GEOLOGY of formation of the Sandstone the area of the island, which existed apparently during Ordovician and Lower Llandovery times, had much decreased. The May Hill Sandstone is a littoral deposit, being composed largely of fragments of rock derived from the island and spread out round its western shore. The succeeding deposit, the equivalent of the Tarannon Shales, is much more shaly and marks the still greater recession of the coast-line — in other words the greater submergence of the land-area. During the Wenlock Epoch a sea, studded with coral-reefs, and teeming with hydrozoans, crusta- ceans, cephalopods, gastropods, echinoderms, pelecypods, and brachiopods, spread over Herefordshire. The earliest known fossils that can be referred with certainty to plants occur in Silurian rocks, and towards the close of the period the graptolites died out and fish made their appearance. In Herefordshire, owing to the great break at the base of the May Hill Sandstone, between it and the underlying Dictyonema Shales, the lower limit of the Silurian System in this county is well defined. But such is not the case with the upper. In Murchison's original classification the lowest division of the succeed- ing Old Red or Devonian System was called the Tilestone.^" Later he divided this into two parts — a lower, the Downton Castle Sandstone, and an upper, to which he restricted the term ' Tilestone.' " The Downton Castle Sandstone, as the result of additional work and criticism on the part of others, he grouped with the Silurian, regarding the Tilestone [sensu stricto) as the basal division of the Old Red. The Downton Castle Sandstone he termed the ' Passage-Beds.' Certain other geologists, however, held that the Tile- stones {sensu stricto) were equally ' Passage-Beds,' and therefore grouped both Downton Castle Sandstone and Tilestone under this denomination. In the Ledbury district the Tilestone of Murchison, or the Temeside Shales of present nomenclature, were called by J. W. Salter the ' Ledbury Shales,' and were grouped by Sir Charles Lyell with the Old Red Sandstone — a view in accordance with Murchison's. But Symonds was always opposed to this classification, holding that in addition to the palaeontological evidence prohibiting such a conclusion, he had found clear indications of a break between the Silurian and the Old Red Sandstone. His contemporary, however, G. H. Piper, who spent much time investigating the railway- cutting near his home at Ledbury, thought that the ' Passage-Beds ' partook much more of the nature of the Old Red Sandstone than of the Silurian. The most recent work in the Ludlow district has confirmed the view held by Symonds, and a layer with plant-remains, called the ' Fragment-Bed ' is regarded as the top bed of the Silurian. The fact that the earlier geologists felt compelled to recognize ' Passage- Beds ' is clear proof of the very gradual transition, as regards lithic structure, from the Silurian into the Old Red. In Herefordshire the Silurian is that system of deposits which intervenes between the Dictyonema Shales and the Fragment-Bed, and has that bed for its top stratum. Of late years in particular it has become recognized that the correlation of beds of different areas can only be satisfactorily accomplished by paying minute attention to the fossils. In the Ordovician and Silurian rocks, as ™ See Proc. Geol. Sac. ii (1834), P- i^- " Si/urian System (1839), P- ^97- I 9 2
