NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 3 1 



as to render it probable that water from the Jurassic sea had access 

 to localities beyond those where marine fossils have been found. 

 Where tracing- of the beds is possible, ultimtae correlations naturally 

 will depend on detailed work. But in many places such tracing- will 

 not be accomplished for many years to come. Also there are places 

 where tracing is impossible because of erosion or because of cover 

 by younger rocks, and other methods must be employed. As gypsum 

 is derived chiefly from sea water, it is difficult to understand where 

 the gypsum which occurs above the White Cliff sandstone in Utah 

 and above the Wingate sandstone in New Mexico came from if not 

 from the waters of the Jurassic sea. The correlation of the gypsum 

 beds above the Wingate sandstone in northwestern New Mexico and 

 above the Exeter sandstone in northeastern New Mexico with the 

 marine Jurassic beds is in harmony with the correlation of these two 

 sandstones with sandstone below the marine beds in Utah and else- 

 where. Also, by means of the gypsum below the Morrison, the corre- 

 lation may be carried northward through regions east of the moun- 

 tains where the Exeter is not known, to southern Wyoming, there 

 to connect again with the marine Jurassic (Sundance). Gypsum 

 occurs in many places east of the mountains in Colorado at or near 

 the same horizon and so near the line of separation between the 

 Morrison and the older formations that in the supposed absence of 

 an intermediate division it seems to have been chiefly a matter of 

 personal judgment whether it should be classed with the beds above 

 it or with those below. The correlations here suggested indicate 

 that the gypsum and associated rocks may be of Jurassic age and 

 represent a distinct stratigraphic horizon between the Morrison and 

 the underlying " Red Beds." 



Throughout the region here described, the gypsum classed as 

 middle La Plata occurs in relatively isolated bodies, as if it had been 

 deposited in separate basins. There are several possible explanations 

 for this manner of occurrence, four of which are suggested below. 



(i) A continuous bed or series of overlapping beds of gypsum 

 may have been formed and later cut away in some places by erosion. 

 The lack of evidence of such erosion renders this explanation 

 improbable. 



(2) Gypsum derived from upland sources, as, for example, from 

 the erosion or solution of older deposits, may have accumulated in 

 inclosed basins in some such manner as gypsum beds are forming 

 now in the vicinity of Alamogordo, N. Mex., where the gypsum is 

 derived from older gypsiferous beds in the surrounding hills. The 

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