CAMBRIAN. 



49 



away, and occasionally migrated when communication between the separate areas 

 was now and again established.^ Mr. Marr has, however, shown that these re- 

 appearances of faunas can be explained by faults. 



It is perhaps needless to observe that the names Cambrian and 

 Silurian are mere terms of convenience in classification : one name 

 might indeed be applied to the whole series of rocks, but as 

 we have no reason to doubt that the history of the earth was a 

 series of regular changes, so the terms of classification, dividing 

 the history into chapters, are necessary to assist the memory and 

 mark out the leading modifications that have taken place over any 

 given area. And the division between Cambrian and Silurian 

 proposed by Sedgwick is one that marks the most important break 

 noticed in the series in England and Wales. Were this classification 

 generally adopted, it would only be an act of justice to the geologist, 

 who first made out the natural divisions of the older Palceozoic 

 rocks, whose early researches have been shown to be in all 

 essential points correct, but whose work has not, until within the 

 last few years, received adequate acknowledgment. The en- 

 thusiastic labours of both Murchison and Sedgwick are now duly 

 appreciated. Nor should we neglect to give credit to the more 

 detailed work of the Geological Survey in Wales, done by A. C. 

 Ramsay, W. T. Aveline, A. R. C. Selwyn, and J. B. Jukes; work 

 which entailed great labour and no little hardship, and which, 

 carried out with the utmost zeal and energy, has formed the basis 

 for all subsequent investigations.^ But, while it is necessary here 

 to give prominence to one form of classification, the alternative 

 groupings of the rocks are likewise given, so that the student can 

 at once understand the views adopted by different authorities.^ 



Table showing the Classification of the older Palaeozoic Rocks. 



' Defense des Colonies, v. iS8i. Apparition et reapparition en Angleterre et en 

 Ecosse des especes coloniales Siluriennes de la Boheme ; see also Lyell, Elements 

 of Geology, edit. 6, p. 569 ; and A. Geikie, Text-Book of Geology, edit. 2, p. 618. 



^ See Letters, etc., of J. B. Jukes, 1871. 



•' See A. C. Ramsay, Geology of North Wales, edit. 2, p. i ; J. E. Marr, 

 The Classification of the Cambrian and Silurian Rocks, 1883 ; and Murchison, 

 Siluria, edit. 5, 1872. See also Life of Murchison, by Dr. A. Geikie. 



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