242 
EOCENE PERIOD. 
[Ch. XVIII. 
suggested the very natural idea that there existed formerly a 
chain of lakes, reaching from the highest part of the central 
mountain-group of France, and terminating in the basin of 
Paris, which he supposes was at that time an arm of the sea. 
Notwithstanding the great changes which the physical geo- 
graphy of that part of France must since have undergone, we 
may easily conceive that many of the principal features in the 
configuration of the country may have remained unchanged, 
or but slightly modified. Hills of volcanic matter have indeed 
been formed since the Eocene formations were accumulated, 
and the levels of large tracts have been altered in relation to 
the sea ; lakes have been drained, and a gulf of the sea turned 
into dry land, but many of the reciprocal relations of the 
different parts of the surface may still remain the same. The 
waters which flowed from the granitic heights into the Eocene 
lakes may now descend in the same manner into valleys once 
the basins of those lakes. Let us, for example, suppose the 
great Canadian lakes, and the gulf into which their waters 
are discharged, to be elevated and laid dry by subterranean 
movements. The whole hydrographical basin of the St. Law- 
rence might be upraised during these convulsions, yet that 
river might continue, after so extraordinary a revolution, to 
drain the same elevated regions, and might continue to convey 
its waters in the same direction from the interior of the conti- 
nent to the Atlantic. Instead of traversing the lakes, it would 
hold its course through deposits of lacustrine sand and shelly 
marl, such as we know to be now forming in Lakes Superior 
and Erie ; and these fresh- water strata would occupy the site 
and bear testimony to the pristine existence of the lakes. Marine 
strata might also be brought into view in the space where an 
inlet of the sea, like the estuary of the St. Lawrence, had once 
received the continental waters ; and in such formations we 
might discover shells of lacustrine and fluviatile species inter- 
mingled with marine testacea and zoophytes. 
Subdivisions of strata in the Paris basin. — The area which 
has been called the Paris basin is about one hundred and 
