156 
THE GEOLOGICAL MIDDLE .AGE. 
at its bottom. Upon these followed the chalk de 
posits of the Cretaceous epoch, until the basin 
was gradually filled, and partially, at least, turned 
to dry land. But at the close of the Cretaceous 
epoch a fissure was formed, allowing the entrance 
of the sea at the western end, so that the con- 
stant washing of the tides and storms wore away 
the lower, softer deposits, leaving the overhang- 
ing chalk cliffs unsupported. These latter, as 
their supports were undermined, crumbled down, 
thus widening the channel gradually. This pro- 
cess must, of course, have gone on more rapidly 
at the western end, where the sea rushed in with 
most force, till the channel was worn through to 
the German Ocean on the other side, and the sea 
then began to act with like power at both ends of 
the channel. This explains its form, wider at 
the western end, narrower between Dover and 
Calais, and widening again at the eastern ex- 
tremity. This ancient basis, extending from tilt 
centre of France into England, is rich in the re- 
mains of a number of successive epochs. Around 
its margin we find the Jurassic deposits, showing 
that there must have been some changes of level 
which raised the shores and prevented later ac- 
cumulations from covering them, while in the 
centre the Jurassic deposits are concealed by 
those of the Cretaceous epoch above them, these 
being also partially hidden under the later Ter- 
