Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/376

 mouthshire, and South Wales are succeeded immediately by Old Red sandstones and marls ; and there is no visible unconformity between them. Occasionally the Silurian rocks are red near the junction with the Old Red Sandstone, as, for example, near Usk, and on the banks of the Sawdde, near Llangadoc, in Carmarthenshire ; but this I regard as due to subsequent infiltration from above having stained the strata. Colouring of this kind is by no means uncommon. Thus the Carboniferous Limestone of North Lancashire, which contains haematite, is overlain by red Permian sandstone, and both the redness of the limestone and the ironstone itself are considered by Sir Roderick Murchison to be due to infiltration from above. Coal-measure sandstones and shales, when immediately underlying red Permian marls and sandstones, are frequently exceedingly red ; and the same is the case with Carboniferous strata underlying the Magnesian Limestone. The last Mr. Ward attributes either to " the action of carbonated water from the limestone above filtering through porous grits and sandstones, and converting the protoxides contained in them into sesquioxides, or by iron being brought from the overlying limstone, in the form of hydrate and carbonate, and redeposited in the rocks below " *. (The Magnesian-Limestone soil is always red, and I consider Mr. Ward's last explanation the most probable.)

The life of the Upper Silurian deposits in Wales, Shropshire, and the adjoining counties continued in full force right up as far as the narrow belt of passage-beds which marks the change from Silurian muddy sands into lower Old Red Sandstone ; and this abundance of life is quite irrespective of the occasional red colour of the uppermost Silurian rocks. In the transition beds, on the contrary, genera, species, and often individuals are generally few in number and often dwarfed in form, with the exception, perhaps, of part of the Tilestone, near Llandovery and elsewhere in Carmarthenshire, and a few other places. The more common genera are Anodontopsis and Modiolopsis of various species, Orthonota angulifera, Cucullella antiqua, Grammysia extrasulcata, various species of Ctenodonta, and some small univalves of the genera Murchisonia, Holopella, Turbo, and Turritella. At Kington and south of Builth, where true passage-beds occur, the fossils are far less numerous, and almost all of small size, including species of Modiolopsis and Modiola, Lingula cornea, Platyschisma helicites, a small Discina, and a small Theca, a few small Crustacea, Leperditia, Cytherellina siliqua, and certain undetermined species. In some districts, as, for instance, at Ludlow and near May Hill, the very uppermost Silurian strata also contain seeds of Lycopodiaceae, and various fragments of undetermined land plants.

The land plants clearly indicate the neighbourhood of land ; and the poverty of numbers, and small size of the shells, a change of conditions in the nature of the waters in which they lived.

The fish-remains found in the passage beds, and in the very base


 * " On Beds of supposed Rothliegende Age, near Knaresborough," by J. Clifton Ward (Quart. Journ. Geol. Soc. 1869, vol. xxv. p. 291).