252 GEOLOGY OF THE DENVEK BASIN. 



the close of Mesozoic time- in the mountainous regions of western North 



America. 



It has been generally assumed that the revolution caused in some 

 way the greal change in life observed in the fossils of the earliest Eocene 



deposits as compared with those of the ( 'retaceous. But recent discoveries 

 show that the nap in lite grows less as knowledge increases, while accu- 

 mulating evidence continually enhances the importance of the orographic 

 movement occurring between the Laramie and Arapahoe epochs. 



Applying tin- criterion of continental development as it has been 

 applied in the past, the pre-Arapahoe movement, which terminated the 

 long- series of conformable Cretaceous sediments, murks the end of Mesozoic 

 time. Applying the criterion of life, and especially of vertebrate life, the 

 post-Laramie epochs may be assigned to the Cretaceous. 



It is not the writer's desire to advocate the establishment of a dual 

 nomenclature for the case under discussion, but simply to recognize this 

 aspect of the question as the logical deduction from the evidence that has 

 thus far been presented. Investigation must be carried on in main' direc- 

 tions before the relative importance of the various factors of this problem 

 can be established. The importance of the pre-Arapahoe movement in 

 comparison with later ones which are not as yet so clearly defined must be 

 carefully demonstrated. The distribution of all classes of fossil remains 

 through the series of formations must be studied, and the extinction of the 

 Mesozoic types of vertebrates must be connected with the great movement 

 of orographic importance by something more tangible than mere assumption 



section rsr.— MONUMENT creek formation. 



By George H. Eldridge. 

 STRATIGRAPHY. 



The name "Monument Creek" was first applied by Dr. F. V. llayden 

 to the series of strata which forms the prominent divide between the Platte 

 and Arkansas rivers, extending from the base of the Colorado Range east- 

 ward. This use of the term is provisionally accepted in this report. About 

 the middle of the series is a well-detiued break in deposition, the divisions 

 above and below which may, upon systematic study, lie found to be 

 distinct formations. 



