On the other hand, the white 

 clays probably gave up their original ferric iron as the carbonate, under the 

 action of the CO, derived from the increased vegetation, and would be 

 redeposited as hematite in the Hmestone forming in a body of shallow water 

 where there was an abundance of oxygen.^ 



A third hypothesis might be considered: that the white clays were origin- 

 ally deposited as red clays with a considerable content of ferric iron, and that 

 the ferric iron was leached out at a m uch later period by the a ction of pe r- 



M'^in Hise, Principles of Metamorphism, Monograph 47, U. S. Geological Survey, p. 844. 



