THE LARAMIE FORMATION AND THE SHOSHONE GROUP 43 



the Shoshone to the great succession of Tertiary local deposits. The 

 retreat of marine waters and the decided uplift of a large continental 

 area marked the appropriate boundary between Cretaceous and 

 Eocene from the stratigraphic side. The plant life seems to show 

 no special reason for drawing the line at any other horizon. 



The argument most effectively used for including the Shoshone 

 beds in the Cretaceous has come from the vertebrate remains of 

 that group. The Mesozoic affinities of that fauna are said to be so 

 pronounced that paleontologists have been unwilling to consider the 

 proposition of referring beds containing that fauna to the Eocene. 

 The intimate relations of the fauna to its ancestors are not surprising 

 and as a large part of the fauna became extinct in Shoshone time 

 the absence of a comparable line of descendants is easily compre- 

 hended, but the stratigrapher naturally raises the question— Why 

 is the extinction of the huge vertebrates of the Shoshone epoch to be 

 considered as determining the Mesozoic-Cenozoic line? A quarter 

 of a century ago it was the common belief of stratigraphers and 

 paleontologists that the "great Rocky Mountain revolution" so 

 changed conditions of environment affecting the living forms of the 

 time that the Mesozoic vertebrate fauna in particular could no longer 

 exist. Little was then known of this revolution and yet an almost 

 catastrophic influence of the uplift upon life was assumed as natural. 

 Now that the change from marine to permanent continental condi- 

 tions is known to have been marked by oscillations and the verte- 

 brate fauna is shown to have survived the very changes once assumed 

 to have been fatal to it the cause of extinction becomes a mystery, yet 

 is held to have the same great significance formerly assigned to it. 

 This conclusion is surely open to question. 



The little mammals fouad in the Converse County beds have been 

 charged with exterminating the huge dinosaurs, but Lull rejects this 

 hypothesis and reverts to orogenic movement as the cause.^* But 

 there is at present no available evidence to show a movement of any 

 unusual importance between the Denver and Fort Union epochs. 



It has been suggested by Peale that the extinction of vertebrate 

 faunas in the Miocene and Pliocene periods was due to widespread 

 showers of volcanic ashes and the same cause has recently been 



2* The Ceratopsia, U. S. Geol. Surv. Mon. XLIX, p. 



