WHITE ON PALEONTOLOGY. 627 



the adjacent parts of Utah aud Colorado, whether vertebrate or inver- 

 tebrate^ will leave no question in the mind. of a paleontologist as to 

 their Cretaceous age. A similar examination of the fossils of the Green 

 Eiver aud Bridger groups will disclose equally conclusive evidence of 

 their Tertiary age. There are two other groups that come in the series 

 between the Fox Hills group below and the Green Kiver group above 

 that contain fossil remains concerning which paleontologists are divided 

 in opinion. These are the Laramie aud Wahsatch groups, the relations of 

 which to the other groups here mentioned are shown in a table embraced 

 in Paleontological Paper No. 4. It is therefore certain that the plane of 

 demarkation between the Cretaceous and Tertiary, if such really exists 

 in nature, must be sought for within the vertical limits of these two 

 groups. It should be remarked, hx)wever, that all geologists aud pale- 

 ontologists, so far as I know, agree in referring to the Tertiary period 

 all of the Wahsatch group as I have defined it, except some 500 to 800 

 feet of strata that form its base. 



These lower strata contain a greater or less proportion of brackish - 

 water species of Mollusks, which are very closely allied with those of 

 the Laramie group, while the upper portion of the former group con- 

 tains only fresh- water species, all of which are closely related to those 

 of the Green Eiver group above it. Therefore the debatable grouud is 

 reduced to the Laramie group, together with the lower ijortion of the 

 Wahsatch group. Some hold the opinion that all the strata thus in- 

 cluded should be referred to the Cretaceous period ; others. have been 

 inclined to refer the whole to the Tertiary; and still others select the 

 upper limit of the Laramie group as the proper plane of separation be- 

 tween the strata of the Cretaceous and Tertiary periods. These differ- 

 ences of opinion would alone suggest that no well-marked plane of sep- 

 aration, either stratigraphical or paleontological, really exists in that 

 region between the strata of the Cretaceous and Tertiary periods, even 

 if more direct evidence were wanting. Dr. Ilayden first,* and very 

 plainly, pointed out the fact that no definite plane of demarkation 

 between the Cretaceous and Tertiary strata could be designated. This 

 is indeed just what we ought naturally to expect where the geological 

 series is complete, and the deposition of sediment was continuous or 

 nearly so. In such a case, there is no reason in nature to expect either 

 the stratigraphical or paleontological transition from the uppermost 

 Cretaceous to the lowest Tertiary group to be any more abrupt than it 

 is between any other groups of those periods respectively. 



The differences of opinion before referred to are understood to be 

 largely due to the special character of the subjects of investigation re- 

 spectively pursued by each observer; for it is evident. that the various 

 evolutional currents of faunal and floral life were not synchronous in 

 the rate of their progression through geological time. For example, it 

 is a well-known fact that the evolutional advance of the vegetable king- 



* See bis Aunual Report for 1870, pp. 1U5-16(5. 

 7 B n 



