566 On the Age of the Laramie Formation. | August, 
led the older geologists to draw their most emphasized line of 
division between them, presupposes of itself a great interval. 
The conception I have formed of this interval is such, that 
should feel no difficulty in referring strata to it, even to a thick- 
ness of 10,000 feet. I will endeavor to explain the reasons for 
this belief. | ; 
In England no break could be more complete, geologically and 
zoologically, than that between our lowest Eocene and the chalk 
upon which it rests, although to the eye but a slightly eroded 
surface and a few flints separate them. The Eocenes are the 
result of a long series of local deposits of shallow estuaries and 
rivers, and the causes which led to their deposition can be traced. 
The chalk contrasts very strongly, being the deposit of wide 
ocean without any indication whatever of the proximity of land. 
We have but a mere fragment left, and its former presence over 
the larger part of Great Britain and Ireland can only be traced 
by the flints which have been left behind. We do not yet even 
know what were the shore lines in any direction of the Creta- 
ceous ocean, for the indications of it in Europe may belong to a 
later period when part of its bed was being elevated into Pre 
eocene land. 3 
No less striking is the completeness of the change which took 
place in the fauna and flora during the interval. In the Eocenes 
the progressive change in the Mollusca, notwithstanding the 
evidently varied physical conditions under which they lived, was 
so small that they are substantially the same throughout. The 
changes underwent by them during the whole Cretaceous period 
is also so small as to be almost entirely of the value now com- 
monly recognized as specific and not generic. When we com- / 
pare the Cretaceous and Eocene faunas of England together, they | 
seem, however, to have hardly anything in common. In the long 
interval between them, genera, families and even orders had 
become extinct, and new types taken their place. We may form 
some estimate of the lapse of time such changes as these imply, 
supposing life forms to have been progressively modified, by 
examining three strata with which I am especially familiar—the 
Neocomian at Atherfield, the Gault at Folkstone, and the Upper 
Green sand at Blackdown. These answer the purpose better than — 
any other, because they contain a very similar assemblage of 
Mollusca, and seem, therefore, to have been deposited by water 
