THE ORIGIN OF RED BEDS 177 



gypsum .... can be changed into anhydrite by a concentrated solution of 



sodium chloride These facts are of interest as pointing to the possibility 



of dehydration of minerals in rocks, in contact with salt solutions, at a tempera- 

 ture considerably below their normal inversion point. 1 



.... The occurrence of red ferruginous sandstone in conjunction with 

 layers containing brown hydrated ferric oxide is less readily explained [than 

 the superficial dehydration of soils, mentioned in the quotation on p. 174, supra], 

 but the dehydration of certain beds may have been effected by contact with 

 salt solutions, as in the case of gypsum already referred to above. 2 



Dehydration by contact with salt solutions presumably would 

 affect the more porous beds first; whereas in the western Red Beds, 

 as already described, 3 it is generally in the more porous beds that 

 the lighter colors occur, and the shales are in general of deeper hue 

 than the sandstones. Just how far the action of salt solutions may 

 have been effective, both during and since sedimentation, in accom- 

 plishing dehydration in the neighborhood of such saline deposits as 

 occur in the Red Beds, it would be difficult to say; but for the group 

 as a whole it appears that this agency cannot have been of general 

 importance. 



In summation, it may be said that while widespread dehydration 

 of iron oxide in the Red Beds since sedimentation cannot, at present, 

 be proved not to have taken place, the greater weight of evidence 

 now at hand is opposed to it; that the opposite process, hydration, 

 may well have been active in the more pervious beds of the series ; 

 and that, therefore, the probabilities are quite as much in favor of a 

 lower degree as of a higher degree of hydration, on the average, in 

 the Red Beds at the time of sedimentation than at present. 



WAS THE COLORING MATTER A CHEMICAL OR A 

 MECHANICAL SEDIMENT? 



Having determined as nearly as the available evidence permits 

 the condition of the coloring matter of the Red Beds at the time of 

 their deposition, we may proceed to inquire as to the geographic 

 conditions which gave rise to sediments so colored. The first 



1 Elsden, op. cit., pp. 85-86. See also H. Stremme, "Zur Kenntnis der wasser- 

 haltigen und wasserfreien Eisenoxydbildungen in den Sedimentgesteinen," Zeit. fiir 

 prakt. Geol., January, 1910, pp. 18-23. 



2 Elsden, op. cit.' p. 97. P. 158-59. 



