418 Professor Ramsay, on Peculiarities of Clunate [April 24, 



rocks. These rocks contain a peculiar suite of fossils, among which 

 Pentamerus ohlongus is conspicuous. They are frequently highly 

 calcareous and conglomeratic, and mixed with the fossils contain 

 pebbles of green and purple grit and slate, derived from the waste 

 of the Longmynd rocks, on the upturned edges of which they rest. 

 These Pentamerus beds form an ancient consolidated beach that 

 surrounded an island of Cambrian and Lower Silurian rocks, at 

 the commencement of the Upper Silurian epoch. Outliers of this 

 old beach lie on the flats and slopes at the Bogmine and elsewhere, 

 near the summits of the Lower Silurian hills west of the Stiper 

 Stones ; and it was shown that during the formation of the beach 

 the island slowly sank and was gradually encased in Pentamerus 

 beds, and these in their turn were buried beneath the Wen lock 

 shale and Ludlow rocks, and probably also the old red sandstone. 

 This part of the subject was illustrated by an account of the 

 gradual submergence of the coral islands of the Pacific. The 

 ancient island was thus not only submerged, but also shrouded 

 beneath many thousand feet of newer strata. While the island 

 stood high above the water the Pentamerus beach began to be 

 formed, but as it slowly sank the beach crept inward and upward at 

 least 800 feet, with a gentle slope, so that finally before complete 

 submergence only the higher summits stood above the sea, sur- 

 rounded by a continuation of the beach. The higher prolongation 

 of the beach was thus shown to be of later date than the parts 

 formed round the earlier margin of the island, and the opposite 

 ends of a continuous stratum may thus be of different ages. This 

 was illustrated by the fossils that the Pentamerus beds of the 

 Longmynd contain. In Wales the Pentamerus beds have been 

 divided by Mr. Aveline, of the Geological Survey, into two sets, 

 the Lower and Upper Llandovery beds, each characterised by its 

 own group of fossils, or by peculiarities of grouping. It is the 

 upper part only that surrounds the Longmynd. At the foot of the 

 Longmynd and Lower Silurian hills the Pentamerus beds among 

 other fossils contain Pentamerus ohlongus in great plenty, and also 

 P. lens. The first is scarce in the Lower Llandovery rocks, and 

 common in the Upper. With the second the reverse is usually the 

 case. It is found in the (geographically) lower part of the beach 

 above described, but in the higher geographical prolongation at the 

 Bogmine it does not occur. Stropho7nena pecten is common to all 

 the Silurian rocks in and below the Wenlock strata, but it is espe- 

 cially abundant in the Wenlock rocks, and is common in the Bogmine 

 outlier. Goniophora cymbceormis is essentially an Upper Silurian 

 species. It is not found in the Upper Pentamerus beds of the 

 ordinary type, but occurs at the Bogmine, and ranges through the 

 Wenlock and Ludlow rocks up to the tilestone, close below the 

 base of the old red sandstone. The same is the case with Bellero- 

 phon trilobatus, also a Bogmine and tilestone species. A trilobite 

 Phacops Dowiiingicc, not known in the ordinary Pentamerus beds, 



