In Scotland the “Old Red” forms a great series of arenaceous and conglomeratic strata, attaining a thickness of many thousands of feet, and divisible into three groups.  Of these, the Lower Old Red Sandstone reposes with perfect conformity upon the highest beds of the Upper Silurian, the two formations being almost inseparably united by an intermediate series of “passage-beds.”  In mineral nature this group consists principally of massive conglomerates, sandstones, shales, and concretionary limestones; and its fossils consist chiefly of large crustaceans belonging to the family of the Eurypterids, fishes, and plants.  The Middle Old Red Sandstone consists of flagstones, bituminous shales, and conglomerates, sometimes with irregular calcareous bands; and its fossils are principally fishes and plants.  It may be wholly wanting, when the Upper Old Red seems to repose unconformably upon the lower division of the series.  The Upper Old Red Sandstone consists of conglomerates and grits, along with a great series of red and yellow sandstones—­the fossils, as before, being fishes and remains of plants.  The Upper Old Red graduates upwards conformably into the Carboniferous series.

The Devonian rocks of Devonshire are likewise divisible into a lower, middle, and upper division.  The Lower Devonian or Lynton Group consists of red and purple sandstones, with marine fossils, corresponding to the “Spirifer Sandstein” of Germany, and to the arenaceous deposits (Schoharie and Cauda-Galli Grits) at the base of the American Devonian.  The Middle Devonian or Ilfracombe Group consists of sandstones and flags, with calcareous slates and crystalline limestones, containing many corals.  It corresponds with the great “Eifel Limestone” of the Continent, and, in a general way, with the Corniferous Limestone and Hamilton group of North America.  The Upper Devonian or Pilton Group, lastly, consists of sandstones and calcareous shales which correspond with the “Clymenia Limestone” and “Cypridina Shales” of the Continent, and with the Chemung and Portage groups of North America.  It seems quite possible, also, that the so-called “Carboniferous Slates” of Ireland correspond with this group, and that the former would be more properly regarded as forming the summit of the Devonian than the base of the Carboniferous.

In no country in the world, probably, is there a finer or more complete exposition of the strata intervening between the Silurian and Carboniferous deposits than in the United States.  The following are the main subdivisions of the Devonian rocks in the State of New York, where the series may be regarded as being typically developed (fig. 67):—­

(1) Cauda-Galli Grit and Schoharie Grit.—­Considering the “Oriskany Sandstone” as the summit of the Upper Silurian, the base of the Devonian is constituted by the arenaceous deposits known by the above names, which rest quite conformably upon the Silurian, and which represent the Lower Devonian of Devonshire.  The Cauda-Galli Grit is so called from the abundance of a peculiar spiral fossil (Spirophyton cauda-Galli), which is of common occurrence in the Carboniferous rocks of Britain, and is supposed to be the remains of a sea-weed.