THE CARBONIFEROUS SYSTEM.
_The Classification._ The British rocks of the Carboniferous system
have been classified according to their lithological characters, but
as the classification has been altered from time to time, we may use
that which seems most acceptable to the majority of British geologists
at the present day. According to this, the beds are grouped as
below:--
{ { Ardwick Stage
Upper Carboniferous { Coal Measures { Pennant Stage
{ { Gannister Stage
{ Millstone Grit
Lower Carboniferous { Carboniferous (Mountain) Limestone
{ Series.
The Lower Carboniferous beds have been further subdivided into:--
Yoredale Series or Upper Limestone Shales,
Mountain Limestone,
Lower Limestone Shales, with Sandstones and Conglomerates,
but as these lithological types are found to be very variable when
traced laterally for comparatively short distances, it is found more
satisfactory to use the terms in a purely lithological sense rather
than with chronological significance.
The somewhat abnormal development of the higher portions of the
Carboniferous rocks of Britain renders the local classification only
partially applicable in other regions, and as our knowledge
progresses, a palæontological classification will probably be adopted.
This has already been done with the more purely open-water sediments
of Russia and Eastern Asia, where the development of the beds is more
normal. There the rocks are classified as under:--
Upper Carboniferous or Gshellian,
Middle Carboniferous or Moscovian,
Lower Carboniferous,
and as this classification has already been found to be applicable
over rather wide areas, it is almost certain that, as in the case of
the rocks of other systems, it will prove more serviceable than one
which is mainly (though not quite exclusively) based upon vertical
variation of lithological characters, especially as the Carboniferous
rocks over large tracts in North America possess faunas which are
similar to those which have been discovered in Russia, Eastern Asia
and North Africa.
_Description of the strata._ The variations in the lithological
characters and fossil contents of the British Carboniferous strata
when traced from north to south have been so frequently described, and
utilised as a means of illustrating the indications as to local
variations in physical conditions which are supplied by those strata,
that little need be said upon the subject. The restoration of the
physical geography of Carboniferous times over the British area will
be found in a chapter by the late Professor Green in the work upon
_Coal_ by various professors at the Yorkshire College of Science and
also in Prof. Hull's _Physical History of the British Isles_. Some
modifications must be made in these restorations as the result of
recent research, the principal being caused by discoveries amongst the
Carboniferous rocks of Devonshire.
Taking the strata in vertical succession, we find evidence of the
occurrence of a complete marine period (the second great marine
period) between the second and third continental periods. The first
shallow-water phase over a great portion of the British Isles is
marked by thin terrigenous sediments, indicating that the period was a
brief one; it was followed by the deep-water phase, probably of some
length, lasting through the greater part of the remainder of Lower
Carboniferous times; while the concluding shallow-water phase was
lengthy as compared with that of the beginning of the period, and is
marked by the accumulation of the great thickness of deposits
belonging to the Millstone Grit and Coal Measures. There is no doubt,
however, that in some parts of the British area minor changes produced
local terrestrial conditions during the period, and accordingly we
find that the deepest water deposits of the system in Britain are
succeeded by an unconformable junction with the sediments of the upper
portion of the system.
The general change in the lithological characters of the beds of the
Lower Carboniferous division when traced from south to north is shewn
in the following diagram (Fig. 20).
It will be seen that the land and open sea areas were in the
respective positions which they occupied during Devonian times, but
that as the result of greater submergence, with which the accumulation
of sediment did not keep pace, the shallow-water marine deposits of
Devonian age are in Devon replaced by open-sea deposits[88], while
shallow-water marine deposits further north replace the anomalous
deposits which were found there during the Devonian period.
[Footnote 88: The Radiolarian Cherts of the Lower Carboniferous rocks
of Devon, and the associated sediments, together with the unconformity
between these and the Upper Carboniferous beds are described by Messrs
Hinde and Fox, Quart. _Journ. Geol. Soc._, vol. LI. p. 609.]
[Illustration: Fig. 20.
_a._ Radiolarian cherts of Devon.
_b._ Mountain limestone of Central England.
_c._ Mechanical sediments of Northern England.
_d._ Freshwater deposits of Southern Scotland.
O.R. Older rocks.
]
Owing to the accumulation of thick masses of sediment, the Lower
Carboniferous sea of the north of England appears to have been largely
silted up, and although the organic deposits of the south are so thin
that they did not render the sea shallow in that region, the general
level of the Lower Carboniferous floor of the south was also uplifted,
and actually converted into land, as the result of the upward movement
which took place in Devonshire and tracts of France; and owing to
silting up in the north, and elevation in the south, a general plane
surface was produced over very extensive areas, not only in Britain
but upon the Continent, upon which the peculiar deposits and
accumulations of Upper Carboniferous times were laid down, sometimes
in shallow water, sometimes upon the land, and often under conditions
which cannot at present be determined with accuracy. That the deposits
of the Millstone Grit and Coal Measure epochs were to a large extent
laid down in water is admitted by all, and in the case of many of the
deposits of the Millstone Grit, and some thin deposits of the Coal
Measures, it is equally clear that the water area was part of an
expanse of ocean, for we find marine fossils, as corals, crinoids, and
cephalopods, in these beds. Associated with them in the Coal Measures
are other beds in which the ordinary Carboniferous genera of marine
invertebrates are absent, and their place is taken by shells which
bear much resemblance to the modern fresh-water mussel, and it has
been maintained with good reason that as the ordinary marine forms are
rarely or never mixed with those resembling recent fresh-water shells,
the latter are truly fresh-water[89]. If this be so, many of the
mechanically formed sediments of the Coal Measures were of fresh-water
origin, laid down in shallow lagoon-like expanses, probably shut off
from the main ocean by a narrow portion of intervening land, which was
occasionally destroyed, thus permitting incursions of salt-water when
some of the ordinary marine invertebrates of the period obtained a
temporary footing in the area.
[Footnote 89: For further information upon this subject the student
should consult the Introduction to a Monograph on _Carbonicola_,
_Anthracomya_ and _Naiadites_ (the shells in question) by Dr Wheelton
Hind, being one of the Monographs of the Palæontographical Society.]
There is not only a difference of opinion as to the mode of
accumulation of many of the mechanical sediments of the Coal Measures,
but also as to that of the coal-seams which accompanied them. Two
different theories have been put forward to account for these
coal-seams, which are usually spoken of as the drift theory and the
growth-in-place theory. According to the former, in its extreme
application, coal is an aqueous deposit formed by the settlement of
drifted masses of vegetation upon the floor of a water-tract, while
those who push the growth-in-place theory to its extreme limits
maintain that coal is the result of growth of vegetation upon the
actual site where the coal is now found. Much apparently conflicting
evidence has been advanced by the advocates of the two hypotheses, and
special cases of coal-formation have been appealed to by each in
support of their views; thus the existence of coal composed largely of
bodies which resemble the spores of modern lycopods,--objects of so
resinous a nature that they float on the surface until they are
decomposed,--is cited by the upholders of the growth-in-place theory,
while the supporters of the other hypothesis can point with equal
force to the occurrence of the finely divided carbonaceous mud
containing remains of fishes which gives rise to cannel coal in some
places. One of the main assertions in support of the growth-in-place
theory was that of the supposed universality of 'underclays' or old
surface soils beneath all coal-seams, but though these are common,
they are far from universal. It is impossible to do justice in small
compass to this question of coal-formation, but it may be pointed out
that much of the difference of opinion can be understood if it be
remembered that the term 'coal' is rather a popular term which has
been admitted into scientific terminology, and therefore used somewhat
loosely, than a strictly scientific term applied to a definite
substance, and accordingly, just as at the present day we find
carbonaceous substances growing in one place on land to form peat, in
other places on a tract sometimes dry and sometimes submerged, to form
the carbonaceous deposits of the cypress-swamps, and once more
accumulated beneath the shallows of a sea as a sediment to form the
carbonaceous muds of the ocean margins where the mangroves grow, so
the diverse substances which are included under the general term coal
may have accumulated in one place on land, in another beneath water,
and in a third on an area alternately dry and submerged. This is not a
question of great importance; the important point is that
accumulations of vegetation on a fairly large scale are found at the
present day on plains, for even if they grow on mountain regions, the
deposits are readily denuded before they are covered up, and also it
must be noted that a moist climate is necessary for the growth of much
vegetation. The conclusion that the accumulations of coaly matter were
formed on plains is borne out by their great horizontal extent as
compared with their thickness, and it is now generally agreed that the
coal vegetation which is found in the normal coal-measures was
essentially a swamp vegetation.
An attempt has been made to prove that an upland vegetation of very
different character existed contemporaneously with it, but reasons
will be given in the sequel for concluding that this supposed upland
Carboniferous flora is everywhere of later date.
The later shallow-water phase of Carboniferous times, as already
stated, was unusually long, it was also very widespread, and appears
to have been accompanied over wide areas by humid conditions during
its continuance, and accordingly the marsh conditions which existed
during Upper Carboniferous times were probably on a larger scale than
that of similar conditions before or after. Special stress is laid
upon this fact, as it is a good illustration of the view which seems
to be gaining ground, that every period possessed peculiar conditions
never to be repeated, which must have left their impress upon the
character of the sediments.
Though the conditions above described were widespread, they were
naturally not universal, and accordingly in many parts of the world,
as previously stated, we find true marine deposits of Upper
Carboniferous times, though even these were sometimes replaced during
part of the epoch, by conditions which were favourable for the
formation of coal-seams in those places. Interruption in the
continuance of a humid temperate climate over the regions of
North-West Europe is also suggested by the discovery of deposits which
are maintained to be of glacial origin amongst the Coal Measures of
France[90].
[Footnote 90: For an account of the numerous volcanic products see Sir
A. Geikie's work on "The Ancient Volcanoes of Great Britain."]
_The Floras and Faunas._ The flora of the Carboniferous rock is so
noteworthy that the period has been termed the Period of Cryptogams;
the remains of ferns, horsetails, and clubmosses predominate, and many
of the forms reached a gigantic size. Though the floras of the various
stages are marked by a general resemblance, there are differences
which enable the palæobotanist to ascertain the stratigraphical
position of the beds by reference to the included plant remains, and a
considerable number of successive floras have been described[91]. The
invertebrate fauna does not differ on the whole very greatly from that
of Devonian times, though the trilobites are now becoming rare, and
the mollusca assume a more prominent position as compared with the
brachiopods. Corals occur in abundance in the calcareous deposits of
the period, and frequently give rise to sheets of reef-formation, but
the foraminifera and crinoidea certainly play the principal part as
limestone-producers, and the influence of the latter in giving rise to
great masses of limestone which are frequently used for ornamental
purposes is too well known to need more than passing reference. The
air-breathers have also been detected in greater abundance, though
they are rare, when we consider the comparatively favourable
conditions for their preservation presented by the Coal Measure rocks.
Myriopods, arachnids, insects and pulmoniferous gastropods have
however been found with tolerable frequency. The danger of arguing
from imperfect data is well illustrated by the great addition to our
knowledge of the insect-fauna of these times due to the exploration of
the beds of one small coal-field, that of Commentry in France, of
which the insects have been described by M. C. Brongniart. The
vertebrates are represented by a considerable variety of fishes, and
less abundant though tolerably numerous remains of Amphibia, which
occur in the Carboniferous rocks of the North of England, Ireland,
France, North America and elsewhere.
[Footnote 91: Consult Kidston, R., "On the Various Divisions of the
Carboniferous Rocks as determined by their Fossil Flora," _Proc. Roy.
Phys. Soc. Edin._, vol. XII. p. 183.]
The existence of definite zones of organisms in the case of the
Carboniferous rocks has been denied, and it appears to be considered
by some that the Carboniferous rocks were accumulated so rapidly as
compared with rocks of some other systems that the fauna remained very
similar throughout. It is very doubtful if this was so. In the case of
other systems, the division into zones has only been accomplished by
means of more detailed researches than those which have been conducted
amongst the Carboniferous rocks of Britain: again, the occurrence of
successive floras suggests that there may have been a similar
succession amongst the faunas, and finally we find that zonal division
has been carried on to some extent amongst the Carboniferous strata
of other regions. The following classification of the Russian type of
sediment may prove useful, as an indication of the possibility of more
detailed separation of our own beds:--
{ Beds with _Spirifera fascigera_, _Spiriferina_
Gshellian { _Saranae_, &c.
(with _Fusulina_ and { Beds with _Producta cora_, _P. uralica_,
_Archimedipora_) { _Camarophoria crumena_, &c.
{ Beds with _Syringopora parallela_ and
{ _Spirifera striata_.
Moscovian { Stage of _Spirifera mosquensis_.
{ Stage of _Spirifera Kleini_.
{ Coals, Sandstones and Shales with _Noeggerathia_
Lower Carboniferous { _tenuistriata_ and _Producta_
{ _gigantea_.
{ Stage of _Producta mesoloba_.
The marine fauna of the Upper Carboniferous beds, which is so poorly
represented in Britain, but is well developed in Spain, Russia, Asia
and North America, is largely characterised by the abundance of
foraminifers of the genus _Fusulina_ and _Fusulinella_ and of bryozoa
of the genus _Archimedipora_. It is very desirable that the truly
marine fauna of the _Spirorbis_ limestone and other marine bands of
the British Coal Measures should be carefully studied to see if they
present any close relationship with that of the Gshellian beds[92].
[Footnote 92: A good idea of the general characters of the
Carboniferous fauna of Britain will be obtained from an examination of
Professor Phillips' _Geology of Yorkshire_, Part I., and Mr (now Sir
F.) M^{c}Coy's _Carboniferous Fossils of Ireland_, while the nature of
the European fauna is well illustrated in Prof. de Koninck's
well-known work _Description des animaux fossiles qui se trouvent dans
le terrain carbonifère de Belgique_. For an account of the characters
of the marine fauna of the Upper Carboniferous rocks the reader should
consult the work on Geology and Palæontology published by the
Geological Survey of the State of Illinois in 1866.]