In Scotland the two divisions of the system are sharply separated by a pronounced unconformability which is probably indicative of a prolonged interval of erosion. In the central valley between the base of the Highlands and the southern uplands lay "Lake Caledonia." Here the lower division is made up of some 20,000 ft. of shallow-water deposits, reddish-brown, yellow and grey sandstones and conglomerates, with occasional "cornstones," and thin limestones. The grey flagstones with shales are almost confined to Forfarshire, and are known as the "Arbroath flags." Interbedded volcanic rocks, andesites, dacites, diabases, with agglomerates and tuffs constitute an important feature, and attain a thickness of 6000 ft. in the Pentland and Ochil hills. A line of old volcanic vents may be traced in a direction roughly parallel to the trend of the great central valley. On the northern side of the Highlands was "Lake Orcadie," presumably much larger than the foregoing lake, though its boundaries are not determinable. It lay over Moray Firth and the east of Ross and Sutherland, and extended from Caithness to the Orkney Islands and S. Shetlands. It may even have stretched across to Norway, where similar rocks are found in Sognefjord and Dalsfjord, and may have had communications with some parts of northern Russia. Very characteristic of this area are the Caithness flags, dark grey and bituminous, which, with the red sandstones and conglomerates at their base, probably attain a thickness of 16,000 ft. The somewhat peculiar fauna of this series led Murchison to class the flags as Middle Devonian. In the Shetland Islands contemporaneous volcanic rocks have been observed. Over the west of Argyllshire lay "Lake Lorne"; here the volcanic rocks predominate, they are intercalated with shallow-water deposits. A similar set of rocks occupy the Cheviot district.
The upper division of the Old Red Sandstone is represented in Shropshire and South Wales by a great series of red rocks, shales, sandstones and marls, some 10,000 ft. thick. They contain few fossils, and no break has yet been found in the series. In Scotland this series was deposited in basins which correspond only partially with those of the earlier period. They are well developed in central Scotland over the lowlands bordering the Moray Firth. Interbedded lavas and tuffs are found in the island of Hoy. An interesting feature of this series is the occurrence of great crowds of fossil fishes in some localities, notably at Dura Den in Fife. In the north of England this series rests unconformably upon the Lower Old Red and the Silurian.
Flanking the Silurian high ground of Cumberland and Westmorland, and also in the Lammermuir hills and in Flint and Anglesey, a brecciated conglomerate, presenting many of the characters of a glacial deposit in places, has often been classed with the Old Red Sandstone, but in parts, at least, it is more likely to belong to the base of the Carboniferous system. In Ireland the lower division appears to be represented by the Dingle beds and Glengariff grits, while the Kerry rocks and the Kiltorcan beds of Cork are the equivalents of the upper division. Rocks of Old Red type, both lower and upper, are found in Spitzbergen and in Bear Island. In New Brunswick and Nova Scotia the Old Red facies is extensively developed. The Gaspé sandstones have been estimated at 7036 ft. thick. In parts of western Russia Old Red Sandstone fossils are found in beds intercalated with others containing marine fauna of the Devonian facies.
Devonian and Old Red Sandstone Faunas.
The two types of sediment formed during this period—the marine Devonian and the lagoonal Old Red Sandstone—representing as they do two different but essentially contemporaneous phases of physical condition, are occupied by two strikingly dissimilar faunas. Doubtless at all times there were regions of the earth that were marked off no less clearly from the normal marine conditions of which we have records; but this period is the earliest in which these variations of environment are made obvious. In some respects the faunal break between the older Silurian below and the younger Carboniferous above is not strongly marked; and in certain areas a very close relationship can be shown to exist between the older Devonian and the former, and the younger Devonian and the latter. Nevertheless, taken as a whole, the life of this period bears a distinct stamp of individuality.
The two most prominent features of the Devonian seas are presented by corals and brachiopods. The corals were abundant individually and varied in form; and they are so distinctive of the period that no Devonian species has yet been found either in the Silurian or in the Carboniferous. They built reefs, as in the present day, and contributed to the formation of limestone masses in Devonshire, on the continent of Europe and in North America. Rugose and tabulate forms prevailed; among the former the cyathophyllids (Cyathophyllum) were important, Phillipsastraea, Zaphrentis, Acervularia and the curious Calceola (sandalina), an operculate genus which has given palaeontologists much trouble in its diagnosis, for it has been regarded as a pelecypod (hippurite) and a brachiopod. The tabulate corals were represented by Favosites, Michelinia, Pleurodictyum, Fistulipora, Pachypora and others. Heliolites and Plasmopora represent the alcyonarians. Stromatoporoids were important reef builders. A well-known fossil is Receptaculites, a genus to which it has been difficult to assign a definite place; it has been thought to be a sponge, it may be a calcareous alga, or a curious representative of the foraminifera.
In the Devonian period the brachiopods reached the climax of their development: they compose three-quarters of the known fauna, and more than 1100 species have been described. Changes were taking place from the beginning of the period in the relative importance of genera; several Silurian forms dropped out, and new types were coming in. A noticeable feature was the development of broad-winged shells in the genus Spirifer, other spiriferids were Ambocoelia, Uncites, Verneuilia. Orthids and pentamerids were waning in importance, while the productids (Productella, Chonetes, Strophalosia) were increasing. The strophomenids were still flourishing, represented by the genera Leptaena, Stropheodonta, Kayserella, and others. The ancient Lingula, along with Crania and Orbiculoidea, occur among the inarticulate forms. Another long-lived and wide-ranging species is Atrypa reticularis. The athyrids were very numerous (Athyris, Retzia, Merista, Meristella, Kayserina, &c.); and the rhynchonellids were well represented by Pugnax, Hypothyris, and several other genera. The important group of terebratulids appears in this system; amongst them Stringocephalus is an eminently characteristic Devonian brachiopod; others are Dielasma, Cryptonella, Rensselaeria and Oriskania.
The pelecypod molluscs were represented by Pterinea, abundant in the lower members along with other large-winged forms, and by Cucullella, Buchiola and Curtonotus in the upper members of the system. Other genera are Actinodesma, Cardiola, Nucula, Megalodon, Aviculopecten, &c. Gasteropods were becoming more important, but the simple capulid forms prevailed: Platyceras (Capulus), Straparollus, Pleurotomaria, Murchisonia, Macrocheilina, Euomphalus. Among the pteropods, Tentaculites was very abundant in some quarters; others were Conularia and Styliolina. In the Devonian period the cephalopods began to make a distinct advance in numbers, and in development. The goniatites appear with the genera Anarcestes, Agoniatites, Tornoceras, Bactrites and others; and in the upper strata the clymenoids, forerunners of the later ammonoids, began to take definite shape. While several new nautiloids (Homaloceras, Ryticeras, &c.) made their appearance several of the older genera still lived on (Orthoceras, Poterioceras, Actinoceras).
Crinoids were very abundant in some parts of the Devonian sea, though they were relatively scarce in others; they include the genera Melocrinus, Haplocrinus, Cupressocrinus, Calceocrinus and Eleuthrocrinus. The cystideans were falling off (Proteocystis, Tiaracrinus), but blastoids were in the ascendant (Nucleocrinus, Codaster, &c.). Both brittle-stars, Ophiura, Palaeophiura, Eugaster, and true starfishes, Palaeaster, Aspidosoma, were present, as well as urchins (Lepidocentrus).
When we turn to the crustaceans we have to deal with two distinct assemblages, one purely marine, trilobitic, the other mainly lacustrine or lagoonal with a eurypteridian facies. The trilobites had already begun to decline in importance, and as happens not infrequently with degenerating races of beasts and men, they began to develop strange eccentricities of ornamentation in some of their genera. A number of Silurian genera lived on into the Devonian period, and some gradually developed into new and distinctive forms; such were Proëtus, Harpes, Cheirurus, Bronteus and others. Distinct species of Phacops mark the Lower and Upper Devonian respectively, while the genus Dalmania (Odontochile) was represented by species with an almost world-wide range. The Ostracod Entomis (Cypridina) was extremely abundant in places—Cypridinen-Schiefer—while the true Cypridina was also present along with Beyrichia, Leperditia, &c. The Phyllocarids, Echinocaris, Eleuthrocaris, Tropidocaris, are common in the United States. It is in the Old Red Sandstone that the eurypterids are best preserved; foremost among these was Pterygotus; P. anglicus has been found in Scotland with a length of nearly 6 ft.; Eurypterus, Slimonia, Stylonurus were other genera.