DEVONIAN SYSTEM, THE. The Devonian rocks re ceive their name from the locality where the stratigraphical and historical position of those of marine origin—between the Silurian below and the Carboniferous above—was first recognized. In some areas they consist of sediments laid down in fresh water or on land, and as these are usually arenaceous and red in colour, they are known as the Old Red Sandstone.
In Devonian times Africa was already an ancient continent, but it lay far south of its present position and extended into the Antarctic. A second continent stretched across northern Europe to the north-east of North America. Between them lay the ocean the geologists call Tethys. In the Western Hemisphere narrow seas existed in the east and west of what is now North America, and low land, submerged later, lay between. In the Old Red Sandstone are the first well preserved remains of vertebrates, comprising many strange types. (The letters D, L, M and U indi cate the division of the period in which they occur—Downtonian [passage beds], Lower, Middle and Upper Devonian.) Palaeospondylus M, was tadpole-shaped, perhaps allied to the lamprey.
An amphibian footprint in the Upper Old Red of Pennsylvania shows that the higher vertebrates had made their appearance before the close of the period.
Among the invertebrates, the "eurypterids" allied to the king crabs and arachnids, include the genera Eurypterus D, L, Ptery gotus D, L. The bivalve crustacean Estheria M, U is of interest as the genus is now confined to saline springs in deserts. Isopods and other crustaceans also occur. Myriapods are met with at different horizons, but true insects are unknown. A fresh-water lamelli branch Archanodon (Amnigenia) M, U is found in South Wales, Europe, North America and, it is said, Bolivia.
The plants are generalized types distantly allied to the ferns. Those best known are grouped as Psilophvtales. In the Rhynie chert, silicified peat of Middle Old Red age, the minute structure of the plants Rhynia, Hornia and Asteroxylon has been preserved. In the Upper Old Red Archaeopteris hibernica, with fern-like foliage, is widely distributed.
The vegetation, like the animal life, was probably confined to streams, lakes and marshes, while the high ground was left un protected by vegetation.
Brachiopods are numerous and the species of Spirifer are of especial value for zoning. Gastropods and lamellibranchs indicate littoral conditions, except those with thin shells, and certain thick shelled lamellibranchs associated with coral reefs. Conical forms such as Tentaculites and Styliolina play an important part. Trilo bites are less numerous than in the Silurian. They are often pro vided with spines.
It is only under special conditions that marine and continental sediments are associated. As the latter are more constant in char acter, they will be taken first. They include conglomerates laid down by torrents escaping from mountain ranges, and sandstones and shales spread out by streams traversing the plains. Some lake deposits are fluviatile in origin, others are formed of fine sand and dust, transported by the wind till they are arrested by sheets of water or by damp soil. Sometimes the sand is rounded by attri tion. Arkose—disintegrated but undecomposed granite materials —is itself evidence of desert conditions. Where organic life was plentiful, the deposits may be bituminous, or coal may occur.
Sometimes there is evidence of alternation of wet and dry sea sons. In the latter, the shallow lakes or moist areas dried up and the calcium carbonate that had been in solution separated out in the form of concretions or "cornstones." Gypsum and salt also occur in places.
On the Welsh border the transition to continental conditions is marked by the Ludlow Bone Bed, a widely extended layer some six inches in thickness consisting of fragments of fish and euryp terids. Then follows the Downton Sandstone and Temeside Shales, the Downtonian of Elles and Slater. This term has been extended by Wickham King to 70o ft. more of marls and sandstones in which Lingula and a few other marine forms are occasionally present. He calls it alternatively the "Anaspida Marls," and makes the Bone Bed the base not only of the Downtonian, but of the Old Red Sandstone. The Downtonian thus defined is charac terized by Cephalaspis lyelli, Cyathaspis, and Anaspida. Pteraspis is absent. It is covered conformably by the "Dittonian" or "Pter aspis Cornstones," distinguished by Cephalaspis murchisoni and Pteraspis. This is followed, also conformably, by the unfossilifer ous "Brownstones," sandstones formerly referred to the Upper Old Red. This, however, overlaps not only them but the Dittonian and Downtonian and rests in Gower on the Silurian. In Cardiff and Bristol, it is conformable on the Lower Old Red. Among the Upper Old Red fossils are Bothriolepis, Holoptychius and Archan odon. It passes up conformably into the marine Carboniferous. In South Wales the Downtonian is represented by the Lower Red Marls, and the Dittonian by the Upper Red Marls and the Senni Beds, fossiliferous sandy beds formerly included in the Brown stones. The Old Red extends to Pembrokeshire where marine strata occur in the upper beds.
Similar marine deposits in South Ireland are known as the Coomhola Beds. Below them are the fresh-water Kiltorcan Beds with Coccosteus disjectus and Archaeopteris hibernica. The Irish Old Red rests unconformably on the Dingle Grits and Slates which are conformable to the Silurian.
Unfossiliferous rocks of Old Red types occur at the base of the Carboniferous of North Wales, the Lake district and Isle of Man.
The Old Red reappears in the Cheviots (where it is largely igneous) and on the margins of the Forth and Clyde syncline. Here both Downtonian and Dittonian are recognizable by their fossils. The former follows the Silurian conformably, but is fre quently separated from the Dittonian by an unconformity. The latter includes conglomerates, lavas, tuffs and lacustrine deposits, with Mesacanthus, Cephalaspis lyelli and Pteraspis.
On the Moray Firth, in Caithness, the Orkneys and Shetlands are strata intermediate in age between the Dittonian and Upper Old Red, which are classed as Middle Old Red, but they are more allied to the Upper than to the Lower. In Caithness an unf ossilif erous basement series is followed by conglomerate, and arkose, and then by thick beds of bituminous and calcareous flags and sandstones. Characteristic fossils are Thursius, Dipterus val enciennesi, Mesacanthus, Coccosteus decipiens, Ptericlithys, Es theria and Palaeospondylus. A higher horizon, the John o' Groats beds, has a distinct fauna.
The Upper Old Red sandstone of Scotland overlies uncon formably the Lower Old Red in the south and the Middle in the north. There are two horizons. The Lower, the Nairn sandstone, contains Psammosteus tesselatus, Asterolepis maxima, and the Upper, the Alves or Scaat Craig beds, has Psammosteus pus tulatus, Bothriolepis major and Holoptychius nobilissimus. Only one form, H. decoratus, is common to both.
The Downtonian appears to occur in Norway and the Dittonian in Spitsbergen. Upper Old Red with Holoptychius and Archaeop teris is found in Bear island, associated with thin coals. In Nova Scotia and New Brunswick the Dittonian with Cephalaspis and Pteraspis rests unconformably on the Silurian. The Upper Old Red also occurs in the latter, and in New York State it is repre sented by the Oneonta beds with Estheria, and the Catskill with Bothriolepis and Holoptychius.
In Antarctica, the Upper Old Red age of the Beacon sandstone is indicated by the same two forms.
The marine succession sometimes includes continental inter calations, and even in the marine beds, different facies occur according to the varying conditions under which deposition took place.
Nearest to land were the brackish water and littoral deposits, which may show by red or purple coloration the desert origin of their materials. In deeper water and farther from shore terri genous mud, now shale or slate, was laid down, and, where condi tions were favourable, coral reefs or lenticular limestones were formed. At still greater depths and distance from land, calcareous ooze accumulated with thin shelled molluscs, goniatites and simple corals. The position of these different facies was modified by changes in the level of the sea and floor movements. To the former has been attributed the recession of the sea throughout the Northern Hemisphere at the beginning of the period. The shore line of the northern continent then passed south of Ireland, through Cornwall and south Devon, entered northern France near Boulogne, passed by way of Fepin to the north of the massives of the Rhine and Harz, and of Bohemia, then southward to the Dniester region and central Russia, and north-westward to the White sea. In the Middle Devonian an important advance of the sea began and reached its maximum at the close of the Middle or beginning of the Upper Devonian. The shore line of the north ern continent then passed across south Ireland, Pembrokeshire, the Bristol channel, Middlesex, north Belgium, the Baltic, Finland, and Lapland. In the later Upper Devonian there was a new retirement of the sea, and the shore line moved southward again— in some places nearly as far as before, in others, as in Bohemia, still farther.
Parallel to the continental shores, the weight of the terrigenous sediments and accumulated limestones caused a sag of the sea bottom in "geosynclines," and this permitted a still heavier accu mulation of deposits.
It is in the Palaeozoic massive of the Ardennes and the Rhine land that the marine Devonian has been most studied. There it has been thrown by the subsequent Armorican movements into folds dipping to the south, and it is in the troughs of these folds that the Middle and Upper Devonian are found as outliers in the Lower. On the Meuse are two main synclines, that of Namur in the north and that of Dinant in the south. On the southern limit of the latter the base of the system is formed by a conglomerate and arkose. These are covered by the Mondrepuits shales with Spirifer sulcatus (mercurei) which are correlated with the Down tonian. They are succeeded by the mottled Oignies shales contain ing Pteraspis and, therefore, presumably Dittonian.
The next horizon, the Siegenian, is represented in Belgium by the Anor grits and the Montigny Greywacke, and in the Rhineland by the Taunus quartzite and the Hunsri ck slates, or their equiva lents, the Siegen slates. Then follows the Emsian with, on the Meuse, the Red shales of Vireux, and on the Rhine, the Lower Coblenz beds. A higher horizon is that of the Hierges Greywacke corresponding to the Coblenz quartzite and Upper Coblenz beds.
On the north of the Dinant syncline and near the former coastline, the Lower Devonian is represented only by the Burnot conglomerate. In the south-eastern Harz, the Lower Devonian has a deep water ("Hercynian") facies with Spirifer togatus, Sp. hercyniae, Capulus and Phacops fecundus. The same fauna ap pears in the Konjeprus limestone of Bohemia, Barrande's F2 zone. The Middle Devonian is divided into the Eifelian and the Givetian. At the base of the former is the Spirifer cultri jugatus zone. Then follow the Calceola Beds. The Givetian is dis tinguished by the large brachiopod Stringocephalus. The Middle Devonian is usually more calcareous than the Lower and the Givetian than the Eifelian. In the Namur syncline the Calceola beds rest directly, except for a conglomerate, on the Silurian. Still further north in Brabant, the Eifelian is absent and the Givetian is only separated from the Silurian by a similar conglom erate. In south-west Westphalia the Eifelian and Lower Givetian consist of sandy clays often more or less calcareous. They have yielded plants and Archanodon and were probably laid down in fresh water. Similarly in the Boulonnais, sandy beds with plants seem to represent the Eifelian as they are overlaid by a limestone with Stringocephalus. In the southern Rhineland the middle division consists of deep-water shales and limestones. In Bohemia the Eifelian is represented by Barrande's zones G,, G2 arid G3 deep-water limestones; the Givetian, on the other hand, by the zones Hr, H2 and H3, shales and sandstones with plants and Stringocephalus. The deep-water Middle Devonian has been zoned by species of goniatites. Of these, Meneceras and Tor noceras are characteristic of the Upper Givetian.
The Upper Devonian has two main divisions, the Frasnian and the Famennian. The former commences by limestone with Buchiola retrostriata, succeeded by the Budesheim or Matagne shales, with characteristic goniatites, including Manticoceras in tumescens and Tornoceras simplex, and the bituminous Kellwas limestone. Other Frasnian fossils are Spirifer verneuili, which ex tends through all the Upper Devonian and Phillipsastrea. The deep-water Famennian is characterized by the prevalence of the Clymenia group. In the lower Famennian it is accompanied by the goniatite genus Cheiloceras which marks the horizon of the Nehden shales. The Cypridina shales, laid down in shallower water, contain the small bivalve crustacean Entomis—formerly Cypridina—serrato-striata in large numbers. In the Namur syn cline continental conditions prevailed in the upper Famennian with Holoptychius and Archaeopteris, though in some places lit toral forms like Cucullaea hardingi are found. In the Boulonnais the Stringocephalus limestone is followed by fossiliferous Frasnian limestone, and then reddish shales with Spirifer verneuili and sandstone comparable with that of Condroz. In Bohemia there is no evidence of the Upper Devonian, but in the eastern Alps the whole formation is well represented.
In the south-west of England, the Devonian is also strongly folded. The lowest rocks in north Devon and west Somerset are the continental Foreland grits—red sandstones and conglomerates, of which the base is not seen as they form an anticline ; they con tain only scanty indeterminable fragments of fish and plants. The succeeding Lynton beds appear to include Dittonian beds, with Pteraspis; and Siegenian with S piri f cr primaevus and Sp.
hystericus. They are overlaid by grits resembling the Forelands, and called collectively the Hangman grits. The lower beds, the Trentishoe grits, have yielded no fossils, but at a higher level are sandstones and shales with plant remains and a scale of Coccos teus, followed by littoral beds with casts of Myalina, Cucullaea and Naticopsis, and by calcareous grits with Stringocephalus which must be Givetian. Above these is a thick succession of grits, slates and limestones, the Ilfracombe beds. They include coral reefs with Cyathophyllum (Phacellophyllum) caespitosum, Endo phyllum, and Pachypora cervicornis. At a somewhat higher horizon occur Heliophyllum helianthoides and Spirifer verneuili. The latter, at least, indicates the Upper Devonian. The higher beds, which are arenaceous, are succeeded by the smooth Morte slates with Spirifer verneuili, and then by the Pickwell Down sandstone, a continental deposit corresponding to the Condroz sandstone.
At the base is a bed of volcanic ash associated with fish remains including Bothriolepis and Holoptychius. The Pickwell Down sandstone passes up into the littoral Baggy and Marwood beds. These contain two fossils Cucullaea unilateralis and Ptychopteria damnoniensis, also found in the marine beds in the Upper Old Red of Pembrokeshire and the Coomhola grits of Ireland. Then follow the Pilton beds which extend up into the Carboniferous, which occupies a great syncline crossing Devonshire from east to west. Beyond this the Devonian reappears in Cornwall and south Devon. In south-west Cornwall it rests on the Silurian, but the rocks are so dislocated that it is impossible to say whether the formations are conformable or not. The Downtonian may be represented by the Portscatho beds, arenaceous slates which pass up into the smooth Falmouth slates, apparently identical with the Dartmouth slates that form an anticline traversing south Devon and Cornwall. They contain Cephalaspis and Pteraspis, and ap pear to be fresh-water deposits of Dittonian age, except in south Devon, where the presence of Bellerophon and Loxonema indi cates brackish conditions. The succeeding Meadfoot beds have yielded Spirifer primaevus, Sp. hystericus, and other marine fossils of Siegenian age. Then follow the Staddon grits which include beds of Emsian age, with Tropidolephis rhenanus and others at the base of the Middle Devonian, with Spirifer cultri jugatus.
Next are slates and slaty limestones with Calceola. There are also great developments of limestone near Plymouth and Torquay in which both Middle and Upper Devonian are represented, for the fossils include Spirifer curvatus, Stringocephalus, Spirifer verneuili and Rhynchonella (Hypothyris) cuboides. The Biides heim horizon is found at Trevona bay, in North Cornwall, and Saltern cove, in South Devon; that of the Nehden shales at Port Quin in north Cornwall. Famennian horizons are also represented by strata with Clymenia, Entomis serrato-striata and Styliolina. The Devonian, south of the syncline, is overlapped by beds high up in the Carboniferous limestone, and the overlap increases to the south, but this relation is obscured by overthursts of the Devonian over the Carboniferous.
Deep under London both continental and marine Upper Devo nian are met with. Many horizons of the Devonian occur in Armorican folds of the north-west of France. In the Devonian rocks of southern Europe, the Lower Devonian and Eifelian are represented by limestones and dolomites laid down at moderate depths; the Givetian and Upper Devonian by limestone accumu lated in deeper water, with pale reddish or greenish tints, believed to characterize calcareous oozes.
In the Baltic and adjoining areas the Downtonian succeeds to the Silurian, but there is then a break (without unconformity) followed by typical Middle Old Red. This is, however, succeeded by marine Frasnian with Spirifer Anossofi, an eastern form. In the Famennian, continental conditions returned. Further south and east, however, there are marine intercalations, and in central Russia and the Urals, the Famennian is wholly marine. Both the Scotch zones of the Upper Old Red are represented in north eastern Europe—the Nairn sandstone by the Wenden deposits on the Aa in Livonia, and the Alves by those of the Sjass river near Leningrad.