CONDITIONS OK THE (iVPS^'^[ DEPOSITS . 223 



4. The beds are often discontinuous. 



5. The beds occur at many horizons, but in western Wyoming ;iiv 

 mainly near the top, and some are near the top in must ot the nivas ot 

 outcrop. 



6. The gypsum is remarkably pure and salt has nevei' been reported 



in association with it. 



Hypothesis por Origin op Gypsum Deposits 

 statement of the hypothesis 



The gypsum deposits of the upper Eed Beds originated Itohi concen- 

 tration of fresh water under arid climatic conditions. 



In order to test this, tlie topography of the area, drainage conditions, 

 content of stream waters, and rate of evaporation were studied. 



TOPOGRAPHY OF DRAINAGE AREA SUPPLYING WATER FOR THE DEPOSITS 



The topography of the region surrounding the Eed Beds basin in 

 which the gypsum was deposited is conjectural, but the topography of 

 the area of the basin itself may be postulated with tolerable certainty. 

 No unconformity that is detectable has been seen in the upper beds, and 

 if the red sandstone was exposed for many thousands of years, it must 

 have been very low and flat to avoid leaving traces of erosion. The Red 

 Beds occur over most of eastern Wyoming, with an area of some 40,000 

 square miles, and the sand-covered plain must have been of about that 

 extent. The source of the red sands is uncertain, but the uniformity in 

 thickness and continuity of the Eed Beds indicates that the places in- 

 vestigated are not near the margin of the area of sedimentation, and 

 thinning to. the eastward indicates that the highlands were to the west. 



RELATION OF RUN-OFF TO PRECIPITATION 



Streams with low gradients flowing through arid regions are likely to 

 carry only a small amount of the rainfall of their drainage basins. At 

 Uva, Wyoming, the run-off carried by the Laramie Eiver is usually les-^ 

 than 10 per cent of the rainfall of its basin, with an area of 3,1?9 square 

 miles.^- In 3896, 1897, and 1898 Ihe run-off of the Arkansas Eiver at 

 Hutchinson, Kansas, with a fall of 5.G feet per mile, was less than .3 

 of 1 per cent of the rainfall of its basin; at Canon City, Colorado, I'M 

 miles from the head of the Arkansas, and above which the fMll langes 

 from 30 to 63 feet per mile, the run-off is 20 to 30 per cent of llic rnin- 

 iidl ; at Oranite, Colorado, 2 I miles from the head of the Arkaii<,is. ,ind 



'* Twentieth Annual Report of U. S. Geological Survey, vol. Iv. p rA. 



