316 Prof. Sedgwick on the Matj Hill Sandstone, 



The establishment of the May Hill sandstone, as a true Silurian 

 group, does away at once with all these three assumptions. 



For several years after the publication of the ' Silurian Sy- 

 stem,' I believed that, between the lower groups of the Silurian 

 system and the upper Cambrian groups, there was an overlap, or 

 alternation of the beds. Hence I have sometimes described the 

 rocks above the Bala limestone as " Cambro-Silurian ;" and, in 

 1843, " for the express purpose of avoiding any collision be- 

 tween the Lower Silurian groups and the Upper Cambrian,^' I 

 proposed the name Protozoic for all the collective groups of 

 Wales and Siluria which are below the Wenlock shale. At the 

 same time I proposed the following general classification of the 

 British palseozoic rocks. They were considered zoologically as 

 one si/stem, separable into four primary divisions, as follows : — 

 '^(1) Permian and Carboniferous; (2) Devonian; (3) Silurian, 

 including (under that name) only the Upper Silurian rocks of 

 Sir R. I. Murchison ; (4) Protozoic, as above defined.'^ (Pro- 

 ceedings of the Geol. Society, vol. iv. June, 1843.) On this 

 classification I may remark — 1st, that 1 never used the word 

 Protozoic as if it were synonymous with Lower Silurian, although 

 this use of the word has been verij erroneously attributed to me* ; 

 2ndly, that the estabhshment of the May Hill sandstone entirely 

 does away with all ambiguity of nomenclature, or the necessity 

 of employing any new terms, such as " Protozoic '^ or Cambro- 

 Silurian; 3rdly, that 1 have never, either before or since 1843, 

 described the rocks under the Bala limestone by any other col- 

 lective name than Cambrian. 



In the vast sequence of the Cambrian deposits, which have a 

 collective thickness of more than 30,000 feet, we find many in- 

 dications of mechanical movements, without any great or sudden 

 change of the organic types ; and it deserves remark, that some 

 of the most violent of these movements, indicated by a succes- 

 sion of vei-y coarse conglomerates, took place in South Wales, 

 far from the immediate action of any plutonic rocks, among the 

 very highest Cambrian groups ; and perhaps within the limits 

 of those groups which may be arranged hereafter with the May 

 Hill sandstone, and in that case must form a part of the Silurian 

 or overlying series. Whatever be the true place of the newest 

 of these coai'se conglomerates (a point I had no opportunity of 

 re-examining), it is obvious that about the end of the Cambrian 



* I here allude to an unwarrantable change made, without my consent or 

 knowledge, in the nomenclature of my older Cambrian groups, during the 

 passage of one of my papers through the press, and of which I was not 

 allowed to see the proofs. See Proceedings of the Geol. Soc. vol. iv. 



L251, and the accompanying map, where I am represented as making 

 wer Silurian the equivalent of Protozoic. 



