THE CLASSIFICATION OF THE STRATIFIED ROCKS.
In the succeeding chapters, a general account of the characters of the
Geological Deposits of different periods will be given, for the
purposes of illustrating the principles to the consideration of which
the earlier chapters have been devoted. It is not proposed to enter
into a description of numberless details, which would only confuse the
student who wished to grasp the main principles, for many facts have
been recorded which it is necessary to notice in a comprehensive
text-book treating of stratigraphical geology, though their full
significance is not yet grasped. The writer, while noting the main
characters of the various subdivisions of the different
stratigraphical systems, will assume that this work is used in
conjunction with some recognised text-book. The stratigraphical
portion of Sir A. Geikie's _Class Book of Geology_ gives an admirable
general account of the British Strata, while the larger text-book by
the same author has a condensed though very full account of the rocks
of the stratigraphical column in all parts of the world, and this is
supplemented by numerous references to the original works wherein
further descriptions may be found. The English edition of Prof. E.
Kayser's _Text-Book of Comparative Geology_, edited by P. Lake, is
also well adapted to the wants of the student, and an excellent
account of the strata is given in Mr A. J. Jukes-Browne's _Handbook of
Historical Geology_, which may be read with the same author's
_Building of the British Isles_.
The reader who refers to different text-books will be struck with the
variations of nomenclature even amongst the larger stratigraphical
divisions, for two authors seldom subdivide the geological column into
the same number of rock-systems. The following classification will be
here adopted:--
Groups. Systems.
{ Recent
{ Pleistocene
Cainozoic or { Pliocene
Tertiary { Miocene
{ Oligocene
{ Eocene
{ Cretaceous
Mesozoic or { Jurassic
Secondary { Triassic
{ Permian
{ Permo-Carboniferous
{ Carboniferous
Palæozoic { Devonian
{ Silurian
{ Ordovician
{ Cambrian.
Precambrian.
A few remarks may be given as to the reason for adopting this
classification.
It is not for a moment suggested that the Systems have the same value,
if the time taken for their accumulation be alone considered. The beds
classified as Recent, for example, were probably accumulated during a
lapse of time far shorter than that occupied for the deposit of some
of the series or even stages of a system like the Silurian, but the
recent rocks acquire a special significance from the fact that we are
living in the period, and the Cainozoic rocks as a whole are capable
of greater subdivision than the earlier groups, on account of the
greater ease with which they can be studied, owing to the small amount
of disturbance which they have usually undergone when compared with
that which has affected older rocks, and the closer resemblance of
their faunas and floras to those of existing times.
With reference to the groups, the writer has already commented upon
the use of the terms Palæozoic, Mesozoic and Cainozoic; below the
lowest Palæozoic rocks (those of the Cambrian system) lie a group of
rocks which have been variously spoken of as Azoic, Eozoic, and
Archæan. There is an objection to the use of any one of these words in
this sense; the objection in the case of the first two is that the
term is theoretical and probably incorrect, whilst the word Archæan,
otherwise suitable, has also been used in a more restricted sense. In
these circumstances the term Precambrian will be used when referring
to any rocks which were formed below Palæozoic times, though no doubt
when this obscure group of rocks is more thoroughly understood a
satisfactory classification will be applied to it.
Taking the other groups into account, the lower systems of the
Palæozoic group will be found to vary greatly according to the views
of different writers; some make only one system, the Silurian, others
two, the Cambrian and Silurian. The three systems are here adopted,
not only because the one, Silurian, is too unwieldy on account of its
size and requires subdivision (and the Cambrian and Silurian however
defined, will be found to be of very unequal importance, whereas the
three systems adopted are of fairly equal value), but especially
because when the term Ordovician is used, the significance of the
other terms Cambrian and Silurian is at once understood.
An attempt has been made to shew that the Devonian system is
non-existent, but the result of modern research is to shew that the
rocks placed in this system are worthy of the distinction, both from
their importance and from the distinctness of the fauna from those of
the underlying and overlying systems.
The Permo-Carboniferous system is adopted, because an important group
of deposits has recently been brought to light which were not
represented either in the Permian or Carboniferous system as
originally defined.
Some authors have advocated the union of the Permian and Triassic
systems into one system placed at the base of the Mesozoic group. This
is unnecessary, and would depart from the classification originally
proposed, which is to be deprecated, unless there is any strong reason
for it.
The Mesozoic systems are classified according to the method generally
adopted. Were a fresh classification to be proposed, a portion of the
Cretaceous system might be included with the Jurassic rocks, but it is
better to adhere to the old classification.
The divisions of the Cainozoic rocks are hardly systems in the sense
in which the term is used in the case of the older rocks, but the
reason for using these smaller subdivisions has already been
mentioned. The addition of the Oligocene to the original divisions
suggested by Lyell has been found useful, and the term will be used
in this work.
The reasons for the adoption of the particular minor subdivisions
(series and stages) in the following chapters will frequently appear
when the rocks of the various systems are described, and need not be
further alluded to in this place.
Although most geologists describe the stratified rocks in ascending
sequence beginning with the oldest, and proceeding towards the newest,
others, and notably Lyell, adopted the opposite method and commenced
with an account of the newest beds. The argument generally used for
the latter method is that it is easier to work from the study of the
known to that of the less known, and as the faunas of the newest rocks
are most like the existing faunas, the student would more readily
follow a description of the rocks in the order which is opposite to
that in which they were deposited.
In practice, the study of the sediments in their proper order, that
is, in the order of deposit, will not be found to task the student to
any great extent, especially if, as is very desirable, he has studied
the main facts and principles of Palæontology before commencing the
study of the rock-systems in detail. There is one reason for beginning
with the study of the older sediments which outweighs any reasons
which can be advanced against it, namely that the events of any period
produce their effect not only upon the strata of that period, but also
on those of succeeding periods.
The task of the stratigraphical geologist is really to learn the
evolution of the earth, in its changes from the simple to the more
complex conditions, and it is quite obvious that it is unnatural to
attempt any study of evolution by working backward. For this reason
the study of the sediments will be here made in the order which is
usually adopted, by passing from the older to the newer, and from the
simple to the more complex.
The British strata will be mainly considered, though references will
frequently be made to their foreign equivalents, and a fuller account
of the latter will be added when the British strata are abnormal, as
are those of Triassic times, and also when a period is not represented
amongst the strata of the British Isles, as for instance, the
Permo-Carboniferous and Miocene periods.
The student is recommended to refer constantly to good geological maps
of the British Isles, of Europe, and of the world. Of maps of the
British Isles, mention may be made of Sir A. Ramsay's geological map
of England, Sir A. Geikie's map of Scotland, and his map of the
British Isles, J. G. Goodchild's map of England and Wales, a map of
Europe by W. Topley and one of the world reduced from that by J.
Marcou, accompanying the first and second volumes of the late Sir J.
Prestwich's _Geology_. For special purposes more detailed maps will be
studied, including the one-inch maps of H. M. Geological Survey, and
the index map on a smaller scale. Lastly, for an account of British
Geology, reference must be made to H. B. Woodward's _Geology of
England and Wales_, where the British formations are described in
order, and to W. J. Harrison's _Geology of the Counties of England and
Wales_, where the stratigraphical geology of the country is given
under the head of the different counties, which are taken in
alphabetical order.
In concluding this chapter, it is hardly necessary to say that every
opportunity of studying the characters of the deposits and their
fossils in the field should be eagerly seized, and that much
information may be acquired even on a railway journey, especially as
to the influence which the deposits exert upon the scenery of a
region[47].
[Footnote 47: In the first edition of H. B. Woodward's _Geology of
England and Wales_, an account of the geology of the main lines of
English railways is given, which is omitted in the later edition. It
is well worth consulting by those who take a long journey, and it will
be found useful to take a geological map with one on the journey so as
to discover when one is passing from one formation to another.]