marked, however,
the great formations are characterised properly by the association
of certain fossils, by the predominance of certain families or
orders, or by an _assemblage_ of fossil remains representing
the "life" of the period in which the formation was deposited.

Fossils, then, enable us to determine the _age_ of the deposits
in which they occur. Fossils further enable us to come to very
important conclusions as to the mode in which the fossiliferous
bed was deposited, and thus as to the condition of the particular
district or region occupied by the fossiliferous bed at the time
of the formation of the latter. If, in the first place, the bed
contain the remains of animals such as now inhabit rivers, we
know that it is "fluviatile" in its origin, and that it must at
one time have either formed an actual riverbed, or been deposited
by the overflowing of an ancient stream. Secondly, if the bed
contain the remains of shellfish, minute crustaceans, or fish,
such as now inhabit lakes, we know that it is "lacustrine," and
was deposited beneath the waters of a former lake. Thirdly, if
the bed contain the remains of animals such as now people the
ocean, we know that it is "marine" in its origin, and that it
is a fragment of an old sea-bottom.

We can, however, often determine the conditions under which a bed
was deposited with greater accuracy than this. If, for example, the
fossils are of kinds resembling the marine animals now inhabiting
shallow waters, if they are accompanied by the detached relics
of terrestrial organisms, or if they are partially rolled and
broken, we may conclude that the fossiliferous deposit was laid
down in a shallow sea, in the immediate vicinity of a coast-line,
or as an actual shore-deposit. If, again, the remains are those
of animals such as now live in the deeper parts of the ocean,
and there is a very sparing intermixture of extraneous fossils
(such as the bones of birds or quadrupeds, or the remains of
plants), we may presume that the deposit is one of deep water.
In other cases, we may find, scattered through the rock, and
still in their natural position, the valves of shells such as
we know at the present day as living buried in the sand or mud
of the sea-shore or of estuaries. In other cases, the bed may
obviously have been an ancient coral-reef, or an accumulation of
social shells, like Oysters. Lastly, if we find the deposit to
contain the remains of marine shells, but that these