Meanwhile, geology had a great deal of sorting out to do, and not all of it went smoothly. From the outset geologists tried to categorize rocks by the periods in which they were laid down, but there were often bitter disagreements about where to put the dividing lines—none more so than a long-running debate that became known as the Great Devonian Controversy. The issue arose when the Reverend Adam Sedgwick of Cambridge claimed for the Cambrian period a layer of rock that Roderick Murchison believed belonged rightly to the Silurian. The dispute raged for years and grew extremely heated. "De la Beche is a dirty dog," Murchison wrote to a friend in a typical outburst.

Some sense of the strength of feeling can be gained by glancing through the chapter titles of Martin J. S. Rudwick's excellent and somber account of the issue, The Great Devonian Controversy. These begin innocuously enough with headings such as "Arenas of Gentlemanly Debate" and "Unraveling the Greywacke," but then proceed on to "The Greywacke Defended and Attacked," "Reproofs and Recriminations," "The Spread of Ugly Rumors," "Weaver Recants His Heresy," "Putting a Provincial in His Place," and (in case there was any doubt that this was war) "Murchison Opens the Rhineland Campaign." The fight was finally settled in 1879 with the simple expedient of coming up with a new period, the Ordovician, to be inserted between the two.