Part 10 (1/2)

The cla.s.s 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 cuira.s.s, 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 cuira.s.s 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 ill.u.s.tration, 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 sh.o.r.e of the Moray Frith. He had picked up a nodular ma.s.s 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 cla.s.s 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.

[Ill.u.s.tration: 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 cuira.s.s, 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.

[Ill.u.s.tration: 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 sh.e.l.l, and which at the present day are probably represented by the _Serpulae_.

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 a.s.sumed 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_, sh.e.l.ls 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_, _Leptaena Murchisoni_, and _Productus subaculeatus_. Among the Cephalopoda we have _Clymenia Sedgwickii_ (Fig. 34), including the _Goniat.i.tes_, ill.u.s.trating the Ammonites, which so distinctly characterise the Secondary epoch, but which were only foreshadowed in the Devonian period.

[Ill.u.s.tration: Fig. 33.--Atrypa reticularis.]

[Ill.u.s.tration: Fig. 34.--Clymenia Sedgwickii.]

Among the Radiata of this epoch, the order Crinoidea are abundantly represented. We give as an example _Cupressocrinus cra.s.sus_ (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.

[Ill.u.s.tration: Fig. 35.--Cupressocrinus cra.s.sus.]

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 pa.s.s into the other by a gradual pa.s.sage, 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 ma.s.ses at the base and the superinc.u.mbent yellow and red rocks become sufficiently striking. In fact, the uppermost beds of the Silurian rocks are the pa.s.sage-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 superinc.u.mbent ma.s.s 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, a.s.suming bold and lofty escarpments in the Vans of Brecon, in Grongar Hill, near Caermarthen, and in the Black Mountain of Monmouths.h.i.+re, 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 a.s.sumes its grandest aspect, wrapping its mantle round the loftiest mountains, and rising out of the sea in rugged and fantastic ma.s.ses, 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 Herefords.h.i.+re, Worcesters.h.i.+re, Shrops.h.i.+re, Gloucesters.h.i.+re, 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 Aga.s.siz recognised as a huge Crustacean.

[Ill.u.s.tration: Fig. 36.--Trinucleus Lloydii. (Llandeilo Flags.)]

Some of the phenomena connected with the older rocks of Devons.h.i.+re 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 Devons.h.i.+re 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 Devons.h.i.+re 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 _faunae_ were created and annihilated in the ma.s.s 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.

CARBONIFEROUS PERIOD.

In the history of our globe the Carboniferous period succeeds to the Devonian. It is in the formations of this latter epoch that we find the fossil fuel which has done so much to enrich and civilise the world in our own age. This period divides itself into two great sub-periods: 1.

The _Coal-measures_; and 2. The _Carboniferous Limestone_. The first, a period which gave rise to the great deposits of coal; the second, to most important marine deposits, most frequently underlying the coal-fields in England, Belgium, France, and America.