HOW WERE THE EOCENES OF ENGLAND DEPOSITED? 289 
place in the mollusca. Over and over again fresh species of 
Cerithiadse, Limnseidae, Melaniadse, and other estuarine shells, 
come in and disappear again. During this period, too, we not 
only have evidence of the diminution of the volume of the 
river, but also of a gradual lowering of the temperature, and a 
more and more wide departure from the tropical conditions 
which prevailed during the middle Eocene and to the close of 
the Headon period. 
From the close of the Hempstead period until the river 
finally became lost, its delta or embouchure must have lain in 
the Solent Channel or in the sea beyond. 
The truly marine mollusca of the highest Hempstead bed 
appear so impoverished and present such a paucity of species 
and these so stunted, that, coupled with the nature and limited 
area of the deposit, we must conclude that they were formed in 
a sea which had shrunk to a mere salt or brackish lake without 
communication with the open sea beyond. That it was com 
pletely isolated is certain, since there is no incoming of the 
Miocene marine forms which existed abundantly elsewhere. The 
forms are characteristically Eocene, but, except such as the 
Corbula which were enabled to withstand the influx of fresh 
water, of a degraded type. We have no evidence of the Hamp- 
shire Basin again becoming submerged ; and although there are 
abundant indications of larger rivers than those at present 
existing having flowed through the district even in human 
times, how far or whether they were in any way connected 
with the great waters whose history I have endeavoured to trace 
is very doubtful. Nor have the oscillations which produced 
these frequent changes of level and at a later period upheaved 
the Chalk altogether ceased, as there is abundant evidence in 
the Isle of Wight and on adjacent coasts of elevation and sub- 
sidence even within historic times. 
Throughout the whole of our Eocene the river action may be 
more or less distinctly traced, and whether we examine the 
structure of the rocks themselves or the fossils they contain, the 
conclusion is irresistibly forced upon us that for countless ages 
the mouth of a great river occupied at first the south-east and 
then the south of what is now England ; and that during the 
whole period its delta was subject to change of level, becoming 
more or less depressed, as we see in recent deltas, permitting 
now the sea to invade it, now the deposits to rise above water and 
become inhabited by plants and animals. 
Having now shown the existence for countless ages of a river 
of far greater magnitude than any which the drainage of our 
present England could furnish, we will briefly consider the 
probable extent and position of the western land surface from 
which it flowed. 
NEW SERIES, VOL. II. — NO. VII. U 
