DENVER FORMATION. 35 



Abundant plant remains arc found in the Laramie and Denver for- 

 mations, which have hitherto been classed together as belonging to one 

 continuous and uninterrupted scries ©f beds. A careful revision of all 

 the fossil plants collected from these and corresponding horizons has 

 shown, however, that the floras of the Laramie and Denver periods were 

 quite distinct. ( If those collected in the Denver Basin (24(1 species in all) 

 only K> per cent are common to the two horizons. In the post-Laramie 

 formations of Middle Park and Montana over 75 per cent of tin- plant 

 remains are common with those of the Denver beds. 



These facts, taken together with the stratigraphical evidence in this 

 and in other fields of a great time-interval and physical break intervening 

 between the original Laramie and these later formations, while as yet there 

 is no evidence of any important physical break or erosion period in the 

 time intervening between the deposition of the Denver and of the suc- 

 ceeding Eocene formations, seem to render it in the highest degree inad- 

 visable to include these two later formations under the general head of 

 Laramie, as has hitherto been done and as some paleontologists would still 

 do. The post-Laramie formations, as they have been provisorily called, 

 constitute a very important part of the geological column, which, up to the 

 time these investigations were undertaken, had either been entirely over- 

 looked by geologists or else confounded with underlying or overlying 

 formations, as the case might be. Consisting, as they generally do, of soft, 

 slightly compacted material, they have been readily eroded, and as their 

 remnants are generally found in regions where the strata occupy a nearly 

 horizontal position — that is, where the unconformities to be observed are 

 those of erosion and not of angle of dip — they are not likely to be recog- 

 nized as distinct from preceding <>r succeeding formations in ordinary 

 reconnaissance work. Beds that occupy a corresponding position with 

 these formations have been recognized stratigraphically by the present 

 observers at so many points on the periphery, as well as in the interim' of 

 the Rocky Mountain uplift in Colorado, as to indicate a general prevalence 

 of similar conditions of sedimentation throughout the region in post- 

 Laramie time. The fossil fauna of most of these exposures is, however, 

 not yet known. On the other hand, at the several exposures from which 



