2. THE DEVONIAN SERIES.-I purpose in this division of the present Lecture to consider the characters and relations of the remaining systems of fossiliferous deposits, namely, the Devonian, Silurian, and Cambrian: all the still more deeply seated rocks, so far as our present knowledge extends, being destitute of any traces of organization, whether of the animal or of the vegetable kingdoms. The Devonian series, formerly called the Old Red Sandstone (p. 205), lies immediately beneath the Carboniferous Limestone, and is largely developed in Devonshire and Cornwall, in Pembrokeshire, Monmouthshire, and Herefordshire; in the south-east border of the Grampians, and over a large portion of the north-eastern part of Scotland. In Ireland there are extensive areas of strata of Devonian age, in the north, south, and central districts. This series consists of many alternations of conglomerates, shales, and sandstones, in various states of induration. The conglomerates are formed of quartz-pebbles, water-worn fragments of slate and other rocks, cemented together either by an argillaceous or a silicious paste, coloured more or less deeply red by peroxide of iron. The quartz has been chiefly derived from the vein-stones of the old slaterocks. These strata have evidently resulted from the degradation of ancient crystalline and schistose rocks; and have been originally ac. cumulated in the state of pebbles, sand, and mud. In Scotland, where the Devonian series was first characterized by being found to contain many peculiar fossils, and where it is of vast thickness, it will probably always be known by its first name of "Old Red Sandstone." In Devonshire it contains shells of a character intermediate to those of the Silurian, on the one hand, and those of the carboniferous rocks on the other, and none of which are known to occur in Scotland. The Devonian or Old Red strata were therefore identified first, from their geological position, by Sedgwick, Murchison, and Lonsdale.* Subsequently the two first-mentioned of these authors compared the Devonian of Britain with the infracarboniferous rocks of the Rhine.+ Lastly, the identification of the group as really one, both by position and organic contents, was established by the survey of Russia, when Sir R. Murchison, Count Keyserling, and M. de Verneuil demonstrated that the ichthyolites of the "Old Red" of Scotland, including many of the *Geol. Trans. 2nd ser. vol. v. p. 633, and p. 721. very same species, and the shells of the Devonian beds of the south of England, are congregated in the same masses of sand and shale.* Whilst there is no doubt of the marine origin of the coralliferous and shelly Devonian limestones and shales of Devon, Belgium, Germany, and Russia, there is a difference of opinion existing as to whether the Old Red strata of Scotland, Ireland, and Hereford have been accumulated in marine or in fresh water,-along the margins of a sea, or in great lakes.† The fossils, so far as the British Isles are concerned, do not decide the question. The Old Red fishes may have been inhabitants of rivers and lakes, like their modern allies, the Bony-pikes of the North American waters, and the Bichirs of the Nile. The local abundance of plant-remains merely indicates, in some cases, the close proximity of land; and in others, probably, the presence of weedy shallows. The only shell-like fossil in the Caithness schists is undescribed, and may be either a Posidonomya, a Cyclas, or an Estheria. On the other hand, the peculiar and characteristic crustacean of the Forfarshire flagstones (the Pterygotus) is associated with some marine shells in the "Tilestones" of Lesmahage and Ludlow (the passage-beds between the Silurian and the Old-Red); though in the Yellow Sandstone of Ireland (Upper Devonian or Lower Carboniferous) it accompanies the probably freshwater mollusc, Anodonta Jukesii. Although usually the conglomerates are supposed to have been the result of an agitated sea acting on the lately upheaved slate-rocks, yet Mr. Godwin-Austen regards these great conglomerate-beds as not being more than equivalent to the enormous shingle-deposits of Lake Superior, and refers to the fact that the whole extent of the Old Red series of Great Britain and Ireland is not equal to the area of the North American lakes. The outflow of the fresh water of the hypothetical Old Red lakesystem would carry its fishes (which might also have been of estuarine habits) into the sea-deposits of the period; and thus, in Mr. GodwinAusten's opinion, account for the intermixture of the Scottish fishes with marine shells in the Devonian strata of Russia. In North America, the Catskill or Old Red Sandstone is rich with the characteristic fish-remains, Holoptychius and Asterolepis, but yields only two kinds of shells,-of the doubtful genus Cypricardites (Vanuxem). Important objections to this hypothesis, however, are advanced by Sir Roderick Murchison, in "Siluria." * See also "Siluria," p. 264. † See Godwin-Austen's remarks, Quart. Journ. Geol. Soc. vol. xii. p. 51, &c "Siluria," p. 259, &c. 3. SUBDIVISIONS OF THE DEVONIAN SYSTEM: HEREFORDSHIRE AND IRELAND.—The uppermost beds of this system, for the most part, dip conformably beneath the mountain-limestone, or other members of the carboniferous series, and the lowermost pass into strata that belong to the upper member of the Silurian series. For the convenience of study, the deposits comprised in this formation in Herefordshire and the neighbouring counties are subdivided into two groups : I. QUARTZOSE CONGLOMERATES, SANDSTONES, AND SHALES. The sandstones are often either of a deep chocolate-red or greenish colour. The shales partake of the same tints, but are frequently mottled with blotches of red and green. Fishes of the genus Holoptychius and plant-remains. II. FLAGSTONES, SHALES, AND CORNSTONE. Sandy flagstones, with intercalated red and greenish shales, containing irregular bands of concretionary limestone, provincially termed "cornstone." Abundant remains of Cephalaspis and other peculiar fishes. [This group passes into the Upper Silurian "Tilestones; " reddish, grey, and yellowish shales and fissile sandstones, which were formerly grouped with Devonian strata, but are now recognised as passage-beds between the two series, though most nearly connected, as to their zoological contents, with the Silurian.*] The total thickness of this system in Herefordshire and South Wales is estimated by Sir R. Murchison at about 9000 feet; and in Brecknockshire it constitutes the loftiest mountains of South Britain.† The red conglomerates of this system are well displayed on the right bank of the Wye,‡ from Monmouth to Tintern Abbey (Medals, p. 865) : and the Devonian sandstone and conglomerate form the base of the Mountain-limestone at the embouchure of the Avon, and the central nucleus or axis of the Mendip Hills (p. 522). *See Quart. Journ. Geol. Soc. vol. xiii. p. 290. "Silurian System," p. 170, &c.; For the section of the Old Red "Manual of Geology," 1856, p. 141. and "Siluria," p. 242. strata of the Wye, see Phillips's In Ireland there are several straggling exposures of reddish sandstones and conglomerates, in the southern, central, and northern districts.* These sandstones are overlaid at places by the "yellow sandstones" and "carboniferous slates." + 4. DEVONIAN STRATA OF DEVONSHIRE AND CORNWALL. -In the south of Devonshire, in many places dipping northward towards and beneath the anthracitic or culmiferous shales and limestones (p. 697), there is an extensive series of strata composed of green chlorite-slates, alternating with quartzose schists and sandstones, with blue and grey limestones, which pass into, or are associated with, red sandstones and conglomerates. Many of these beds abound in organic remains. These slaty rocks of Devonshire were formerly regarded as belonging to the earliest or most ancient fossiliferous strata-the Transition rocks, as they were termed, until the labours of Professor Sedgwick and Sir R. Murchison, aided by the paleontological research of Mr. Lonsdale, ascertained their true position and relations, and the unity of type which prevails in the organic remains of the entire system, in places very distant from each other, and under very dissimilar conditions of mineral character. The Devonian strata of South Devon extend westward into Cornwall, and the beautiful coralline marbles of Bab * See Griffith's Geol. Map of Ireland; and the Map accompanying Mr. Godwin-Austen's Memoir in the Geol. Journ. vol. xii. p. 46. † See Portlock's "Report on Londonderry," &c.; Report Brit. Assoc. '843, rep. sect. p. 42, and p. 47; ibid. 1852, sect. pp. 43, 47, and 51; Mr. Kelly's paper in the Journ. Geol. Soc. Dublin, vol. xii. p. 115; and particularly Portlock's Presid. Address, 1857, Geol. Soc. Journ. vol. xiii. p. 120. To this late summary, by Gen. Portlock, of the labours of geologists among the Carboniferous and Devonian rocks of Ireland, the reader is especially referred for a philosophical review, not only of the relations of these rocks, but of the principles by which geologists should be guided in working out the correlation of presumed equivalent deposits in different districts. See also Austen, Rep. Brit. Assoc. 1838, sect. p. 93. bicombe, Torquay, &c. (pp. 650 and 652) belong to this formation. In North Devon* also there are thick series of gritty, sandy, and shaly rocks, coming out from under the black anthraciferous strata of the central district, and presenting many characters in common with the Devonian rocks of South Wales, from which they are separated by the Bristol Channel. From careful and extended examinations of these rocks and their fossils in North and South Devon and in Cornwall, a serial order has been recognised, according to which the northern and southern bands of strata are more or less uniformly arranged, although great differences in their mineral composition and in the variable abundance or paucity of fossils, and the frequent metamorphic changes the rocks have undergone, have made this a difficult study.+ Beneath the culm-limestones and black shales with Posidonomyæ, succeed (1.) The Barnstaple or Petherwin group of strata; chiefly consisting of shales and sandstones, associated with a limestone containing a peculiar cephalopod, the Clymenia, of the Nautiloid family. These beds contain numerous plant-remains (Aspidaria, &c.), and comprise a series of strata (the Pilton group) which are the equivalents of the "Yellow Sandstones" of Ireland and the Boulonnais. They constitute the beds of passage between the Carboniferous to the Devonian systems, being very closely related by conformability and fossils to the former. This group is equivalent also to the Cypridinen-schiefer and Clymenien-kalk of Germany. * See the description of a section across North Devon, in “Siluria,” p. 256. † Sedgwick, Murchison, Lonsdale, Austen, De la Beche, Phillips, Sharpe, Giles, Peach, Pattison, and many others have elucidated these strata and fossils in the publications of the Geological Societies of London and Cornwall, and in other works. |