Time-scale in millions of years PERIODS Since beginning of period Duration of period PLEISTOCENE _ca_ 1 .... _ca_ 1 .... PLIOCENE 10 .... 11 .... MIOCENE 14 .... 25 .... OLIGOCENE 15 .... 40 .... EOCENE 20 .... 60 .... PALEOCENE 10 .... 70 ± 2 .... Upper } CRETACEOUS 65 Lower } .... 135 ± 5 .... Upper } JURASSIC 45 Mid. & Lower } .... 180± 5 .... TRIASSIC 45 .... 225 ± 5 .... PERMIAN 45 .... 270 ± 5 .... Upper } CARBONIFEROUS 80 Lower } .... 350 ± 10 .... Upper } DEVONIAN 50 Lower } .... 400 ± 10 .... SILURIAN 40 .... 440 ± 10 .... ORDOVICIAN 60 .... 500 ± 15 .... CAMBRIAN 100 .... 600 ± 20 .... Using measurements made in many laboratories, and interpolating between them by using the relative thicknesses of sediments, the great British geologist, Arthur Holmes, established the time scale that is in general use today. His original scale is shown on the preceding page. Small changes can be expected to be made in this scale from time to time, but major alterations are not likely, except perhaps in the Cambrian Epoch where the present data are unreliable because they are not complete. (A scale showing the epochs or periods as often given now is on page 4.) Precambrian Stratigraphy