By the year 1839 Murchison had mapped and described the four groups that made up his Silurian System, defining each on the basis of fossils he had collected during five years fieldwork in South Wales and the Borders. His friend Professor Sedgwick had given only a brief lithological description of the Cambrian rocks of North Wales by this date. As it became clear the two great divisions did not lie one on the other but overlapped at their boundary, Murchison extended his Silurian terminology into Sedgwick's domain. The Professor's inability to give a palaeontological definition of even a restricted Cambrian System led Murchison to extend the Silurian System down to include the oldest fossiliferous strata.

Arguments over the limits and validity of these two Systems scarcely abated with the death of the two chief contestants. British geologists remained in opposing camps until the end of the century, when Lapworth's Ordovician System of 1879 was generally accepted as a compromise for the disputed strata.

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