GEOLOGICAL TIME
“How old are they?” “How can you learn their names from the rocks?” These are typical examples of questions most frequently asked concerning fossils. The second question follows the usual reply to the first, for prehistoric plants and animals are as old as the rocks in which they are found. The answer, as to age, must come from the rocks and what we have learned about them through many years of hard work, thoughtful observation, and careful study. Names, however, come from a different source. Nature, apparently, managed for a long time to carry on without the use of words. Since man began talking he has had no trouble inventing names for things which interest him.
Early students of rocks and fossils likewise accomplished a great deal without being able to date events in terms of years although many of their efforts and interests centered on the problem of discovering a continuous sequence of events in the fragments of evidence that had been uncovered. This relatively simple problem has not been fully worked out, and some of the breaks in the record are recognized as “time gaps” which may never be converted into history.
The question of time, expressed in years, has been a puzzle which attracted some attention even in the earliest days of investigation. Its solution was attempted by several methods long before there was sufficient information to make them work satisfactorily, which accounts in part for the extreme variation in results of the calculations. Even now it is to be expected that changes will have to be made as long as pertinent studies are continued. Two of the most promising methods of investigation in late years have been producing figures which are surprisingly large. More accuracy than ever before is probably present in modern estimates but, except for comparatively recent time, there is yet no way of knowing within a range of millions of years when a creature lived.
Astronomy and physics were used in early calculations but, although taken seriously by some geologists, it was soon recognized by others that certain events revealed by earth history could not be explained with so short a time allowance as these methods indicated. One of the first estimates provided a total of only twenty-five millions of years and included a great stretch of time during which the earth, according to prevailing theory, was more sun-like than rock-like, a time when planets were being born and the earth could not have been in its present physical condition, which is the chief concern of the geologist. Since those earlier conditions could not have supported life as we know it, our knowledge of cosmic history renders small service in the study of fossils.
Among the methods suggested by astronomy and the laws of physics is one which is based on the probable rate at which the earth cooled from its molten condition to present temperature. It is believed now that the heat of the earth is not necessarily due to an original molten state and that a steady rate of cooling cannot be ascertained. Any figures based on such procedure, therefore, are discredited today.
The amount of salt in the oceans, and the time required for its concentration there by natural processes, offers another way of attacking the problem. It is a well known fact that salt is being added to the seas at a fairly constant rate; sea water, then, must become saltier from year to year. The salt comes from rocks exposed on land surfaces and is transported by the rivers which drain these areas. By analyzing the river waters it is possible to estimate the amount of salt annually dumped into the oceans and, also by chemical analysis, it is a comparatively simple matter to figure the total amount now present in the oceans. Some recent calculations indicate that thirty-five million tons of salt are being added each year, and this figure divided into the total amount for all the years places the age of the oceans at three hundred sixty millions of years.
However, there are certain other factors which complicate the problem. For instance, it is known that land areas exposed to surface drainage have not always been of their present size, and the annual production of salt by the different types of rocks exposed at various times in the history of the earth has not always been as it is now. The rocks also must be older than the oceans, but how much older cannot be determined by means of figures obtained in this way.
Until the beginning of this century there was little anticipation of a better measuring stick than one in use at the time which placed its reliance on the total thickness of the sedimentary deposits and the length of time required to produce this great accumulation of material which is known as the geological column. Since the total thickness, or height of the column, was not accurately known, and with recognized time gaps to bridge, there was little hope of working out a complete chronology by this device, but it has supplied highly desirable and reliable information concerning parts of the record.
The system has been somewhat improved since its earliest use, and one of its latest applications gives us an age, for known sedimentary rocks, of nearly half a billion years, this being based on a total thickness of one hundred miles and an average rate of 880 years for the building up of one foot of sediments. Its greatest weakness is due to the absence of a reliable factor to take care of long stretches of time in which the sedimentary rocks are known to have been subjected to destructive processes. A yardstick of this character cannot be applied to rocks that have been destroyed, and there are excellent reasons for believing that these interruptions may account for several times the lapse of years indicated by the amount of rock remaining in the column which has been pieced together.
Following the discovery of radium, however, the present century provided a new field of knowledge which has contributed greatly to the measurement of geologic time. The penetrating rays produced by radium and other radioactive substances are due to extremely slow but violent disintegration of the material. Uranium and thorium are radioactive elements which occur in the rocks of many parts of the world. There is little or no loss of material as the so-called disintegration proceeds; instead there is a complicated series of transformations in which other elements are produced, radium itself being one of these. Helium and lead eventually take the place of the less stable elements and the known rate at which these products accumulate provides the highly desired key to the age of the rocks.
Part of the gas, helium, may escape, but except in rare instances where chemical alteration might occur, there probably is no loss of lead. Fortunately, when this metal is produced by radioactivity it differs slightly in atomic weight from ordinary lead; otherwise the presence of the latter would introduce a misleading factor. Since the speed at which the change goes on cannot be increased or decreased, it is assumed that throughout past ages it has never been faster or slower. The amount of such change that has been completed in any body of radioactive minerals may be measured by techniques employed in physics and chemistry. If it is found that the amount of helium or lead present requires a hundred million years for its production at the working speed of the parent elements, the mineral deposit must be at least that old.
Certain conditions of course complicate the problem seriously: knowing the age of a piece of rock which happens to contain some radioactive element is of small service in historical studies unless the rock can be definitely associated with a flora or fauna, or some outstanding event disclosed by geological investigations. But there have been a few instances in which most of the necessary conditions were present, and more and better opportunities to apply this method will no doubt appear. Other elements, or their radioactive isotopes, are already being employed with good results. Some of these, such as carbon 14, are more sensitive indicators for the accurate dating of events in comparatively recent time.
When it can be used, this type of measurement is far less subject to uncertainties than any other. It promises to eliminate all need for guessing, and comes close to a degree of accuracy which is satisfactory to the scientist, a person who thoroughly dislikes uncertainties of any kind. If suitable material can be found in just the right places it should accomplish what the preceding method cannot do—the accurate measurement of the great time breaks which interrupt the geological record in many places. Something along this line already has been accomplished, for radioactive material has been found in some of the oldest of the rocks. Regardless of the destruction going on in other localities, these rocks have continued to register the passing of time, and a tremendous antiquity for the earth and some of its first inhabitants has been indicated.
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.
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.