285 



As has already been said and is well known, the base of the Eocene 

 was established just below the Thanetian of England and its continental 

 equivalents; and this line of separation of the Cenozoic from the Mesozoic 

 has been recognized by practically all geologists since Lyell's time. Con- 

 sidering the great gap between the two systems, as known in Europe at 

 that time, the separation did not appear to be at all an arbitrary one. In 

 his "Text-book of Geology," edition of 1896, Geikie placed the Montian in 

 the Eocene, but in the edition of 1903 this formation is restored to the 

 Upper Cretaceous. Lapparent, too, draws the line above the Montian. 

 Nor does this manner of division appear to arouse objections on the part 

 of the paleontologists. 



If, therefore, American geologists and paleontologists wish to have the 

 boundary line between the Mesozoic and the Cenozoic of their country 

 coincide with that of Europe, the type continent of the base of the Eocene, 

 it will be necessary, unless there are compelling reasons for the contrary, 

 to make the base of our Eocene the equivalent of the Thanetian of Europe. 

 I believe that geologists and paleontologists generally will give assent to 

 this proposition. 



It is well understood that in the determination of the level of any 

 geological formation not all kinds of fossils are of equal value ; some are 

 indeed of little value. It is agreed that the marine animals record most 

 accurately the progress of geological time, because of their abundance, 

 their wide distribution, the slow and steady changes which they undergo 

 during geological periods, and the facility with which they become en- 

 tombed in accumulating sediments. Furthermore, of marine species the 

 pelagic forms are of greater value, because their remains are dropped in- 

 discriminately into deposits of all kinds, thus enabling geologists to cor- 

 relate formations widely separated and composed of very different ma- 

 terials. Terrestrial animals are of less value. They are subject to rapid 

 and extreme changes in their environment through changes in climate and 

 through sudden migrations. They surfer accordingly rapid modifications 

 in their structure or sudden extinction. They are also less likely to be 

 preserved in the rocks. Every shell in an oyster bed may be preserved, 

 while from a million horses but a single tooth may escape destruction. 

 In an interesting address at the meeting of the British Association at 

 Montreal, in 1S84, Blanford gave it as his opinion that determinations of 



