[3] Manchester Phil. Mem., vol. ix, 1851.
[4] Trans. of Bot. Soc. of Edinburgh, vol. i, 1844.
[5] For figures of these corals, see Palæontographical Society’s Monographs, 1852.
[6] Quart. Geol. Journ., vol. xxiii, p. 674, 1867.
CHAPTER XXV.
DEVONIAN OR OLD RED SANDSTONE GROUP.
Classification of the Old Red Sandstone in Scotland and in Devonshire. — Upper Old Red Sandstone in Scotland, with Fish and Plants. — Middle Old Red Sandstone. — Classification of the Ichthyolites of the Old Red, and their Relation to Living Types. — Lower Old Red Sandstone, with Cephalaspis and Pterygotus. — Marine or Devonian Type of Old Red Sandstone. — Table of Devonian Series. — Upper Devonian Rocks and Fossils. — Middle. — Lower. — Eifel Limestone of Germany. — Devonian of Russia. — Devonian Strata of the United States and Canada. — Devonian Plants and Insects of Canada.
Classification of the two Types of Old Red Sandstone.—We have seen that the Carboniferous strata are surmounted by the Permian and Trias, both originally included in England under the name “New Red Sandstone,” from the prevailing red colour of the strata. Under the coal came other red sandstones and shales which were distinguished by the title of “Old Red Sandstone.” Afterwards the name of “Devonian” was given by Sir R. Murchison and Professor Sedgwick to marine fossiliferous strata which, in the south of England, occupy a similar position between the overlying coal and the underlying Silurian formations.
It may be truly said that in the British Isles the rocks of this age present themselves in their mineral aspect, and even to some extent in their fossil contents, under two very different forms; the one as distinct from the other as are often lacustrine or fluviatile from marine strata. It has indeed been suggested that by far the greater part of the deposits belonging to what may be termed the Old Red Sandstone type are of fresh-water origin. The number of land-plants, the character of the fishes, and the fact that the only shell yet discovered belongs to the genus Anodonta, must be allowed to lend no small countenance to this opinion. In this case the difficulty of classification when the strata of this type are compared in different regions, even where they are contiguous, may arise partly from their having been formed in distinct hydrographical basins, or in the neighbourhood of the land in shallow parts of the sea into which large bodies of fresh-water entered, and where no marine mollusca or corals could flourish. Under such geographical conditions the limited extent of some kinds of sediment, as well as the absence of those marine forms by which we are able to identify or contrast marine formations, may be explained, while the great thickness of the rocks, which might seem at first sight to require a corresponding depth of water, can often be shown to have been due to the gradual sinking down of the bottom of the estuary or sea where the sediment was accumulated.
Another active cause of local variation in Scotland was the frequency of contemporaneous volcanic eruptions; some of the rocks derived from this source, as between the Grampians and the Tay, having formed islands in the sea, and having been converted into shingle and conglomerate, before the upper portions of the red shales and sandstones were superimposed.
The dearth of calcareous matter over wide areas is characteristic of the Old Red Sandstone. This is, no doubt, in great part due to the absence of shells and corals; but why should these be so generally wanting in all sedimentary rocks the colour of which is determined by the red oxide of iron? Some geologists are of opinion that the waters impregnated with this oxide were prejudicial to living beings, others that strata permeated with this oxide would not preserve such fossil remains.