NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS LEE 33 



the gypsum beds in many places, but occurs at some localities where 

 the gypsum is absent. Darton found this limestone so persistent and 

 its peculiar character so constant that it proved valuable in tracing 

 the formations over wide areas. He describes its occurrence as far 

 east as Las Vegas, N. Mex. 1 An impure limestone of somewhat 

 different character occurs near the gypsum in northeast New Mexico. 2 

 Although this was examined long before there was any suspicion that 

 special significance might be attached to it, it was found to be 

 enough different from the limestones of the overlying Morrison to 

 attract attention. It seems probable that by reexamination of sec- 

 tions described years ago, this limestone and gypsum horizon in 

 eastern New Mexico and Colorado may be identified as middle La 

 Plata in age. 



INVASION OF JURASSIC SEA 

 EXTENT AS DETERMINED BY FOSSILS 



The sea water entered the interior of the North American conti- 

 nent in late Jurassic time, submerging a large part of the Northern 

 Rocky Mountain Province and extending southward through Utah 

 to Arizona. The waters of this sea apparently represent the maxi- 

 mum submergence of land during the Jurassic period. It is possible, 

 of course, that this advance of the sea water may have been due to 

 a subsidence of land, but it is equally possible, and in my opinion more 

 probable, that the submergence was due to a rise of sea-level and 

 that the water flowed over the lower parts of the old valley in much 

 the same way that a rise at the present time would cause the Gulf 

 waters to submerge the lower part of Mississippi Valley. 



The area covered by this Jurassic sea has been outlined by W. N. 

 Logan 3 (fig. 1, p. 11) to include all localities where marine Jurassic 

 fossils have been found. There is a tendency to assume that the 

 boundary line marks the maximum extent of the sea, whereas in 

 reality it denotes only the extent of fossiliferous strata. Sediments 

 were probably accumulating under conditions which were not favor- 

 able for marine life, over a much larger area. 



Relatively little can be learned now of the physiographic conditions 

 west of this sea because of erosion since Jurassic time, but in the 

 southern and eastern parts of the area occupied by it there is 

 abundant evidence that the water spread out in a thin sheet over 



1 Personal communication. 



1 Lee, W. T., Jour. Geol., vol. 10, p. 46, 1902. 



3 Logan, W. N., Jour. Geol., vol. 8, pp. 241-273, 1900. 



