MOOK, STUDY OF THE MORRISON FORMATION 171 



lowest beds of the McElmo formation probably represent this period of 

 sedimentation. The exact geological age of these lower McElmo beds 

 depends upon the decision of the question regarding the post-Sundance 

 deposition and erosion. If these beds were deposited immediately after 

 the highest Sundance, they are upper Jurassic in age; if they were de- 

 posited after a post-Sundance-pre-Morrison erosion interval, they may be 

 upper Jurassic or basal Comanchean. To which of these two periods the 

 lower beds of the McElmo really belong is not especially important from 

 the point of view of the present paper. 



Prom the greater thickness of the Morrison beds to the west, and from 

 the larger amount of coarse material in the formation in its western 

 occurrences, conditions which are distinctly shown in the descriptions of 

 the formation, though there are local variations from them, it seems to 

 the writer almost certain that the source of the materials comprising the 

 Morrison came from mountain areas to the west of the present area of 

 the formation. 



After the lower beds of the McElmo phase of the Morrison were de- 

 posited, the formation was extended to the east, northeast and southeast. 

 On such a plain as the one above indicated, it would be possible for a 

 drainage system similar to that now existing in eastern China to develop. 

 Such a drainage system, with large overloaded rivers, swamps, lakes and 

 interlacing connecting streams was inaugurated. ^ZEolian sedimentation 

 no doubt accompanied the stream deposition to a certain extent. As the 

 formation increased in thickness it also spread farther out, the upper beds 

 overlapping the lower ones. It seems probable that the lower beds of the 

 Morrison formation in its eastern areas are not to be correlated in age with 

 the lower beds of the McElmo in the western areas, but are later. This 

 extension of the formation undoubtedly took a considerable length of 

 time. The section shown in a given locality at present does not usually 

 represent continuous deposition in that area, but in many cases at least 

 represents an alternation of erosion and deposition, with deposition pre- 

 dominating in the long run. A given thickness laid down under such 

 conditions may represent as long a time interval as a thickness three or 

 four times as great, or more, deposited under conditions of continuous 

 deposition. 



The Unkpapa sandstone, in the Black Hills region, may represent an 

 seolian deposit laid down on the Morrison plain after the retreat of the 

 Sundance sea and before the Morrison sediments had been extended to 

 that point. 



Under the conditions of alternating deposition and erosion indicated 

 above, it would not be necessary for the beds in every section to eorre- 



