TEE ORIGIN OF RED BEDS 247 



The extraordinary development of gypsum in the Permian 

 Red Beds deserves more than passing comment. In association 

 with salt, deposits of gypsum are interpreted as indicating aridity 

 of climate at the time of deposition, and of formation in at least 

 partially inclosed basins by the evaporation of bodies of water 

 not freely connected with the open sea. 1 This occurrence of 

 gypsum, supported by independent evidences of continental origin 

 for the Red Beds, has been the one strongest influence in establish- 

 ing the idea that the red color itself is an indication of aridity. 

 The absence of gypsum from many series of Red Beds, and its 

 occurrence in series free from Red Beds, make it necessary to investi- 

 gate the two problems on their own independent merits. 



Rock salt is of relatively rare occurrence in the group of sedi- 

 ments under discussion. Since the saturation point of gypsum in 

 aqueous solution is much lower than that for common salt, it is 

 logical to suppose that the deposition of gypsum unaccompanied 

 by rock salt signifies a condition of aridity and of continuous or 

 intermittent supply of normal sea-water or of fresh water such as 

 to maintain a degree of salinity more moderate, for example, than 

 that of Great Salt Lake at present, but sufficient to cause the con- 

 tinued precipitation of gypsum. Such a condition might be kept 

 up by a limited or intermittent connection between the open sea 

 and the basin of deposition. 



The relation of the gypsum and salt deposits of the West to 

 the Red Beds proper suggests a relation similar to the relation 

 between marine limestones and terrigenous sediments. The 

 Rustler dolomite and the Castile gypsum of the Texan portion of 

 the Pecos Valley give place northward to typical Red Beds with a 

 few interbedded strata of dolomite and gypsum. May not the 

 gypsum, as well as the dolomite, be but the complements of the red 

 clastic sediments, deposited in the clear central waters of an inland 

 sea, or in lagoons near or at sea-level but partly or wholly cut off 

 from the sea ; while clastic sedimentation went on nearer the shores 

 and on river deltas or flood-plains yet nearer to the sources of 

 sediment ? 



1 Cf. Wilder, op. cit. 



