Marsh — Jurassic formation on the Atlantic Coast. 439 



The same conclusion, based upon a review of the Wealden 

 plants, has recently been reached bj A. C. Seward, likewise an 

 eminent authority, who states the case as follows: *' The evi- 

 dence of paleobotany certainly favors the inclusion of the 

 Wealden rocks in the Jurassic series." * 



Age of the Laramie. 



The problem before us to-day has a strong family resem- 

 blance to another with which geologists were face to face twenty 

 years ago ; namel}- , the geological age of the great lignite series 

 of the West. Then as now, the plants and the animal remains 

 seemed to tell a different story, and I was thus led to investi- 

 gate the question with considerable care. It may perhaps aid 

 in solving the present problem if I repeat what I then said so 

 far as it relates to the value of different kinds of fossils as evi- 

 dence of geological age. In an address before the American 

 Association for the Advancement of Science, in 1877,t 1 stated 

 the case as follows : 



"The boundary line between the Cretaceous and Tertiary 

 in the region of the Rocky Mountains has been much in dis- 

 pute during the last few years, mainly in consequence of the 

 uncertain geological bearings of the fossil plants found near 

 this horizon. The accompanying invertebrate fossils have 

 thrown little light on the question, which is essentially whether 

 the great Lignite series of the West is uppermost Cretaceous 

 or lowest Eocene. The evidence of the numerous vertebrate 

 remains is, in my judgment, decisive, and in favor of the 

 former view. 



Relative Importance of Fossils. 



" This brings up an important point in paleontology, one to 

 which my attention was drawn several years since ; namely, 

 the comparative value of different groups of fossils in mark- 

 ing geological time. In examining the subject with some 

 care, I found that, for this purpose, plants, as their nature 

 indicates, are most unsatisfactory witnesses: that invertebrate 

 animals are much better ; and that vertebrates afford the most 

 reliable evidence of climatic and other geological changes. 

 The subdivisions of the latter group, moreover, and in fact all 

 forms of animal life, are of value in this respect, mainly 

 according to the perfection of their organization or zoological 

 rank. Fishes, for example, are but slightly affected by changes 

 that would destroy reptiles or birds, and the higher mammals 

 succumb under influences that the lower forms pass through in 

 safety. The more special applications of this general law, and 

 its value in geology, will readily suggest themselves." 



* Catalogue British Museum, Wealden Flora, p. 290, 1895. 

 f This Journal, vol. liv, p. 338-378, November, 1877. 



