Tests made on radioactive minerals from Gilpin County, Colorado, have established the age of late Cretaceous or early Cenozoic rocks at sixty million years, providing a convenient and reasonably accurate date for the beginning of the Age of Mammals. In Russia, one of the oldest mineral deposits yet studied in this way and regarded as early Pre-Cambrian, produced the astonishing figure of 1,850,000,000 years; what we commonly refer to as geological history may therefore be regarded as covering a range of approximately two billions of years. The earth, in some form or other, has in all probability passed through an earlier history of another billion years or more. Wherever we may roam, a portion of the prehistoric record is to be found in the rocks underfoot and not far from the surface. Formations as already mentioned may be regarded as the pages—often torn and badly scattered—of nature’s own book, in which the geological periods are chapters. But instead of numbering these pages and chapters we have _named_ them, in order to get the parts reassembled in orderly fashion and restored to a condition which makes the book legible. However, the names cannot render the service intended except in connection with a time chart and an outline of earth history. [Illustration: GEOLOGICAL TIME Figures to the left denote millions of years that have elapsed up to recent time] CENOZOIC Age of Man RECENT Man and his Culture 1 PLEISTOCENE Last of Mammoths & Mastodons Age of Mammals 7 PLIOCENE Horses modernized 20 MIOCENE Grasses and Grazing Animals Three-toed Horses, Rhinos, Camels 35 OLIGOCENE Specialization of Primitive Ancestors 60 EOCENE Decline of archaic types Mammals flourishing MESOZOIC Age of Reptiles 125 CRETACEOUS Last of Great Reptiles Specialization of Dinosaurs 160 JURASSIC Bony Fishes thriving Flowering plants advance Cycads Birds and Flying Reptiles 200 TRIASSIC Few small mammals of lower orders Dinosaurs become prominent PALEOZOIC Age of Amphibians 225 PERMIAN Reptiles advancing Amphibians dominant insects 300 CARBONIFEROUS Dense forests of spore-bearing plants Age of Fishes 350 DEVONIAN Shark-like Fishes Land floras established 375 SILURIAN First land animals (scorpions) Armored Fishes prominent Age of Invertebrates 425 ORDOVICIAN Corals and Bryozoa