93. The fossiliferous strata, as they are generally termed, have been
chronologically arranged in a series of _formations_, each of which is
characterised by its own peculiar suites of fossils. Their relative age
has been determined, as we have indicated above, by their fossils, and
also by certain physical tests, the chief of these being
_superposition_. It holds invariably true that a formation, A, found
resting upon another series of strata, B, will always occur in precisely
the same position, wherever these two deposits occur together. If B
should appear in some place as resting upon A, we may be sure that the
beds have been inverted during the contortion of the strata consequent
upon subterranean action (see fig. 11, page 34). Again, another useful
test of the relative age of strata lies in the circumstance that one is
often made up or contains fragments of the other. In this case, then, it
is quite clear which is the more recent accumulation. These tests have
now been applied to the strata in many parts of the world, and the
result is that geologists have been able to arrive at a chronological
arrangement or classification, and so to construct a table shewing the
relative position which would be occupied by all the different
formations, if these occurred together in one place. In the British
Islands the long series of strata is well developed, but many of the
formations are much more meagrely represented than their equivalents in
other countries. But even when we attempt to fill up the blanks in our
own series by dovetailing with them the strata of foreign countries,
there yet remain numerous breaks in the succession, pointing to the fact
that the stony record is a very fragmentary one at the best. No doubt
there are many large tracts of the earth's surface which have not yet
been investigated, and when these are known we may hope to have our
knowledge greatly increased. But no one who reflects upon the mode of
origin of the fossiliferous strata, and the wonderful mutations which
the earth has undergone, can reasonably anticipate that a perfect and
complete record of the geological history of our planet shall ever be
compiled from the broken and fragmentary testimony of the rocks.
94. The following table gives the names of the different formations
arranged in the order of their superposition, the youngest being at the
top, and the oldest known at the bottom:
IV. POST-TERTIARY OR QUATERNARY--
Historical or Recent.
Pleistocene.
III. TERTIARY OR CAINOZOIC--
Pliocene.
Miocene.
Eocene.
II. SECONDARY OR MESOZOIC--
Cretaceous.
Jurassic.
Triassic.
I. PRIMARY OR PALAEOZOIC--
Permian.
Carboniferous.
Devonian and Old Red Sandstone.
Silurian.
Cambrian.
Laurentian or Pre-Cambrian.
95. The PRIMARY formations are so called because they are the oldest
known to us: they are not necessarily the first-formed aqueous deposits.
Dr Hutton said truly: There is no trace of a beginning, and no signs of
an end. In the PRIMARY or PALAEOZOIC (ancient-life) formations are found
the earliest traces of life. The forms as a rule depart very widely from
those with which we are acquainted now. The _Laurentian_ rocks have
yielded only one fossil--a large foraminifer named _Eozoon Canadense_.
The _Cambrian_ formation contains but few fossils--crustaceans,
molluscs, zoophytes, and worm-tracks. The _Silurian_ strata are often
abundantly fossiliferous. All the great classes of invertebrates are
represented, and fish remains also occur. The _Devonian_ and _Old Red
Sandstone_ are also characterised by the presence of an abundant fauna.
In the Old Red Sandstone are numerous fish remains; it appears to have
been an estuarine or lacustrine deposit; the Devonian, on the other
hand, was marine, like the Silurian and Cambrian. The _Carboniferous_
formation is the chief repository of coal in Britain. It consists of
terrestrial, fresh or brackish water, and marine deposits. The fauna and
flora of the _Permian_, which is partly a marine and partly a
fresh-water formation, are allied, upon the whole, to those of the
Carboniferous, but offer at the same time many contrasts.
96. The SECONDARY OR MESOZOIC (middle-life) formations contain
assemblages of fossils which do not depart so widely from analogous
living forms as those belonging to Palaeozoic times. The _Triassic_
strata yield abundance of rock-salt. In Britain they contain very few
fossils, but these are more abundant in the Triassic deposits of foreign
countries. The oldest known mammals first appear in this formation. The
_Jurassic_ formation is very highly fossiliferous. It is distinguished
by the occurrence of numerous reptilian remains. Nearly all the beds of
this formation are marine, but there are associated with these the
remains of a forest or old land surface, and a considerable accumulation
of estuarine or fresh-water deposits; impure coals also occur in this
formation. The _Cretaceous_ strata are almost wholly marine, and chiefly
of deep-water origin. But some land-plants are found, chiefly ferns,
conifers, and cycads. Near the base of the formation occurs a great
river deposit (Weald clay) with numerous remains of reptiles.
97. Among the oldest strata of the TERTIARY or CAINOZOIC (recent-life)
division we meet with the _dawn_ of the existing state of the testaceous
fauna--the _Eocene_ (_eos_, dawn, and _kainos_, recent) containing three
and a half per cent. of recent species among its shells. The proportion
of recent species increases in the _Miocene_ (_meion_, less, and
_kainos_, recent), although the majority of the molluscs entombed in
that formation belong to extinct species. In the _Pliocene_ (_pleion_,
more, and _kainos_, recent), however, the extinct species are in a
minority.
The POST-TERTIARY or QUATERNARY division comprises the concluding
chapters of geological history. The _Pleistocene_ (_pleistos_, most, and
_kainos_, recent) contains no extinct species of shells, but a number of
extinct mammalia. In the _Recent_ deposits all the species of animals
and plants are living. The Tertiary and Quaternary formations are partly
of marine and partly of terrestrial and fresh-water origin. At the close
of the Tertiary period the 'glacial epoch' of Pleistocene times began,
and the British Islands and a large part of northern Europe and North
America were then cased in snow and ice. Traces of glacial conditions
have also been met with in the Eocene and Miocene. The evidence
furnished by Palaeozoic and Mesozoic formations points chiefly to mild,
genial, and sometimes tropical conditions. But traces of ice action are
occasionally noted (namely, in the Silurian, Old Red Sandstone,
Carboniferous, Permian, and Cretaceous formations), pointing, perhaps,
in some of the cases, to former alternations of cold and warm periods.
Indeed, the belief is now gaining ground, that the so-called glacial
epoch of Pleistocene times was not one long continuous age of ice, but
rather consisted of an alternation of warm and cold periods. And it is
not improbable, but highly likely, that similar alternations of climate
have happened during every period of great eccentricity of the earth's
orbit.