Its lower portion,
for a thickness of from 500 to 1000 feet, is arenaceous, and is
known as the Hastings Sands. Its Upper portion, for a thickness
of 150 to nearly 300 feet, is chiefly 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.
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