O. C. Marsh — Different Kinds of Fossils, etc. 483 



Art. L. — The Comparative Value of Different Kinds of 

 Fossils in Determining Geological Age /* by O. C. Marsh. 



More than twenty years ago, my attention was called to the 

 subject of the difference between the value of fossil Plants, 

 Invertebrates, and Vertebrates, as evidence of the geological 

 age of the strata in which they were preserved. On the com- 

 parative value of these different groups of fossils then depended 

 the solution of some grave problems in the geology of the 

 Rocky Mountains. I therefore began a systematic investiga- 

 tion of the subject, and gave the results in an address before 

 the American Association for the Advancement of Science, 

 in 1877. t I 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. 



"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 unsatisfactory witnesses; that invertebrate animals are 

 much better ; and that vertebrates afford the most reliable 

 evidence of climatic and other geological changes. The sub- 

 divisions 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." 



In the statement I have quoted, I had no intention of reflect- 

 ing in the slightest degree on the work of the conscientious 

 paleobotanists who had endeavored to solve the problem with 

 the best means at their command. I merely meant to suggest 

 that the means then at their command were not adequate to 

 the solution. 



* Abstract of Communication made to Section C, British Association for the 

 Advancement of Science, Bristol Meeting, September 9, 1898. 

 f This Journal, vol. xiv, pp. 338-378, November, 1877. 



