1. Northumberland and Durham coalfield.
2. South Lancashire coalfield.
3. Derbyshire coalfield.
4. Leicestershire and Staffordshire coalfields.
5. South Wales coalfield.
6. Bristol and Somerset coalfields.
_b._ _Millstone grit_ or _Farewell-rock_. The former term explains
itself, the latter designation has been applied to it in the southern
districts, because when it is reached, then good-bye to all workable
coal-seams.
It consists of coarse sandstones, shales, and conglomerates with a few
small seams of coal. Fossils are not very common in it.
_c._ Yoredale Rocks, a series of flagstones, gritstones, limestones and
shales, with seams of coal, occurring in the northern counties. It is
underlain by--
_d._ _Carboniferous_ or _Mountain Limestone_, which in places is upwards
of 1,000 feet thick, and full of fossils. The stems of encrinites, or
"stone-lilies," corals, brachiopods (_e.g._ _Productus_, _Orthis_,
etc.), and Mollusca, including some Cephalopods, like _Goniatites_ and
the straight Nautilus (_Orthoceras_), with fish teeth, etc., go to
compose this tough, bluish-grey limestone which is largely quarried for
marble mantlepieces, etc.
_e._ The _Tuedian group_ in the north, and _Lower Limestone Shale_ in
the south, follow next, and consist of shales, sandstones, limestones,
and conglomerates, varying greatly in different districts, and
containing few fossils.
3. Devonian or Old Red Sandstone. To this age are assigned a perplexing
series of strata, the principal members of which consist of (_a_) a
thick limestone, well seen in the cliffs and marble quarries of south
Devon, and full of fossil-corals (_e.g._ _Favosites polymorpha_ [or
_cervicornis_]) Brachiopods, and Mollusca, etc.
_b._ A series of sandstones, slates, and limestones in North Devon
containing Trilobites (_Phacops_, _Bronteus_, etc.), Brachiopods, and
other fossils.
_c._ The _Old Red Sandstone_ of Wales, the North of England, and
Scotland, consisting of red and grey sandstone and marly beds, with
remains of fish.
These fish, unlike most now living, were more or less covered with hard
external plates, and possessed merely a cartilaginous skeleton. In one
set of individuals, indeed (_Pterichthys_), the armour plates formed
quite a little box. These creatures propelled themselves by means of two
arm-like flippers, rather than fins. They were but a few inches long,
and appear pigmies in contrast to the strange half-lobster-like
crustacean, _Pterygotus_, that lived with them, and attained sometimes
as much as five feet in length.
4. Silurian. Named by Sir Roderick Murchison after a tribe of Ancient
Britons that dwelt in that part of Wales, where these rocks were first
observed. Some of Murchison's Lower Silurian beds were included by
Professor Sedgwick in his Cambrian, of which we shall have to speak
next; and as these two geologists never could agree on a divisional line
between their respective formations, and since succeeding observers have
followed sometimes one and sometimes the other method of classification,
considerable confusion has resulted. Here, however, for several reasons,
we propose to follow Sedgwick's arrangement; and hence, under the term
Silurian, retain only Murchison's Upper beds. They consist of a series
of sandstones, gritstones, conglomerates, shales, limestones, etc.
Amongst the more important fossils, which are very abundant in the
limestones, are various corals (_e.g._ the Chain-coral _Halysites_),
Star-fish, Crinoids, Trilobites (_Phacops_, etc.), Polyzoa, Brachiopods
and Mollusca, especially Cephalopoda (_Orthoceras_, _Nautilus_, etc.).
These rocks occur principally in the border land between England and
Wales, and the adjacent counties; but are also represented in
Westmoreland, Scotland, and Ireland. Their principal subdivisions are
given in the Table on p. 16.
[Illustration: Trilobite (_Asaphus candatus_), (from the Silurian).]
[Illustration: _Orthoceras subannulatum_ (from the Silurian).]
5. Cambrian. Under this term, derived from the old name for Wales, are
included many sandstones, grits, slates and flags, with here and there a
limestone band. They form the greater part of the western counties of
Wales, where they rise to a considerable height above the sea level. The
highest hills of Westmoreland and more than half of Scotland are
composed of beds of this age.
The fossils, save in the limestone bands, are not easy to find, but in
places they are fairly abundant. Brachiopods are far more numerous than
the Mollusca properly so-called. Of these, the genus _Orthis_ was most
abundant at about the close of this period. Certain beds of this age
have received the name of Lingula Flags, owing this prevalence in them
of the curious Brachiopod _Lingula_ so like the species now living in
some of the warm seas of the tropics. The Trilobites included several
forms, and one species (_Paradoxides Davidis_) attained the length of
nearly two feet. A few star-fish, some Hydrozoans (_Graptolites_), and
the tubes and casts of Annelides and tracks of Trilobites, complete the
list of more remarkable fossils. The subdivisions of the Cambrian rocks
will be found in the table on p. 16.
6. Pre-Cambrian.--Near St. David's Head and some other places in Wales,
in Anglesea, Shropshire, etc., some yet older rocks have been found.
They are probably for the most part of volcanic origin, but they have
been so much changed since they were first deposited, and as hitherto no
fossils have been found in them, little is known concerning them.
Parts of the western coast of Northern Scotland and the Hebrides are
composed of a crystalline rock called Gneiss, and supposed to be the
oldest member of the British strata. No fossils have been found in it.
[Illustration: Skull of _Deinotherium giganteum_, a huge extinct
animal, related to the elephants (from the Miocene of Germany).]
VOLCANIC ROCKS. Although there are fortunately no volcanoes to disturb
the peace of our country at the present day, there is abundant evidence
of their existence in the past. Not only are some of the beds,
especially those of Paleozoic age, composed of the dust and ashes thrown
out of volcanoes, with here and there a lava flow now hardened into
solid rock, but the stumps of the volcanoes themselves are left to tell
the tale. The cones indeed are gone, carried off piecemeal by the rain
and frosts, and other destructive agencies, in the course of countless
ages: not so the once fluid rock within; _that_ cooled down into
Granite, and though originally below the surface, it now, owing to the
removal of the overlying softer strata, forms raised ground overlooking
the surrounding country. The granite masses of Cornwall, of Dartmoor, in
the south-west of Mt. Sorrel; the variety called Syenite at Malvern and
Charnwood Forest; the Basalts of the Cheviot Hills and of Antrim; the
volcanic rocks of Arthur's Seat, Edinburgh, and of the islands of Skye
and Mull, etc., are examples of this class of rock. They are of
different ages, and belong to different periods of the earth's history,
from early Palaeozoic down to Miocene times.
TABLE OF THE PRINCIPAL DIVISIONS OF THE ANIMAL KINGDOM, TO SHOW THE
ORDER IN WHICH THE FOSSILS SHOULD BE ARRANGED.