.]

_Cypris vidua_, a living species, greatly magnified.[1100]]

The recent strata of lacustrine origin above alluded to are of very
small extent, but analogous deposits on the grandest scale are forming
in the great Canadian lakes, as in Lakes Superior and Huron, where beds
of sand and clay are seen inclosing shells of existing species.[1101]
The Chara also plays the same part in the subaqueous vegetation of North
America as in Europe. I observed along the borders of several freshwater
lakes in the state of New York a luxuriant crop of this plant in clear
water of moderate depth, rendering the bottom as verdant as a grassy
meadow. Here, therefore, we may expect some of the tough seed vessels to
be preserved in mud, just as we detect them fossil in the Eocene strata
of Hampshire, or in the neighborhood of Paris, and many other countries.

_Imbedding of freshwater Species in Estuary and Marine Deposits._

_In Lewes levels._--We have sometimes an opportunity of examining the
deposits which within the historical period have silted up some of our
estuaries; and excavations made for wells and other purposes, where the
sea has been finally excluded, enable us to observe the state of the
organic remains in these tracts. The valley of the Ouze between Newhaven
and Lewes is one of several estuaries from which the sea has retired
within the last seven or eight centuries; and here, as appears from the
researches of Dr. Mantell, strata thirty feet and upwards in thickness
have accumulated. At the top, beneath the vegetable soil, is a bed of
peat about five feet thick, inclosing many trunks of trees. Next below
is a stratum of blue clay containing freshwater shells of about nine
species, such as now inhabit the district. Intermixed with these was
observed the skeleton of a deer. Lower down, the layers of blue clay
contain, with the above-mentioned freshwater shells, several marine
species well known on our coast. In the lowest beds, often at the depth
of thirty-six feet; these marine Testacea occur without the slightest
intermixture of fluviatile species, and amongst them the skull of the
narwal, or sea unicorn (_Monodon monoceros_), has been detected.
Underneath all these deposits is a bed of pipe-clay, derived from the
subjacent chalk.[1102]

If we had no historical information respecting the former existence of
an inlet of the sea in this valley and of its gradual obliteration, the
inspection of the section above described