Old Red Sandstone and Devonian System, the name given to certain series of strata that are intermediate in age between the Silurian and Carboniferous systems. These, known respectively as 'Old Red Sandstone' and 'Devonian,' are nowhere seen together, but the one is believed to be the equivalent of the other.
Old Red Sandstone.—This series, which underlies the Carboniferous system, was so called to distinguish it from another set of red sandstones which rests upon the Carboniferous strata, and was formerly known as the New Red Sandstone (see PERMIAN, and TRIASSIC). In the British Islands the Old Red Sandstone is confined to Scotland, Wales, and Ireland. In Scotland it comprises two groups of strata, the upper resting unconformably on the lower. The lower group attains a great thickness (20,000 feet as a maximum), and consists of coarse red, gray, brown, and purplish and sometimes yellowish sandstones, gray flagstones, clays, and shales, coarse conglomerates, and local beds of limestone and cornstone. Associated with these strata are interbedded lavas (porphyrites, diabase, &c.) and tufts, which in some regions (Sidlaws, Ochils, Pentlands, Cheviots, Ayrshire, &c.) reach several thousand feet in thickness. The fossils of the Lower Old Red Sandstone consist chiefly of fishes and crustaceans and badly-preserved plants, which are mostly lycopodiaceous. In Lanarkshire a thin bed of shale in the group has yielded a few Upper Silurian fossils. The upper group comprises red sandstones, clays, conglomerates, and breccias, the sandstones in some areas being gray, yellow, or white. Few fossils occur, and these are chiefly the remains of ganoid fishes. In Arran the group contains a limestone which has yielded marine Carboniferous fossils. In some places this group passes upwards conformably into the lower member of the Carboniferous system.
In Wales the Old Red Sandstone appears to graduate downwards into the Upper Silurian, and to be likewise conformable with the overlying Carboniferous strata. In Ireland, as in Scotland, there appears to be an unconformity between the upper and lower groups of the series, the former passing conformably upwards into the Carboniferous, and the latter ('Glengariff Grits') graduating downwards into the Upper Silurian.
In Scotland the Old Red Sandstone strata are developed chiefly in the Lowlands, but here and there they rise to considerable elevations. They flank the Palæozoic strata of the southern uplands and the Highlands, and are probably more or less continuous underneath the overlying Carboniferous strata throughout the whole breadth of central Scotland. Other wide areas occur in the lower basin of the Tweed, along the borders of the Moray Firth, in Caithness, Orkney Islands, &c. In Wales the Old Red Sandstone is well developed in the region watered by the Usk and the Wye. In Ireland it is met with chiefly in the west and south-west.
Devonian.—In Devon and Cornwall we meet with a very different series of strata occupying the same stratigraphical position as the Old Red Sandstone. The Devonian strata pass up conformably into the Carboniferous system, but the base of the series is not seen, so that the relation of the strata to the Silurian is not known. The English Devonian probably does not exceed 10,000 or 12,000 feet in thickness. It consists of three groups (Lower, Middle, and Upper), the rocks being principally gray and brown slates, brown, yellow, red, and purple sandstones, grits, conglomerates, calcareous slates, and limestones. The calcareous members of the series are generally well charged with fossils of marine types, and are developed chiefly in the middle group.
Devonian rocks occupy wide areas at the surface on the Continent. They appear in the north of France, and extend from the Boulonnais eastwards through Belgium to Westphalia. In northern Russia they extend over more than 7000 miles, and crop up along the flanks of the Urals. But the areas exposed to view probably bear but a small proportion to those which lie buried underneath later formations. In central Europe the strata have the general aspect of the English Devonian, and contain relics of the same marine fauna. In Russia the strata are remarkable for showing alternations of calcareous and arenaceous rocks—the former of which contain an assemblage of fossils of a Devonian facies, while the latter are charged with the remains of a fish fauna resembling that of the Scottish Old Red Sandstone. It may be noted that volcanic rocks are here and there associated with the Devonian strata of central Europe. In North America both types of strata appear; the arenaceous type occurring in Nova Scotia and New Brunswick, while the Devonian type is met with in the state of New York and the Appalachian region, and is largely developed in the Mississippi basin.
Life of the Period.—Fucoidal markings are not uncommon in the Devonian strata, but land-plants rarely occur. These latter, however, are met with now and again in the Old Red Sandstone, more especially in that of Nova Scotia and New Brunswick. The chief forms are tree-ferns and small herbaceous ferns, lycopods (lepidodendroids), great horsetails (Calamites), and sigillarioids. The vegetation would thus appear to have been for the most part flowerless. Here and there, however, remains of large conifers have been detected. Among the lower forms of life that swarmed in the seas of the period were rugose and tabulate corals. Of the former the most characteristic were Cyathophyllum, Cystiphyllum, Calceola, &c., while the honeycomb corals (Favosites) are the most common of the tabulate forms. Echinoderms abounded, especially crinoids (Cupressocrinus, Cyathocrinus) and pentrem- ites. Trilobites, which formed so marked a feature in the life of the Silurian seas, were now reduced in number and variety—among the more notable forms being Phacops, Homalonotus, and Bronteus. Some of the eurypterids (most of which are small) attained a large size, one of these (Pterygotus) being 5 or 6 feet long. They occur chiefly in the Old Red Sandstone. From the same strata in North America have come the remains of insects—neuropteroid and orthopteroid wings of ancestral forms of May-fly, &c. Myriopods have also been recognised. Brachiopods are among the most common Devonian fossils; indeed this group appears to have reached its maximum development in the seas of that period. Very characteristic forms are Uncites, Stringocephalus, and Rensseleria. Lamellibranchs were well represented, some of the notable genera being Pterinea, Megalodon, Cucullaea, Avicula, &c. The marine gasteropods call for no particular mention, for they belong chiefly to types which had come down from earlier Palæozoic times. One may note, however, that the earliest pulmonates (Snails, &c.) come from the Old Red Sandstone. The straight Orthoceras and other old genera of cephalopods continued to flourish, but coiled forms (Clymenia, Goniatites) began to predominate in Devonian times. From the Old Red Sandstone chiefly come the remains of numerous ganoid fishes—a group feebly represented in existing waters. Among these are the placaganoids Cephalaspis, Pteraspis, Pterichthys, and Coccosteus, and the lepidoganoids Osteolepis, Diplopterus, Holoptychius, Acanthodes, &c. The largest placaganoid was the Dinichthys of North America—the armoured head of which was 3 feet in length. According to Dr Newberry, this fish was probably not less than 15 feet long, 'encased in armour, and provided with formidable jaws, which would have severed the body of a man as easily as he bites off a radish.' Other forms (such as Dipterus, and possibly Phaneropleuron) appear to have relations with the modern mud-fish (Ceratodus) of Australia.
It is obvious that in the Old Red Sandstone and Devonian we have two distinct types of sedimentation; the two series must have accumulated under different physical conditions. The Devonian strata are unquestionably of marine origin, while the Old Red Sandstone beds are believed to have been deposited in large lakes or inland seas. Hence we meet with the latter in a few more or less isolated basins, while the former extends over enormous regions. From the geographical distribution of the marine Devonian in Europe we gather that during the period in question the sea covered the south of England and the north-east of France, whence it extended eastwards, occupying the major portion of central Europe, and sweeping north-east through Russia, and how much farther we cannot tell. North of that sea stretched a wide land surface, in the hollows of which lay great lakes and inland seas, which seem now and again to have communicated with the ocean. It was in these broad sheets of water that the Old Red Sandstone strata were accumulated. Several of these old lakes in Scotland were traversed by lines of volcanoes, the relics of which are seen in many of the hill-ranges of the central and southern regions of that country. Volcanic action also at the same time manifested itself in some parts of Germany, but on a smaller scale apparently than in the Scottish area. The land, as we have seen, was clothed for the most part with a monotonous flowerless vegetation, but large pines grew on the higher and drier uplands, whence they were occasionally carried down by rivers to the lakes and seas. Very little is known of the terrestrial animal life of the period; most of the fossils met with in the lacustrine sediments of the period consisting of the remarkable ganoids and eurypterids already referred to. These (the fishes especially) appear to have abounded in the lakes, whence, however, they now and again descended by the rivers to the sea. The general facies and the geographical distribution of the life of the Devonian and Old Red Sandstone are suggestive of genial climatic conditions. Some geologists, however, have thought that the coarse breccias and conglomerates which occur in the Old Red Sandstone may be indicative of somewhat cold conditions; for these masses have often quite the aspect of morainic accumulations. It is possible, therefore, either that local glaciers may have existed in certain regions, or that the temperature may have been lowered for some time over wider areas. However that may be, the presence of the Devonian fauna in the Arctic regions seems to show that the temperature of the ocean must have been more equable in Devonian times than it is now.