494 J. Starliie Gardner — Relative Ages of 



as tbe Lignitic or Coal strata, or Laramie group, yarying in thick- 

 ness from 1000 to 5000, and even 10,000 feet. There does not 

 appear to be any physical break between this and the underlying 

 series, though here and there a slight erosion near the junction 

 seeUiS to have been observed. A great change takes place, however, 

 in the character of the flora, the palmate leaves of dicotyledons, 

 such as Sassafras, Liqnidambar, Platanus, Acer, etc., suddenly 

 giving place to Palms and a series of dicotyledons more characteristic 

 of European Eocenes. The evidence of the mollusca was, formerly 

 at least, nil or but trivial ; but the discovery of Mosasaurus in it has 

 led to a very keen discussion as to whether it should be classed as 

 Cretaceous or Tertiary. Professors Cope and Marsh still, I believe, 

 classify the whole series with the former, and Dr. Lesquereux with 

 the latter. Dr. Hayden formerly considered it to be part Cretaceous, 

 part Eocene, — a true passage bed, — and the term " Post-Cretaceous" 

 was, I think, proposed by Dr. White as a compromise.^ 



The American " Eocene Series." — This is represented by the 

 Wasatch beds, some 2000 feet thick, and the Fort Union group, 

 of purely freshwater origin. It is conformable with the last, and 

 no doubts have hitherto been expressed regarding its age. The 

 American " Miocene " is represented by the Green Eiver, Bridger, 

 and other groups, whose aggregate thickness seems to fall not far 

 short of 4000 ft. ; and the " Pliocene " by the Uinta and Salt Lake, 

 700 ft., and the Niobrara and Wyoming, 1300 ft. thick. 



We thus have a continuous series of littoral and terrestrial de- 

 posits in America, commencing somewhere in the Cretaceous age 

 and enduring to the close of the Tertiaries, without any perceptible 

 break in the sedimentation, but with a very considerable break in 

 both the Fauna and Flora. It discloses in fact a sudden transition 

 from a relatively temperate Flora to a sub-tropical one, and from 

 Cretaceous Reptilia to Tertiary Mammalia. 



My object is to show, that the data upon which the lowermost 

 beds of the series are correlated with our English Cretaceous series 

 are wholly insufficient ; and to set forth grounds which, when more 

 fully investigated, will, I believe, tend to place them very much 

 higher in the series. 



We have now to consider at somewhat greater length the Cre- 

 taceous and Eocene formations in Europe. 



But before attempting to deal with the Cretaceous series, as de- 

 veloped in Europe, we must not forget that the sequence and agws of 

 the rocks composing it were determined originally from a study of a 

 small area only. The classic Cretaceous series is to be met with in 

 England only, I believe, but at all events not beyond Western France 

 and a portion of Germany. The relative ages of all Cretaceous rocks 

 beyond these areas have been inferred from comparisons of their 

 fossils with those of the typical series ; and these inferences may be 

 fallacious. When the subdivisions of the Cretaceous system were 



1 An account has been published since this paper of the non-marine mollusca of 

 the series, probably strengthening my views. — J. S. G. 



