AGE OF THE ABAPAHOE AND DENVER. 209 



relation of such evidence to the problems of stratigraphical and historical 

 geology. 



Phases of the problem. Tl 1( ' (l'lS( • I ISsitll 1 < -f tllO age of tilt' A l\l ] l.'l 1 1< It ' ,'111(1 I >. 1 1- 



ver formations is thus seen to involve many points of interest which may 

 be summarily stated as follows: 



1. Age of the two local formations. 



"_'. Age of numerous allied formations. 



3. Relations tit', and weight to be assigned to, various classes of 

 evidence. 



4. Condition of knowledge of the faunas and floras of the formations 

 in question. 



5. History of the period between Mesozoic and Cenozoic times in 

 the Rocky Mountains. 



6. Designation of the boundary between these great divisions in the 

 Rocky Mountains. 



7. Relation between physical changes and fauna] modifications in 

 the general period involved. 



8. Relations between the chronological systems necessary to express 

 the history of physical changes in the earth, and of the development of 

 life upon it, i. e., the desirability of a dual nomenclature. 



It would be manifestly out of place to discuss all these broad questions 

 in detail in a volume of essentially local character, and such a discussion is 

 in this instance at present impossible, because of imperfect knowledge in 

 many directions, but it will be the aim in the following pages to present the 

 evidence in the special case in such a way as to indicate or suggest its value 

 in respect to some of the broader problems which naturally develop from 

 the one of local importance. 



EVIDENCE OF LITHOLOGIC CONSTITUTION. 

 Conglomerates of the Arapahoe beds. The conglomerates of the A HI] IJll It le contain 



evidence of events in the long interval that separated their deposition 

 from that of the sandstones and clays of the Laramie — events which can 

 not be disregarded in spite of the apparent conformability of the two 

 formations as now exposed in many places. Mr. Eldridge has described the 

 MON XXVII It 



