efly argillaceous, consisting of
clays with sandy layers, and occasionally courses of limestone.
The geological importance of the Wealden formation is very great,
as it is undoubtedly the delta of an ancient river, being composed
almost wholly of fresh-water beds, with a few brackish-water
and even marine strata, intercalated in the lower portion. Its
geographical extent, though uncertain, owing to the enormous
denudation to which it has been subjected, is nevertheless great,
since it extends from Dorsetshire to France, and occurs also in
North Germany. Still, even if it were continuous between all
these points, it would not be larger than the delta of such a
modern river as the Ganges. The river which produced the Wealden
series must have flowed from an ancient continent occupying what
is now the Atlantic Ocean; and the time occupied in the formation
of the Wealden must have been very great, though we have, of
course, no data by which we can accurately calculate its duration.

The fossils of the Wealden series are, naturally, mostly the
remains of such animals as we know at the present day as inhabiting
rivers. We have, namely, fresh-water Mussels (_Unio_), River-snails
(_Paludina_), and other fresh-water shells, with numerous little
bivalved Crustaceans, and some fishes.

II. _Lower Greensand_ (_Neocomien_ of D'Orbigny).--The Wealden
beds pass upward, often by insensible gradations, into the Lower
Greensand. The name Lower Greensand is not an appropriate one,
for green sands only occur sparingly and occasionally, and are
found in other formations. For this reason it has been proposed
to substitute for Lower Greensand the name _Neocomian_, derived
from the town of Neufchatel--anciently called _Neocomum_--in
Switzerland. If this name were adopted, as it ought to be, the
Wealden beds would be called the Lower Neocomian.

The Lower Greensand or Neocomian of Britain has a thickness of
about 850 feet, and consists of alternations of sands, sandstones,
and clays, with occasional calcareous bands. The general colour
of the series is dark brown, sometimes red; and the sands are
occasionally green, from the presence of silicate of iron.

The fossils of the Lower Greensand are purely marine, and among
the most characteristic are the shells of _Cephalopods_.

The most remarkable point, however, about the fossils of the
Lower Cretaceous series, is their marked divergence from the
fossils of the Upper Cretaceous rocks. Of 28