EXTENT OF GAP BETWEEN CHALK AND EOCENE IN ENGLAND. 57 
reasons to infer that the lapse of time was of vast duration, 
from the striking and complete changes which took place in 
the fauna and flora in the interval. From the commencement 
to the close of the Eocene period, as it is represented in England, 
the aggregate amount of progressive change is, notwithstanding 
repeatedly varied physical conditions, by contrast trifling, and 
it seems to have been effected so gradually that the types of 
life in the oldest and the newest Eocene are substantially the 
same. 
The change in the types of the fauna throughout the Cretaceous 
period is also so small as to he almost entirely of the value now 
commonly recognised as specific and not generic. From the 
Neocomian to the Grey Chalk, wherever like conditions pre- 
vailed, the same groups of genera, especially if we take the 
Mollusca, are seen to reappear. The fauna and flora of each 
of these periods are, although much interrupted, continuous and 
similar, and notwithstanding that unrepresented intervals are 
indicated, there is no great break in geological time. 
The Cretaceous and the Eocene faunas, on the contrary, seem, 
when compared together, to have nothing in common, and so 
complete a change had taken place in them, that even the gap 
between the Jurassic and Cretaceous periods, judged by their 
organic remains, was small compared to this. How or when 
the Cretaceous fauna disappeared, we cannot trace in this area ; 
we only know that it existed in seas which deposited the Chalk, 
and had ceased to exist here when the Eocene time arrived. 
Yet we know that the extinction must have been a natural 
and gradual one, since we see in America, and also in many 
other countries, that chambered Cephalopoda and Inocerami , 
for example, of late Cretaceous types lived long after the time 
in which we have any record of them in this country, mingled 
with mollusca, whose general features are strikingly similar 
to those of our Eocene. 
The inference from all this is necessarily, that great as the 
lapse of time must have been during the accumulation of 
Cretaceous or Eocene, immeasurably greater was the period, of 
which we have no record here, which elapsed between the close 
of the one series and the commencement of the other. We 
might reasonably, therefore, expect to find in other parts of the 
world, deposits thousands of feet in thickness, belonging to the 
intermediate or Cretaceo-tertiary age, as Hector has named it, 
without, perhaps, even then having the gap entirely filled up. 
We believe that ocean spread widely over Western Europe, at 
least, in the Chalk portion of Cretaceous times ; and it would 
therefore be almost impossible that any but the merest traces of 
the land vegetation of that period could be met with in our own 
or even in any adjacent country. 
