Another great period in the Earth’s history opens on us--the Devonian or
“Old Red Sandstone,” so called, because the formation is very clearly
displayed over a great extent of country in the county of Devon. The
name was first proposed by Murchison and Sedgwick, in 1837, for these
strata, which had previously been referred to the “transition” or
Silurian series.
The circumstances which marked the passage of the uppermost Silurian
rocks into Old Red Sandstone seem to have been:--First, a shallowing of
the sea, followed by a gradual alteration in the physical geography of
the district, so that the area became changed into a series of mingled
fresh and brackish lagoons, which, finally, by continued terrestrial
changes, were converted into a great fresh-water lake; or, if we take
the whole of Britain and lands beyond, into a series of lakes.[40]
[40] “On the Red Rocks of England,” by A. C. Ramsay. _Quart. Jour.
Geol. Soc._, vol. xxvii., p. 243.
Mr. Godwin Austen has, also, stated his opinion that the Old Red
Sandstone, as distinct from the Devonian rocks, was of lacustrine
origin.
The absence of marine shells helps to this conclusion, and the nearest
living analogues of some of the fishes are found in the fresh water of
Africa and North America. Even the occurrence in the Devonian rocks of
Devonshire and Russia of some Old Red Sandstone fishes along with marine
shells, merely proves that some of them were fitted to live in either
fresh or salt water, like various existing fishes. At the present day
animals that are commonly supposed to be essentially marine, are
occasionally found inhabiting fresh water, as is the case in some of the
lakes of Sweden, where it is said marine crustacea are found. Mr.
Alexander Murray also states that in the inland fresh-water lakes of
Newfoundland seals are common, living there without even visiting the
sea. And the same is the case in Lake Baikal, in Central Asia.
The red colour of the Old Red Sandstone of England and Scotland, and the
total absence of fossils, except in the very uppermost beds, are
considered by Professor Ramsay to indicate that the strata were
deposited in inland waters. These fossils are terrestrial ferns,
_Adiantites_ (Pecopteris) _Hibernicus_, and a fresh-water shell, _Anodon
Jukesii_, together with the fish _Glyptolepis_.[41]
[41] “On the Red Rocks of England,” by A. C. Ramsay. _Quart. Jour.
Geol. Soc._, vol. xxvii., p. 247.
The rocks deposited during the Devonian period exhibit some species of
animals and plants of a much more complex organisation than those which
had previously made their appearance. We have seen, during the Silurian
epoch, organisms appearing of very simple type; namely, zoophytes,
articulated and molluscous animals, with algæ and lycopods, among
plants. We shall see, as the globe grows older, that organisation
becomes more complex. Vertebrated animals, represented by numerous
Fishes, succeed Zoophytes, Trilobites, and Molluscs. Soon afterwards
Reptiles appear, then Birds and Mammals; until the time comes when man,
His supreme and last work, issues from the hands of the Creator, to be
king of all the earth--man, who has for the sign of his superiority,
intelligence--that celestial gift, the emanation from God.
Vast inland seas, or lakes covered with a few islets, form the ideal of
the Old Red Sandstone period. Upon the rocks of these islets the
mollusca and articulata of the period exhibit themselves, as represented
on the opposite page (PLATE IX.). Stranded on the shore we see
armour-coated Fishes of strange forms. A group of plants
(_Asterophyllites_) covers one of the islets, associated with plants
nearly herbaceous, resembling mosses, though the true mosses did not
appear till a much later period. _Encrinites_ and _Lituites_ occupy the
rocks in the foreground of the left hand.
The vegetation is still simple in its development, for forest-trees seem
altogether wanting. The Asterophyllites, with tall and slender stems,
rise singly to a considerable height. Cryptogams, of which our mushrooms
convey some idea, would form the chief part of this primitive
vegetation; but in consequence of the softness of their tissues, their
want of consistence, and the absence of much woody fibre, these earlier
plants have come down to us only in a fragmentary state.
[Illustration: IX.--Ideal Landscape of the Devonian Period.]
The plants belonging to the Devonian period differ much from the
vegetation of the present day. They resembled both mosses and lycopods,
which are flowerless cryptogamic plants of a low organisation. The
Lycopods are herbaceous plants, playing only a secondary part in the
vegetation of the globe; but in the earlier ages of organic creation
they were the predominant forms in the vegetable kingdom, both as to
individual size and the number and variety of their species.
[Illustration: Fig. 30.--Plants of the Devonian Epoch. 1. Algæ. 2.
Zostera. 3. Psilophyton, natural size.]
In the woodcut (Fig. 30) we have represented three species of aquatic
plants belonging to the Devonian period; they are--1, _Fucoids_ (or
_Algæ_); 2, _Zostera_; 3, _Psilophyton_. The Fucoid closely resembles
its modern ally; but with the first indications of terrestrial
vegetation we pass from the _Thallogens_, to which the _Algæ_ belong
(plants of simple organisation, without flower or stem), to the
_Acrogens_, which throw out their leaves and branches at the extremity,
and bear in the axils of their leaves minute circular cases, which form
the receptacles of their spore-like seeds. “If we stand,” says Hugh
Miller, “on the outer edge of one of those iron-bound shores of the
Western Highlands, where rock and skerries are crowned with sea-weeds;
the long cylindrical lines of _chorda-filum_, many feet in length, lying
aslant in the tideway; long shaggy bunches of _Fucus serratus_ and _F.
nodosus_ drooping from the sides of the rock; the flat ledges bristling
with the stiff cartilaginous many-cleft fronds of at least two species
of _Chondrus_; now, in the thickly-spread Fucoids of this Highland scene
we have a not very improbable representation of the Thallogenous
vegetation. If we add to this rocky tract, so rich in Fucoids, a
submarine meadow of pale shelly sand, covered by a deep-green swathe of
_Zosteræ_, with jointed root and slim flowers, unfurnished with petals,
it would be more representative still.”
Let us now take a glance at the animals belonging to this period.
The class of Fishes seem to have held the first rank and importance in
the Old Red Sandstone _fauna_; but their structure was very different
from that of existing fishes: they were provided with a sort of cuirass,
and from the nature of the scales were called _Ganoid_ fishes. Numerous
fragments of these curious fishes are now found in geological
collections; they are of strange forms, some being completely covered
with a cuirass of many pieces, and others furnished with wing-like
pectoral fins, as in _Pterichthys_.
Let any one picture to himself the surprise he would feel should he, on
taking his first lesson in geology, and on first breaking a stone--a
pebble, for instance, exhibiting every external sign of a water-worn
surface--find, to appropriate Archdeacon Paley’s illustration, a watch,
or any other delicate piece of mechanism, in its centre. Now, this,
thirty years ago, is exactly the kind of surprise that Hugh Miller
experienced in the sandstone quarry opened in a lofty wall of cliff
overhanging the northern shore of the Moray Frith. He had picked up a
nodular mass of blue Lias-limestone, which he laid open by a stroke of
the hammer, when, behold! an exquisitely shaped Ammonite was displayed
before him. It is not surprising that henceforth the half-mason,
half-sailor, and poet, became a geologist. He sought for information,
and found it; he found that the rocks among which he laboured swarmed
with the relics of a former age. He pursued his investigations, and
found, while working in this zone of strata all around the coast, that a
certain class of fossils abounded; but that in a higher zone these
familiar forms disappeared, and others made their appearance.
He read and learned that in other lands--lands of more recent
formation--strange forms of animal life had been discovered; forms which
in their turn had disappeared, to be succeeded by others, more in
accordance with beings now living. He came to know that he was
surrounded, in his native mountains, by the sedimentary deposits of
other ages; he became alive to the fact that these grand mountain ranges
had been built up grain by grain in the bed of the ocean, and the
mountains had been subsequently raised to their present level by the
upheaval of one part of its bed, or by the subsidence of another. The
young geologist now ceased to wonder that each bed, or series of beds,
should contain in its bosom records of its own epoch; it seemed to him
as if it had been the object of the Creator to furnish the inquirer with
records of His wisdom and power, which could not be misinterpreted.
[Illustration: Fig. 31.--Fishes of the Devonian Epoch. 1. Coccosteus,
one-third natural size. 2. Pterichthys, one-fourth natural size. 3.
Cephalaspis, one-fourth natural size.]
Among the Fishes of Old Red Sandstone, the _Coccosteus_ (Fig. 31, No. 1)
was only partially cased in a defensive armour; the upper part of the
body down to the fins was defended by scales. _Pterichthys_ (No. 2), a
strange form, with a very small head, furnished with two powerful
paddles, or arms, like wings, and a mouth placed far behind the nose,
was entirely covered with scales. The _Cephalaspis_ (No. 3), which has a
considerable outward resemblance to some fishes of the present time, was
nevertheless mail-clad, only on the anterior part of the body.
Other fishes were provided with no such cuirass, properly so called, but
were protected by strong resisting scales, enveloping the whole body.
Such were the _Acanthodes_ (1), the _Climatius_ (2), and the
_Diplacanthus_ (3), represented in Fig. 32.
[Illustration: Fig. 32.--Fishes of the Devonian epoch. 1. Acanthodes. 2.
Climatius. 3. Diplacanthus.]
Among the organic beings of the Devonian rocks we find worm-like
animals, such as the _Annelides_, protected by an external shell, and
which at the present day are probably represented by the _Serpulæ_.
Among Crustaceans the _Trilobites_ are still somewhat numerous,
especially in the middle rocks of the period. We also find there many
different groups of Mollusca, of which the _Brachiopoda_ form more than
one-half. We may say of this period that it is the reign of
Brachiopoda; in it they assumed extraordinary forms, and the number of
their species was very great. Among the most curious we may instance the
enormous _Stringocephalus Burtini_, _Davidsonia Verneuilli_, _Uncites
gryphus_, and _Calceola Sandalina_, shells of singular and fantastic
shape, differing entirely from all known forms. Amongst the most
characteristic of these Mollusca, _Atrypa reticularis_ (Fig. 33) holds
the first rank, with _Spirifera concentrica_, _Leptæna Murchisoni_, and
_Productus subaculeatus_. Among the Cephalopoda we have _Clymenia
Sedgwickii_ (Fig. 34), including the _Goniatites_, illustrating the
Ammonites, which so distinctly characterise the Secondary epoch, but
which were only foreshadowed in the Devonian period.
[Illustration: Fig. 33.--Atrypa reticularis.]
[Illustration: Fig. 34.--Clymenia Sedgwickii.]
Among the Radiata of this epoch, the order Crinoidea are abundantly
represented. We give as an example _Cupressocrinus crassus_ (Fig. 35).
The Encrinites, under which name the whole of these animals are
sometimes included, lived attached to rocky places and in deep water, as
they now do in the Caribbean sea.
The Encrinites, as we have seen, were represented during the Silurian
period in a simple genus, _Hemicosmites_, but they greatly increased in
numbers in the seas of the Devonian period. They diminish in numbers, as
we retire from that geological age; until those forms, which were so
numerous and varied in the earliest seas, are now only represented by
two genera.
[Illustration: Fig. 35.--Cupressocrinus crassus.]
The Old Red Sandstone rocks are composed of schists, sandstone, and
limestones. The line of demarcation between the Silurian rocks and those
which succeed them may be followed, in many places, by the eye; but, on
a closer examination, the exact limits of the two systems become more
difficult to fix. The beds of the one system pass into the other by a
gradual passage, for Nature rarely admits of violent contrasts, and
shows few sudden transitions. By-and-by, however, the change becomes
very decided, and the contrast between the dark grey masses at the base
and the superincumbent yellow and red rocks become sufficiently
striking. In fact, the uppermost beds of the Silurian rocks are the
passage-beds of the overlying system, consisting of flagstones,
occasionally reddish, and called in some districts “tile-stones.” Over
these lie the Old Red Sandstone conglomerate, the Caithness flags, and
the great superincumbent mass which forms the upper portion of the
system. Though less abrupt than the eruptive and Silurian mountains, the
Old Red Sandstone scenery is, nevertheless, distinguished by its
imposing outline, assuming bold and lofty escarpments in the Vans of
Brecon, in Grongar Hill, near Caermarthen, and in the Black Mountain of
Monmouthshire, in the centre of a landscape which, wood, rock, and river
combine to render perfect. But it is in the north of Scotland where this
rock assumes its grandest aspect, wrapping its mantle round the loftiest
mountains, and rising out of the sea in rugged and fantastic masses, as
far north as the Orkneys. In Devon and Cornwall, where the rocks are of
a calcareous, and sometimes schistose or slaty character, they are
sufficiently extensive to have given a name to the series, which is
recognised all over the world.
In Herefordshire, Worcestershire, Shropshire, Gloucestershire, and South
Wales, the Old Red Sandstone is largely developed, and sometimes attains
the thickness of from 8,000 to 10,000 feet, divided into: 1.
Conglomerate; 2. Brown stone, with _Eurypterus_; 3. Marl and cornstones,
with irregular courses of concrete limestone, in which are spines of
Fishes and remains of _Cephalaspis_ and _Pteraspis_; 4. Thin
olive-coloured shales and sandstone, intercalated with beds of red marl,
containing _Cephalaspis_ and _Auchenaspis_. In Scotland, south of the
Grampians, a yellow sandstone occupies the base of the system;
conglomerate, red shales, sandstone and cornstones, containing
_Holoptychius_ and _Cephalaspis_, and the Arbroath paving-stone,
containing what Agassiz recognised as a huge Crustacean.
[Illustration: Fig. 36.--Trinucleus Lloydii. (Llandeilo Flags.)]
Some of the phenomena connected with the older rocks of Devonshire are
difficult to unravel. The Devonian, it is now understood, is the
equivalent, in another area, of the Old Red Sandstone, and in Cornwall
and Devonshire lie directly on the Silurian strata, while elsewhere the
fossils of the Upper Silurian are almost identical with those in the
Devonian beds. The late Professor Jukes, with some other geologists, was
of opinion that the Devonian rocks of Devonshire only represented the
Old Red Sandstone of Scotland and South Wales in part; the Upper
Devonian rocks lying between the acknowledged Old Red Sandstone and the
Culm-measures being the representatives of the lower carboniferous rocks
of Ireland.
Mr. Etheridge, on the other hand, in an elaborate memoir upon the same
subject, has endeavoured to prove that the Devonian and Old Red
Sandstone, though contemporaneous in point of time, were deposited in
different areas and under widely different conditions--the one strictly
marine, the other altogether fresh-water--or, perhaps, partly
fresh-water and partly estuarine. This supposition is strongly supported
by his researches into the mollusca of the Devonian system, and also by
the fish-remains of the Devonian and Old Red Sandstone of Scotland and
the West of England and Wales.[42] The difficulty of drawing a
sharply-defined line of demarcation between different systems is
sufficient to dispel the idea which has sometimes been entertained that
special _faunæ_ were created and annihilated in the mass at the close of
each epoch. There was no close: each epoch disappears or merges into
that which succeeds it, and with it the animals belonging to it, much as
we have seen them disappear from our own fauna almost within recent
times.
[42] For fuller details on this subject, see J. B. Jukes’ “Manual of
Geology,” 3rd ed., p. 762. Also, R. Etheridge, _Quart. Journ.
Geol. Soc._, vol. 23, p. 251.