40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



these layers may occur below or even be interbeddecl.with the gypsum. 

 Careful observation is needed to determine, first, whether the unfos- 

 siliferous limestone is, in fact, of fresh-water origin and, second, 

 whether it is actually at the same horizon as neighboring beds of 

 gypsum. It is conceivable that bodies of fresh water and of salt 

 water existed side by side. It is also conceivable that the bodies of 

 shallow water shifted from place to place as the surface was built up. 



If the significance of the gypsum is correctly interpreted, the saline 

 water extended south into New Mexico and east over much of central 

 and eastern Colorado. Because of later erosion, it is not possible now 

 to determine how much of the present mountainous region was cov- 

 ered by it, but the occurrence of the gypsum on both sides of the 

 Rocky Mountains indicates that the marine water may have covered 

 considerable portions of the present mountainous area. The large 

 areas where no La Plata rocks are known render the occurrence of 

 Jurassic " islands " possible. Also, because of erosion, the maximum 

 southern extent of the sea cannot now be determined and because 

 of cover by younger rocks its eastern extent is not known. 



It is not easy to picture the physiographic conditions of the 

 Southern Rocky Mountain Province in late Jurassic time. I doubt 

 if there is an area of any considerable size in the world to-day that 

 exhibits an approximation to them. The whole province seems to 

 have been so far degraded that a change of a few feet in the water 

 level of the sea would have shifted the strand line many miles. An 

 advance of the sea over such a peneplain would produce an intricate 

 pattern of shallow, interlacing channels, bays, lagoons, gypsum pans, 

 and salt marshes. 



Continental deposits probably continued to accumulate around the 

 sea during the whole period that it occupied the interior of the con- 

 tinent. Following its retreat they must have still continued to 

 accumulate and to cover the marine beds in some places. These 

 accumulations constitute the upper part of the La Plata group, 

 which seems to form a wedge entering from the west and thinning 

 eastward between marine Jurassic beds and those of the McElmo 

 or Morrison formation. The rocks of this wedge are not as regular 

 in thickness as those of the lower La Plata. They thin out to the 

 south and east and also in some places in the midst of the area 

 where the upper La Plata is typically developed. Where these beds 

 are absent, the Morrison and its age equivalents rest on the marine 

 beds or Sundance wherever this formation is present; on the beds 

 of gypsum which are believed to be the age equivalents of the Sun- 



