This would not, however, represent the age of the stratified rocks. It would only represent the time required to deposit the rocks which we have assumed to be now in existence. The greater mass of sedimentary rocks has been formed out of previously existing sedimentary rocks, and these again out of sedimentary rocks still older. The oldest known sedimentary rocks are the Laurentian; but these are believed by geologists to have been formed from still older sedimentary rocks. It is therefore evident that the materials composing our stratified beds must have passed through many cycles of destruction and re-formation. The materials of some of the recent formations, for example, may have passed through denudation and deposition a dozen of times over.[31] The time required to have deposited at a given rate the present existing mass of sedimentary rocks is probably but a small fraction of the time required to have deposited at the same rate the total mass that has actually been formed. Few geologists, I think, who will duly reflect on the subject, will deem it too much to say that the present existing stratified rocks have on an average passed at least thrice through the cycle of destruction and re-formation. If this be admitted, then the 1,000 feet of stratified rock represent, not a period of 24,000,000 years, but a period three times as great, viz. 72,000,000 years. [Footnote 31: It is this destruction of the stratified rocks which makes it so difficult to detect the marks of former glacial epochs, and which has led to such prevailing misconceptions regarding the evidence which we ought to expect of those epochs. See paper read before the Geological Society, “On Prevailing Misconceptions regarding the Evidence which we ought to expect of former Glacial Periods,” January 23, 1889.] It is impossible to tell from geological data the actual age of the stratified rocks; but this is not required. What we require is, as already stated, not their _actual_ age, but an _inferior limit_ to that age.