1880. | On the Age of the Laramie Formation. 567 
of approximately the same depth, although the sea bottom was 
of different natures, and further there may be gaps of age between 
them. In all these the genera of Mollusca are the same, and the 
species marvelously similar, yet when compared together they 
are found in nearly every case to be slightly modified. Here we 
have merely specific changes of minute extent, although it would 
be impossible to estimate the thickness of the deposits from Neo- 
comian to the Upper Green sand, inclusive, comprising also Ap- 
tien and Gault, at less than one thousand feet, and the probability 
is that they far exceed this. This is merely an example, but 
similar beds, either of Jurassic, Cretaceous or Eocene formations, 
if contrasted together separately, lead to the same result, pro- 
vided, of course, that those’examined were deposited under like 
conditions. : 
Now to compare, for a moment, Cretaceous and Eocene strata. 
The Gault and the London clay are both massive deposits of clay 
which seem to have been formed under almost similar conditions 
and on the same area. The utter dissimilarity of the Mollusca of 
each, however, is such that the time required to have effected 
such a change (always admitting progressive modification) would 
have permitted the accumulation of very many times more sedi- 
ment than the amount which actually separates them. It may be 
objected that a part of the difference which we see in these littoral 
faunæ took place, to an unknown extent, during the deposition of 
the chalk itself, but this does not seem to have been so to any 
great degree, since but a slightly modified Gault fauna can be 
traced through the Gray chalk into the lower White chalk, whilst 
Ammonites and Belemnites are met with in the highest chalk 
rocks of England without any mixture of Tertiary forms. There 
is nothing, therefore, to indicate that any extensive modifications 
had taken place up to the close of the chalk period in England. 
‚Taking the floras of Cretaceous age in England, whose hori- 
zons are absolutely known, we see that they point to an even 
greater interval than the fauna. Of course I leave out of the 
question the so-called Upper Cretaceous floras of Europe, whose 
age, not based upon stratigraphical evidence, is even more a mat- 
ter of doubt than those of America. The Neocomian flora, from 
the little we know of it, was similar to that of the Wealden, and 
the Wealden to the Jurassic; plants would seem, therefore, to 
have become modified even more slowly than animals. My own 
