FAUNAL CONSIDERATIONS 305 



There i.s one other consideration connected with the fauna that T wish 

 to submit before re\ie\\ing the physical evidence. It is well known that 

 the dinosaur fauna of the ]\Iorrison is so extensive and varied that it has 

 been regarded as the (■ulininating fauna of tlie age of reptiles (5, page 

 97). It is not strange tliat hind faunas, developed as they are in regions 

 of general lock destruction, appear in maximum development Avithout 

 the preliminarv stages where some exceptional condition makes })0ssible 

 their preservation. A fauna like that of the Morrison warrants the belief 

 in a long period immediately })receding, during which physical conditions 

 wer(! favorable for its development. If the j\Iorrison is Jurassic in age, 

 the time available for the extraordinary differentiation of reptilian types 

 seems insufficient and makes maximum development coincident witli min- 

 imum land expansion. If, however, the Morrison is Lower Cretaceous 

 and late Lower Cretaceous, as the physical evidence seems to indicate, 

 ample time for this differentiation is afforded by the long intersystemic 

 interval of maximum expansion of land areas which preceded its deposi- 

 tion. Furthermore, a natural consequence of such extensive and long- 

 enduring land conditions is a corresponding extensive prevalence of sur- 

 face erosion. Both the causal conditions of maximum land growth and 

 its consequent erosional effects constitute criteria of generally admitted 

 taxonomic value, the former being generally regarded as approximately 

 delimiting geologic periods, while the latter results in the major uncon- 

 formities which are properly used in separating geologic systems. That 

 the results of this erosion are not more obvious in some places is doubt- 

 less due to the fact that the lands of the Kocky Mountain region were 

 approaching the final stage of baseleveling. 



Physical Considerations 

 in general 



I'x'caiisc the |)ah'oii(()higic evidence will be presented by others, 1 shall 

 luive little to say of il aside from the above brief remarks and shall 

 approach Hie piohleiii fi-inii a physical standpoint, taking whai iiia\ he 

 ealleil ;i iilii/siiit/ni/iliic \ iew. 1 iiasimieh as many geologists maintain that 

 it is ini|ii-o))er to appioaeh a question of geologic age I'roni a physical 

 standj)oint, it seems advisable to make a brief statement of some of the 

 princi|)les on which my views are based. 



recogrnizcd as the Morrison In Moiituiia. Hecause of tho close association and lithologlc 

 similarity of tlipso formations, thorc is a foelins on the part of some KeoloRlsts, among 

 them ('ami)l)ell and lierry, who have studied the question, that there is no essential 

 difference in age between them. 



