30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



spread out over extensive areas on this plain proves that a large 

 part of it was practically unaffected hy warping during the deposi- 

 tion of the Jurassic sediments. 



The old valley seems to have been so near sea-level that a slight 

 land movement would shift the courses of the streams or even reverse 

 the direction of their flow, much the same as a relatively slight move- 

 ment now in central North America would modify the drainage 

 between the Hudson Bay and the Gulf of Mexico. Except in a few 

 localities where the sedimentary rocks of Jurassic age are thick, as 

 previously noted, the uniform thinness of the marine Jurassic rocks 

 (Sundance) indicates that the waters of the Jurassic sea spread out 

 over a nearly level floor. 



If the red color of sedimentary rocks and the occurrence of gypsum 

 are sufficient indications of aridity, southwestern America had not 

 recovered in La Plata time from the arid conditions that seem to have 

 prevailed there in Permian and Triassic time. Indeed the strong 

 colors of the Vermilion Cliff seem to have been responsible for the 

 early reference of this sandstone to the Triassic system. It seems 

 probable, however, that the arid conditions under which the sedi- 

 ments of the La Plata group gathered (pi. 1 ) were caused by the west- 

 ern mountains, as already suggested. For some reason not all of the 

 sediments brought into this old valley in La Plata time were carried 

 away. The physiographic conditions there may have been such as 

 would obtain in the Mississippi Valley should the climate for any 

 reason become so arid that the Mississippi River and its tributaries 

 would be unable to transport the material delivered to them. The 

 streams would then bring debris from the highlands and spread 

 it out over the lowlands, there to be reworked by the winds and 

 the local streams on a large scale. In this way relatively coarse 

 debris is now being spread out in the bolsons of the semiarid south- 

 west and in the " dead " valleys of the Great Basin. Perhaps still 

 better illustrations of these conditions are to be found in the great 

 deserts of North Africa, Asia Minor, and central Australia. To 

 complete the picture the desert should be practically at sea-level and 

 the water have easy access to it so that a slight subsidence of land or 

 a rise of water level would shift the strand line far up the valley. 



PROJECTION OF THE MARINE HORIZON BEYOND THE LIMITS 

 OF THE FOSSILIFEROUS BEDS 



Gypsum is commonly derived from sea water, and its occurrence 

 in fossiliferous marine Jurassic rocks proves that conditions were 

 favorable for the deposition of gypsum around the Jurassic sea. 

 The gypsum in the tin fossiliferous rocks is so near the same horizon 



