Page:VCH Herefordshire 1.djvu/67

 GEOLOGY Murchison made three divisions of the Old Red Sandstone in Hereford- shire, in ascending order, Tilestone, Cornstone and Marl, and Quartzose Conglomerate and Sandstone. The Tilestone, as has been pointed out, is now classed with the Silurian. Of the two remaining divisions Symonds " made three, separating the ' Cornstone and Marl ' into Brownstones (upper part) and Cornstones (lower part). In some of his essays Symonds grouped as Brownstones only the strata which merited the designation by virtue of their colour ; but in others he classed therewith certain cornstone-bands. In yet another of his communications he called these cornstones the ' Upper Cornstones,' to distinguish them from the Lower Cornstones, which, together with the underlying Pteraspis Beds, made up his Lower Division of the Old Red. Symonds was inclined to parallel the Brownstones proper with the richly fossiliferous Osteokpis Beds of Scotland — the Middle Devonian of Murchison. He admitted, however, that the Brownstones had yielded no fossils of correlation value, and that it was mainly from the test of stratigraphical position that he suggested their probable contemporaneity. In most parts of the world where Old Red rocks or their equivalents occur, it has been found that they will admit of a dual division — a lower, characterized by such fish as Pteraspis, Cephalaspis and Phlyctoenaspis, and an Upper, characterized by Holoptychius, Sauripterus, Bothriolepis and Asterolepis. In this county a similar division may be made, and scanty as are the remains of Pteraspis and Cephalaspis that have been found in the Brownstones, they are sufKcient to show that the beds in which they occur belong to the Lower and not to the Upper Division. In Scotland between the Upper and Lower Divisions is a marked break, but no such non-sequence has yet been discovered in Herefordshire or in the neighbouring county of Monmouthshire during the later geological survey. Symonds, however, held that he had evidence of a break between the top beds of the Cornstone Series and the succeeding beds of the Brown- stones, for the latter ' overlap the Rowlstone Beds [the top division of the Cornstone Series] both on the Scyrrid and the Sugar-Loaf Confirmatory evidence of this is required. In 1840 Murchison and Sedgwick published the term 'Devonian' for certain truly marine beds which occur in South Devon, and paralleled them with the very dissimilar Old Red Sandstone of Herefordshire. There is no doubt about their contemporaneity ; but while it is well known under what conditions the rocks of South Devon were formed, this is not the case with regard to their equivalents in Herefordshire. Dr. J. Fleming sug- gested a lacustrine origin, and Godwin Austen, basing his conclusions on the fact that the Old Red fish resemble in many respects those of the American lakes of the present day, also advocated a lacustrine origin — a view that was upheld by Sir Andrew Ramsay and later by Sir Archibald Geikie. The late Robert Etheridge and the Rev. La Touche held that, whatever were the conditions of deposition, the two areas of sedimentation were quite distinct — the Devonian of Devon was laid down in one hydrographic area, the Old Red of Herefordshire in another. In 1904 Dr. A. Smith Woodward ^^ " Records of the Rocks (1872), p. 212 ; see also for a good general description, Old Stones (ed. 2), pp. 91- 115; and Edin. New Phil. Journ. 1859, p. 232. ^'' Proc. Geol. Assoc, xviii (1904), p. 434. 19