Fossils: A Story of the Rocks and Their Record of Prehistoric Life

by Harvey C. Markman

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.