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 |
| Progress among Mollusks | ||
| 500 | CAMBRIAN | Brachiopods gaining |
| Trilobites dominant | ||
| Advance of shelled animals | ||
| PROTEROZOIC | ||
| EARLIEST LIFE | ||
| 1000 | UPPER PRE-CAMBRIAN | Small marine invertebrates |
| Lowest Forms of Plant and Animal Life | ||
| Few Fossils | ||
| ARCHEOZOIC | ||
| 2000 | LOWER PRE-CAMBRIAN | Some chemical evidence of life |
| No fossils | ||
Such aids have been devised and revised from time to time. No figures have been offered as final or absolutely “right” since the beginning of scientific investigations. Time divisions have been proposed that are not yet in common use while others have been abandoned or modified. Sources of information are so numerous that appropriate credit cannot be given fairly for anything that is up-to-date. The combined chart and outline here provided is based on time calculations of recent date but with figures slightly rounded off for the sole purpose of making them easier to remember. In view of the still existent probability of error it is felt that the slight alteration of figures may justify itself. It need not be regarded as misleading if the present purpose be considered—the stimulation of a natural history interest which is not vitally concerned with the little difference between a thousand million years and nine hundred ninety-nine million years.
EXPLANATION OF THE TIME CHART
The whole of geological time has been divided and subdivided according to varying practices. The development of life is perhaps the one outstanding feature of the time divisions, but for the most part the changes in floras and faunas have been gradual rather than abrupt, and this makes it very difficult to draw sharp lines or to visualize beginnings and endings of the various stages of development. Occasionally there is good excuse for drawing a line, where the record is broken and resumed again after a long lapse of time. The principal cause of such breaks is the elevation of great land masses, which brings on an interval of erosion and surface destruction for the areas uplifted.
These movements of parts of the earth’s crust have been exceptionally pronounced at certain times, often culminating in the production of mountain systems, and because of the extreme changes they introduce are known as revolutions. The major divisions of prehistoric time have been established, at least in part, by such revolutions; crustal, climatic, or other disturbances, on a smaller scale and recurring with greater frequency, may be regarded as establishing boundaries for the minor divisions. Hence we have five great Eras of geological history, and these are divided again into Periods. The time chart shows an arrangement commonly used in America. In the first column the names of the Eras are stated in technical form. Closely coinciding with these terms are the popular names of the Ages which appear in the second column. These names, describing the dominant life of each age, are very convenient. The more scientific terms used for the eras, while serving essentially the same purpose, are a little more systematic and generalized in that they refer to ancient life (Paleozoic), middle life (Mesozoic), and recent life (Cenozoic), without being specific as to any class of animals or plants for any one division of time.
The period names, in the central column, have been derived from miscellaneous sources, some of them from geographical districts, some from descriptive references to prominent features of the rocks, others indicating a degree of approach to recent time. In paleontology (fossil study) it has long been a practice to cut the periods into lower, middle, and upper divisions, and in a few cases it has been found desirable to make two periods out of an old one. What was once known as the Lower Carboniferous is now commonly recognized as the Mississippian period while the upper portion has become the Pennsylvanian. The Lower Cretaceous is now the Comanchean of some authors.
Both old and new practices are responsible for a little confusion at the present time. A former division into Primary, Secondary, Tertiary, and Quaternary eras has been partly abandoned, but the term “Quaternary” still applies to the Age of Man, while “Tertiary time” remains in good usage for the balance of the Cenozoic era. Among the newer introductions may be mentioned the use of a Paleocene period which precedes the Eocene. Geologists are not entirely in agreement as to the necessity for this addition and many would not give it equivalent rank with other periods. In the interest of simplicity these modern refinements have been omitted from the chart.
The figures appearing in the third column, between the Ages and Periods, indicate the millions of years that have elapsed up to present time. They denote the age of the rocks at the beginning of each period. The age of a plant or animal which lived in Eocene time would be, according to this scale, somewhere between 35 million and 60 million years. In practice it is usually possible to determine whether a fossil was embedded in the rocks during an early or late portion of the period, and thus its age may be established within a shorter range, but it is impossible to be exact, even in terms of millions of years, with regard to anything as far back in prehistory as the Eocene period.