285 
As has already been said and is well known, the base of the Hocene 
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 abuve 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 Hocene, 
it will be necessary, unless there are compelling reasons for the contrary, 
to make the base of our Hocene 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 suffer 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 1884, Blanford gave it as his opinion that determinations of 
