OLD RED SANDSTONE, the name given to a large series pakeozoic rocks, of which red sandstones are the most conspicuous portions, but which contains also white, yellow, or gi'een sandstones, as well as beds of clay and limestone. The group lies below the ear boniferious strata, and was called " " to distinguish it from a newer series of similar beds which occur above the coal measures. The discovery that the highly fossiliferous calcareous rocks of Devonshire and the continent occupied the same geological horizon, showed that the name was very far from descriptive of all the deposits of the period, and suggested to-Murchison and Sedgewick the desirableness of giving them a new designation. They consequently proposed Devonian, which has been extensively adopted; but it is liable to the same objection as that urged against the name it was intended to supplant, inasmuch as it incorrectly limits geographically what the other limits lithologicaily. Many names used by geologists are similarly at fault; there is therefore no good reason why the old name should be given up. especially as it has been rendered classical by the labors and writings of Hugh Miller, the original monographer of these rocks.
The position of the old red sandstone series is easily determined, though the sequence of the various beds which form it is somewhat obscure. All the rocks are situated between the beds of the Silurian and. carboniferous periods. In Wales, Scotland, and Ireland it has been observed that there is an old series of red sandstones which are more or less conformable with the underlying Silurians, and a newer series uneonformable with the older, strata. but conferthable With rocks. The great interval represented by this break has been believed to be that dUribg which the calcareous Devonian rocks were deposited. The recent researches, however, Of Mr. Salter show that the one set of beds do not alternate with the other, but that they are really contempo raneous—the coarse shallow water deposits of conglomerate and sandstone having been formed on the shores of that sea in whose depths-the deposits of thicker mass, finer grain, and lighter color, full of marine shells and corals, were at the same time being aggre gated.
The strata of the period have been arranged in four groups. 1. Upper old red sand stone. including the _Norwood and Petherwiu groups. 2. Middle old red sandstone, including the Dartmouth and Plymouth groups. 3. Lower old red sandstone, including
the North Foreland and Torbay groups. 4. Tilestones or Ledbury Shales.
1. The upper old red sandstones are conformable with the interior strata of the coal measures, and differ so little petrologically, or even paleoutologically from them, that have been considered as the basement series of that period. T hey consist of yellowish and light-colored sandstones, which are at Dura Den, in Fifeshire, remarkably rich in some of their layers in the remains of Holoptychius, Ptericthys, Dendrodus, etc. In the south of Ireland. and at Dunse, similar beds contain a fresh-water shell very like the modern anodon, and fragments of a fern called cyclopteth labernieus. Mr. Salter has shown, from the intercalation of the marine beds with the red. sandstone, and from the identity of the fossils, that the Devonian representatives of these beds are the Marwood and Potherwin groups. These consist of dark-colored calcareous and argillaceons beds, and gray and reddish sandstones. The fossils found in them are shells and land-plants, many of them belonging to the same genera, but different species to those which are found in the carboniferous system. The little crustacean Cypridina and Clymenia are so characteristic of this division, that in Germany the strata are known as the Cypridinien Schielter and Clymenien Balk.
2. The middle old red sandstone is represented in the north of Scotland by the Caith ness flags, a series of dark-gray bituminous schists, slightly micaceous or calcareous, and remarkably tough and durable. Throughout their whole thickness they arc charged with fossil fish and obscure vegetable remains. The characteristic fishes belong to the genera Coccosteus, Asterolepis, and Dipterus. The corresponding beds in Devonshire are the Dartmouth and Plymouth groups, which consist of extensive deposits of lime stones and schists, all of them abounding in the remains of corals, trilobites, and shells. In the German equivalent, the Eifel limestone, but especially in the Russian, the char acteristic invertebrate fossils of the Devonshire calcareous beds have been found asso ciated with the remains of. Coecosteus, showing beyond doubt the identity of these various beds. The Calceola Schieffer of German geologists belongs to the middle old red; it receives its name from abundance in it of a singular brachiopod (cceceola sari. thilina).