MORRISON FORMATION. 23 



Its thickness in the Denver field is somewhat variable from point to point, 

 the variation being assumed to be mainly the result of erosion. 



The Morrison formation is chiefly remarkable for the abundant 



remains of gigantic saurians and other reptiles found in its beds, together 

 with fishes, birds, and a few diminutive mammals. The abundance of the 

 land animals and the discovery of fossil plants in the heels is further proof, 

 if anv were needed, that they were formed along the shores of a large 

 land mass. Molluscan remains, of fresh-water habit, indicating that the 

 sediments were deposited in an inclosed lake, are also found in these beds. 



As both molluscan and plant remains found at this horizon have 

 too wide a range to be of value in the determination of the age of the 

 inclosing beds, this determination has been based exclusively upon the 

 vertebrates. Although the latter have some affinities with the European 

 Wealden or Lower Cretaceous, to which Professor .Marsh was at first 

 inclined to assign the horizon, he found the evidence in favor of late 

 Jurassic age to be so much stronger that he assigned to this period not 

 only these but other beds with a similar fauna, notably the Potomac beds 

 of the East, which on other grounds had been considered early Cretaceous 

 and which present many structural analogies with the Morrison beds. 



From the point of view of the stratigrapher, the assignment of the 

 Morrison beds to the Lower Cretaceous rather than to the Upper Jurassic 

 is much more desirable, not only because it accords better with the sequence 

 of sedimentation thus far disclosed in the adjoining regions of Kansas and 

 Texas, but because it places the physical break whose effects are recognized 

 over the whole continent between these two great time divisions rather 

 than in the midst of one of them. 



EARLY CRETACEOUS MOVEMENT. 



It has been hitherto assumed by the writer and others that a move- 

 ment must have occurred in the Rocky Mountain region between the time 

 of deposition of the Morrison or Atlantosaurus beds and those of the 

 succeeding Dakota formation, now considered as the base of the Upper 

 Cretaceous. In this interval a considerable thickness of earlier Cretaceous 

 beds has been deposited in other regions, notably the Comanche series 



