II. UPPER OLD RED SANDSTONE.
Red Sandstone conglomerates, of considerable thickness, but occupying an inconsiderable area, appear on the flanks of the Cheviots at Roddam and Biddlestone, in Northumberland, at elevations from 500 to 700 feet above the sea level. In the deep, narrow dene of Roddam, they are exposed for upwards of a mile, consisting principally of conglomerates formed of rounded pebbles of Cheviot porphyry, from the size of a pea to that of the human head, scattered through a flesh and brick-red clay and sand, loosely bound together by peroxide of iron. Interstratified with these are thin beds of harder conglomerate with smaller pebbles, and thin beds of greenish chloritic, calciferous sandstones, some of which contain as much as forty per cent, of carbonate of lime. Above the loose conglomerates are soft thin bedded red sandstones, and below them are hard red sandstones, with large ripple marks. These beds are not less than 500 feet thick. Organic remains I have not found in them; but as their mineral characters and geological position correspond with the Old Red conglomerates of Berwickshire and Roxburghshire, they may, without much doubt, be grouped with this formation. In Biddlestone Burn they are close upon the porphyry of the Cheviots, and they are overlaid conformably by Tuedian strata.
The red conglomerates of the border counties are more connected with the Carboniferous, than with the Devonian system. In some parts of Scotland there is a physical break between them and the Lower Old Red Sandstone. Their relations are best seen in the section from Siccar Point to the northern extremity of Berwickshire, where they distinctly rest on the upturned edges of the Greywacke or Cambro-Silurian strata, and are conformably overlaid by beds of the Tuedian age, the line between the two being marked by the occurrence of Holoptychius nobilissimus in the red conglomerate, and of Stigmaria ficoides in the Tuedian beds. In this Upper Old Red Sandstone one determinable plant, Adianthoides Hibernicus, has been found in Berwickshire; and from similar beds in Roxburghshire I have seen casts of pretty large stems, probably belonging to Sigillaria. A dry fertile soil is produced by the disintegration of these rocks.
