PHYSICAL CONSIDERATIONS 307 



1 II ii.xi Ilium Avithdra^A•al of oceanic wateis from land ai'eas, and that tlic 

 succeeding system might proj^eiiy begin witli the initial return of these 

 waters. 



During a retreat of the sea the emerging land is likel}- to he eroded, 

 and a non-marine deposit laid down during its return is likely to be un- 

 cniirormablo witli tlic underlying fornuilions and to he closely related 

 st.nici nrally 1o the overlying ('(U-nuitions. Wliili' this prijiciple is aj)- 

 plicahlr in oi'(lin;iry cases, tlic |)hysiogra|)liic conditions under which the 

 ^rorrison wiis formed arc so cxtraordimiry that the possibility of an ex- 

 ception should he considered, [nasmuch as the Eocky Mountain region 

 was h)w in Morrison time, it seems possible that slight changes in the 

 ahitude of land relative to sealevel would change aggrading to degrading 

 streams, and vice versa. It is theoretically possible that a retreat of the 

 sea into which such streams discharged might have so lengthened their 

 courses as to reduce their gradients and cause them to deposit sediments. 

 There is, however, a limit to the possible thickness of deposits laid down 

 in this wa}^, and this limit seems to preclude the possibility that Morrison 

 deposition could be due alone to withdrawal of the sea. However, if the 

 unexpected did happen in this case and the Morrison was formed during 

 a time of retreat, it should be nlore closely related structurally and other- 

 wise to the underlying than to the overlying formations. 



I f the diastrophic movement that caused the retreat of the Jurassic 

 sea l)e accepted as terminating the period, the Morrison must have been 

 formed either during a time of emerging land — or retreat of the sea — 

 in which case it is Jurassic, or during a time of subsidence of land — or 

 advance of the sea — in which case it is Cretaceous. Inasmuch as the sea 

 did not return to the Eocky Mountain region until the latter part of the 

 Lower Cretaceous- (Comanchean of Cham1)erlin and Salisbury), it re- 

 mains to inquire whether the ]\Iorrison is more closely related to the over- 

 lying Cretaceous or to the underlying Jurassic. For this inquiiy it is 

 necessaiT to consider a gi-ou[) of formations that are either equivalent in 

 age to the Morrison or are closely related to it. 



equivalents; and AffSOCIATES OF THE MORRISON 



In character the Aloni-on formation Ihrcuighoiit Wyoming and eastern 

 Colorado corresponds essentially lo llic hods at the type locality at Morri- 



^ The reference of the Pm-gatoire formation, or I.owj^r rrctaceous, of the Rocky Moun- 

 tain region to the Wasliita epoch of the Lower rrotacoous Is in accordance with the 

 olasslflcation used liy the Geological Survey (:.'0). If. liowever, the Washita is Upper 

 • Tetaceous, as Berry asserts (-1, pages i:{0-1.37) and as Ilaug (Text-booit, pages 1100 

 iind 1293) and other European geologists l)elk've, the Purgatoire is also Upper Cretaceous 

 and the overlying Dakota sandstone therefore holds a position somewhat above the base 

 of this series. 



