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bedded in the limestone of the neighbourhood, in evolving 

 a definite system of rocks, until he reached a stratum in 

 ■which even his industrious researches f;nled to distinguish a 

 single fossil. Meanwhile Professor Sedgwick was investi- 

 gating the slaty rocks of North Wares, and the result of 

 his labours was to bring to light an entirely new system of 

 rocks, and apparently underlying those which Sir Roderick 

 Murcbison was investigating. These were called provi- 

 sionally and apparently Cambrian ; but when a comparison 

 of the fossils which were unaltered by slaty cleavage was 

 made, they were found to be one and the same 

 with Murcbison' s Silurian fossils. Subsequent in- 

 vestigations pressed this still more strongly, and 

 now it was a generally accepted doctrine that they were 

 the same, but it was agreed as a matter of convenience to 

 separate the lower portion of Professor Sedgwick's from 

 the Silurian system of Sir Roderick Murchison's, and re- 

 strict the term Cambrian to it. Dr. Mitchinson then pro- 

 ceeded to speak of the classification of the Silurian rocks, 

 at length. The Longmynd was the basis of the 

 Silurian system— so called from its geographical position. 

 The Longmynd mountain was then described with great 

 felicity of language by Dr. Mitchinson. This formation 

 disappeared under the mountains of North Wales, until 

 it cropped out again in large masses between Barmouth in 

 Merioneth, and Festiniog in Carmarthenshire. They then 

 appeared again in the great slaty rocks which were quarried 

 on the eastern shores of the lake of Llanberris and in the 

 Bethesda quarries, belonging to Lord Fenrhyn, out of which 

 the finest roofing slates in England were got. These slates 

 •were Cambrian rocks, identical with the Longmynd 

 rocks of Sir Roderick Murcbison. In England these 

 rocks had never yielded any fossils. They again 

 dipped under the sea at Holyhead, and reappeared at 

 Bray, in Ireland. There they had yielded one or two species 

 of fossils of a very low organisation— one a polype resem- 

 bling the horny-textured Sertularia. Oldhamia was the 

 name of the genus, of which two species had been described ; 

 and Sir Rodctick Murohison, speaking of this animal, called 



