nce, during some part or other of this
period, in all the districts distinguished by ruled lines, are of a most
unequivocal character; for the area thus described is now covered by
deposits containing the fossil remains of animals which could only have
lived in salt water. The most ancient part of the period referred to
cannot be deemed very remote, considered geologically; because the
deposits of the Paris and London basins, and many other districts
belonging to the older tertiary epoch, are newer than the greater part
of the sedimentary rocks (those commonly called secondary and primary
fossiliferous or paleozoic) of which the crust of the globe is composed.
The species, moreover, of marine testacea, of which the remains are
found in these older tertiary formations, are not entirely distinct from
such as now live. Yet, notwithstanding the comparatively recent epoch to
which this retrospect is carried, the variations in the distribution of
land and sea depicted on the map form only a part of those which must
have taken place during the period under consideration. Some
approximation has merely been made to an estimate of the amount of _sea
converted into land_ in parts of Europe best known to geologists; but we
cannot determine how much land has become sea during the same period;
and there may have been repeated interchanges of land and water in the
same places, changes of which no account is taken in the map, and
respecting the amount of which little accurate information can ever be
obtained.

I have extended the sea in some instances beyond the limits of the land
now covered by tertiary formations, and marine drift, because other
geological data have been obtained for inferring the submergence of
these tracts after the deposition of the Eocene strata had begun. Thus,
for example, there are good reasons for concluding that part of the
chalk of England (the North and South Downs, for example, together with
the intervening secondary tracts) continued beneath the sea until the
oldest tertiary beds had begun to accumulate.

A strait of the sea separating England and Wales has also been
introduced, on the evidence afforded by shells of existing species found
in a deposit of gravel, sand, loam, and clay, called the northern drift,
by Sir R. Murchison.[199] And Mr. Trimmer has discovered similar recent
marine shells on the northern coast of North Wales, and on Moel Tryfane,
near the Menai Straits, at the height of 1392 feet