38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



the water ? Also, did this withdrawal occur within the period or at 

 its close? At this point a definite answer is needed to the question 

 previously asked, Was the sea drained quickly or slowly? If the 

 Jurassic was a period of continental repose, it would seem appro- 

 priate to regard any diastrophic movement which had sufficient 

 magnitude to cause the drainage of the submerged portions of the 

 continents, as appropriately marking the close of the period. It 

 seems probable that the draining of the sea was due to the first or 

 introductory movement — perhaps a relatively slight one — of the 

 diastrophism which characterized the Cretaceous period. 



In western North America the Jurassic period was closed by 

 crustal movements called the Sierra Nevada disturbance by Schu- 

 chert; the Sierra Nevada movement by Whitney; the Nevadian 

 movement by Blackwelder ; and the Cordilleran revolution by Smith. 

 It seems probable that the Jurassic movement to which S. F. 

 Emmons * ascribed the expulsion of the Jurassic sea from the Rocky 

 Mountain region was part of this diastrophic movement which closed 

 the period ; for there seems to be good reason for doubting that this 

 so-called Jurassic movement was a local movement of land in the 

 Southern Rocky Mountain Province. The draining of the sea may 

 have been due to a general lowering of sea-level, caused perhaps by 

 the sinking of some part of an ocean basin. And it seems reasonable 

 to believe that this movement was a part of the diastrophic dis- 

 turbance which terminated the long period of continental repose and 

 introduced the equally long period of diastrophic activity which fol- 

 lowed. If the land in the Southern Rocky Mountain Province was 

 elevated at all, the elevation must have been slight. The sediments 

 of the Morrison formation were spread out over a nearly level plain 

 only slightly above sea-level and nearly 1,000 miles long and 500 

 miles wide. It is inconceivable that the nearly base-leveled area 

 covered by the Jurassic sea could have been elevated to any great 

 extent and then have settled back to form the extremely regular sur- 

 face on which the Morrison sediments were deposited. Such a plain 

 could have been formed only under the gradational processes of 

 standing or running water. In this, as in the maximum advance of 

 the sea, there seems to be an analogy between America and Europe, 

 for in Europe the period is described as closing with a retreat of the 

 sea from the continent without notable disturbance of land. 



1 Emmons, S. F., U. S. Geol. Survey Mon. 27, p. 21, i* 



