CONCLUSIONS AND REFERENCES 313 



('<)N(!H'SIONS 



"^riK^ wciyiit (if ]iliysinur;)))liic ;iiiii other evidence liere c'oiisidered vseems 

 to wainiiit the iissiynmoiit of the JMorrison I'ormation hi I ho Lower Cre- 

 taceous or ( 'oiiiaiiche seiies for the following reasons: 



1. lis |)hinl and \-ei'tehi'a(e fossils indicate close i-ehil ionship to the 

 Arunih'l and I'alii.xent iorniations of the I'otoniac and to tlie Wealden, 

 both of which are now general]}' regarded as Lower Cretaceous. 



8. The evidence of a long period of erosion preceding the Morrison, 

 together with the culmination of the reptilian fauna during this land 

 stage, agrees best with the assignment of the Morrison to Lower Cre- 

 taceous. 



3. It is much more closely allied structurally with overlying formations 

 of mid-Cretaceous age — late Lower Cretaceous and early Upper Cre- 

 taceous — than it is to the underlying formations. 



4. The overlap of the Morrison on a variety of older formations is 

 indicative of a long interval of erosion. 



5. The sea which occupied portions of the Eocky Mountain region in 

 late Jurassic time was expelled by a movement that is regarded as a part 

 of the diastrophism which brought the Jurassic period to a close. 



. 6. The physical character of the ^Morrison and its relations to con- 

 tiguous formations indicate that it was deposited on a peneplain at a 

 time soon after the beginning of the Cretaceous subsidence, when the 

 surface was too near sealevel for further degradation, but not yet low 

 enough for marine sul)mergenee. It is therefore the first sedimentary 

 expression in tlie Eocky Mountain region of the new order of events that 

 culminated in the occupancy of the interior of Xorth America by sea- 

 waters in Cretaceous time. It is a non-marine forerunner of the Cre- 

 taceous niai'ine formations and thei-efore of Cretaceous age. 



References 



1. E. O. Ulricli : T\w Ordovician-Silurian honndary. Congre.s Geologuiue In- 

 ternational. XIIo session, Can.-idii, ]!)i;;, pages 59.3-667, 1914. 



'2. T. W. Stanton : A comparative study of the Lower Cretaceous formations 

 and faunas of the United States. .Journal of Geology, volume v, 1897. 



.'5. K. S. Lull: Tjic Keptilia of the Annidel formation. Maryland (^.eological 

 Survey. Lower Cretaceous. 1911. 



4. Kdwai'd W. Hei-ry : Correlation of the Potomac formations. .Maryland 



Geological Surve\. L.iwcr Cretaceous. 1911. 



5. T. C. Cliamlierlin ;;nd R. 1). Salisbury: Text-book, volume iii, 1906. 



(». S. F. Emmons: (Jeology of the Denver Basin in Colorado. U. S. Geological 

 Survey Monograph 27, 1896. 



