242 PKOCEEBINGS OF THE GEOLOGICAL SOCIETY. [Mar. 8, 



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 consi- 

 dered 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 over- 

 lying limstone, in the form of hydrate and carbonate, and rede- 

 posited 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 SUurian 

 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 Anodontojp- 

 sis and Modiolopsis of various species, Ortlionota angulifera, Cucut- 

 lella aniiqua, Grammysia extrasulcata, various species of Ctenodonta, 

 and some small univalves of the generaMurchisonia, Holopella, Turbo, 

 and Turritella. At Kington and south of Builth, where true pas- 

 sage-beds occur, the fossils are far less numerous, and almost all 

 of small size, including species of Modiolopsis and Modiola, Lin- 

 gula cornea, PlatyscTiisma helicites, a small Discina, and a small 

 Theca, a few small Crustacea, Leperditia, Gytherellina siliqua, and 

 certain undetermined species. In some districts, as, for instance, 

 at Ludlow and near May HiU, the very uppermost Silurian strata 

 also contain seeds of Lycopodiacese, and various fragments of unde- 

 termined 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 Eothliegende Age, near Knaresborough," by J. Clif- 

 ton Ward (Quart. Journ. Geol. See. 1869, toI. xxv. p. 291). 



