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ANNALS NEW YORE ACADEMY OF SCIENCES 



The presence of small stratigraphic breaks at many localities and levels 

 in the Morrison formation emphasizes the force of the statements of 

 Hatcher, that in the production of a continental formation of the char- 

 acter of the Morrison, the main process of deposition is not continuous 

 for any given area. The process is rather one of alternating deposition 

 and erosion, deposition being the dominant factor. The situation is 

 analogous with the conditions at the front of a glacier, v^^here the ice 

 front may stand still, while the actual ice advances, through melting at 

 the front. If melting goes on faster than ice advance, the front retreats; 



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I o I a I "I ° I ■" ■ ir-r 



. [ o I * I o I A I o 1 o I oi o I o I o I — -a I — o \ o~r 



Pig. 83. — Diagrammatic section of the exposure of the Morrison formation at the Marsh- 

 Hatcher dinosaur quarry, near Canon City, Colorado. 



if melting goes on slower than ice advance, the front advances. In the 

 case of a continental formation ultimate deposition of a considerable 

 thickness would be brought about by excess of deposition over erosion. 

 If erosion predominated over deposition, there would be no formation 

 produced, but a great stratigraphic break. 



In considering the age of such a formation as the one under considera- 

 tion, it must be remembered that deposition under the conditions indi- 

 cated above will be much slower in producing a great thickness of beds 

 than under conditions of continuous deposition. A total thickness of 



