CONTENTS

Page

Introduction 5

The Prehistoric Record 5

Varieties of Fossils 8

Fossilization 9

Floras and Faunas 13

Formations 16

Geological Time 18

Explanation of the Time Chart 23

The Geological Section 25

Before the Age of Reptiles 31

The Pre-Cambrian Complex 31

Cambrian Life 33

The Ordovician Record 35

Silurian Events 36

Devonian Progress 37

Carboniferous Forests 40

Permian Hardships 43

The Age of Reptiles 47

Dinosaurs 48

Plant Life and Climate 56

Coal and Fossil Footprints 57

Mesozoic Invertebrates 58

Extinct Birds 60

Ancestors of the Mammals 61

The Age of Mammals 64

Prehistoric Horses 70

Mastodons and Mammoths 80

The Rancho La Brea Fossil Pits 88

The Age of Man 93

Supplementary Reading 95

LIST OF ILLUSTRATIONS

Page

At Work on a Dinosaur Skeleton 2

Fossil Bones in Rock Formation 7

Insect Fossils 10

Restoration of Rhinoceros 12

Dinosaur Tracks 17

Time Chart 22

Geological Section Showing Positions of Formations 26

Marine Beds of the Benton Formation 28

Plesiosaur Bones in Place 28

Invertebrate Fossils 34

Modernized Fishes 38

Prehistoric Plants 41

Marine Reptiles 46

Diplodocus 49

Trachodon 51

Stegosaur 52

Sea Turtle 54

Murals, Hall of Mammals 63

Uintatheres and Contemporary Life 65

Moropus 67

Titanotheres 69

Oligocene Mammals 71

Pleistocene Horse 73

Structure of Molar Teeth 75

Grazing Type of Molar Teeth 77

American Mastodon 79

Long-Jawed Mastodont 81

Molar Tooth of Mammoth 83

Nebraska Mammoth 85

Rancho La Brea Fossils 87

Folsom Bison 90

Man and Mammoth 92

FOSSILS

INTRODUCTION

In the recent growth of knowledge there has been rapid progress in two directions. The commercial exploitation of natural resources, being fundamental to modern civilization, attracts a liberal share of the talents and energies of workers trained for the industrial professions. A second trend has specialized in the further development of the sciences which are characteristic of our time. Such activities, in the natural history field, deal largely with the refinements of exact definition, nomenclature and classification, all of which means little or nothing to the layman who is otherwise engaged.

For the latter, however, there is a quality of interest which may be described as a wholesome curiosity about what has happened, how it happened, how we know it happened, and what it may signify to one who is neither industrialist nor scientist. This booklet is intended for the many who feel that there is more to be obtained from a natural history museum than an occasional glimpse of a bewildering “marvel.” In addition to being a guide to fossil exhibits it supplies parts of a great story which specimens alone can not relate.

THE PREHISTORIC RECORD

All that is known of the extinct plants and animals which inhabited the earth before man began the practice of recording his observations has been obtained from a study of the rocks. The few possible exceptions to this rule, in which animal and plant remains have been preserved by freezing or drying, are so unusual as to be hardly worth mentioning.

Explanation of this is that plant and animal tissues quickly decay under ordinary conditions when life ceases. Unless protected from destructive agencies which are especially active at the surface of the ground, even the heavier bones of animals and the large trunks of fallen trees will soon crumble into shapeless masses. The usual method employed by nature to prepare a fossil specimen is so closely related to the natural process of rock making that a little knowledge of that subject will be necessary in order to know what fossils are and how they are preserved for so long a time.

It should be understood first that a fossil is some record, commonly preserved in rock, of a kind of plant or animal which no longer exists as a living type. This, at least, is the ordinary sense of the word and more elaborate definitions are of small service to anyone. It may be necessary to add, however, that all things which have lived at any time are regarded as either plants or animals.

Nature’s way of producing rocks and fossils remains a mystery to many of us because we are so wrapped up with the importance of finding names for things and materials that we frequently neglect the consideration of sources and histories. Everyone knows a rock when he sees it in a large mass, but when he looks at sand, mud, dust, or soil, he seldom thinks of it as related in any way to rocks. Although the difference is almost entirely a matter of size, our use of words makes it seem unreasonable to speak of the finer particles as rock.

There can be no reality or meaning in the natural record for an individual who has failed to observe a few simple facts which involve changes going on in all parts of the world at the present time. With regard to rocks, it is supposed that what happens in our day also occurred under like circumstances ages ago. Anyone wishing to do so may see for himself that rock masses break down wherever they are exposed to the elements, that the larger pieces are reduced to smaller fragments, and that the final product is sand or dust.

He may also note that this finely ground material is being moved and sorted, by rain, wind, and streams, transported to lower levels and accumulated in great quantities wherever it finds a resting place. Along with it go sticks and leaves, bugs, shells, bones and carcasses of animals, some of which in time may become fossils. In large lakes and seas there is a steady distribution of such materials over broad areas, yesterday’s accumulation of sediments being buried by the contributions of today, the most recent of the settlings always resting upon older ones until something happens to disturb that arrangement.

Not so readily observed are other parts of the process, such as the consolidation of sands and muds into the firm sandstones and shales which we again recognize as rocks. Much of this requires more than the few score years of a human lifetime for its accomplishment, but many of us have seen muds become so solidified, by merely drying, that they could hardly be distinguished from prehistoric shales. It is to be noted also that some ancient fossils come from sandstones which are scarcely more rock-like than the loose sands of an ocean beach. Thus we learn that firm consolidation of rock-making materials is not always a sign of great antiquity, and that hardness of rock is not always essential to the preservation of imbedded plants and animals.

The rocks themselves must explain the many things which have happened during the course of millions of years, and this they do remarkably well when carefully studied, for many of the factors involved in their histories leave characteristic marks. Changing climates, the draining of seas, the uplifting of mountain ranges, all have ways of registering their occurrence which are as convincing and reliable as anything ever written by man. Piece by piece the story has been patched together through the efforts of thousands of investigators. Parts of the narrative remain buried at inaccessible depths, and whole chapters, no doubt, have been destroyed by the same forces that composed this tremendous record of prehistoric times.

VARIETIES OF FOSSILS

It would be a serious mistake to regard nature as divided into a number of distinct and independent schools of fossil making, each refusing to use the methods and devices of another. There are, however, certain features which stand out so prominently that a little classification becomes helpful. While this procedure brings out differences it should be understood that processes actually work together, several of them usually being involved in the production of any individual specimen.

(1) Impressions of animals and plants, or parts of these, are frequently left in soft sand or mud which later becomes converted into more durable rock. This type of fossils is represented by animal foot-prints and the imprints of leaves, flowers, insects, and like objects which may be mingled with the finely ground materials of the common sedimentary rocks.

(2) Parts of plants and animals may be gradually replaced by mineral matter with little or no change from original form and texture. Fossils of this class are said to be petrified or turned to stone. They are also known as replacements. The fleshy parts of animals do not petrify.

(3) Many animals among the invertebrates use mineral substances for protective or supporting structures. Small plants of various kinds follow a similar practice. These structures, being produced in stony materials, are readily converted into fossils. The shells of mollusks are the best known illustrations in this field, and all that is required for a shell to become a fossil is the extinction of the species of animal that produced it. Fossils of this type are extremely abundant.

(4) Preservative substances other than those which produce common rocks may be mentioned among fossil-making possibilities. Bones are known to have been preserved in asphalt, and insects in resins, but such cases are few in comparison with the products of other methods.

(5) In rare instances there has been preservation of extinct creatures by the process of drying or by refrigeration. Occasional mummies are found with shriveled flesh and skin still in place, but better preservation of all tissues occurs when the temperature is quickly reduced below freezing point and held there without interruption. This can happen only in the colder parts of the earth and is always subject to climatic change. The effect of drying also may be undone at any time by a slight increase in the amount of moisture.

(6) Coal beds often produce fossils of an unusual sort. In the formation of coal, plant material gradually loses some of its more perishable substances but retains carbon which has better lasting qualities and slowly accumulates to produce the seams and beds that are mined. In the early stages of the process the original vegetation undergoes little change in appearance but eventually practically all of its character is lost. Many fossil leaves are found as thin layers of carbon, bedded in the clays which are commonly associated with coal deposits.

(7) Concretions, which are hardened lumps of mineral substances occurring commonly in sandstones and shales, are often mistaken for fossils because of their peculiar shapes. However, there are localities in which the mineral solutions have been concentrated and deposited around shells, leaves, seeds, or similar objects, thus producing an abundance of fossils which may be obtained by opening the concretions. Fossils of this type are well known from Mazon Creek and other districts in Illinois, Kansas, Colorado, and elsewhere.

FOSSILIZATION

Footprints need little explanation other than a consideration of the factors which make it possible for them to be preserved. The sand or mud must be neither too soft nor too hard to take the form of the foot and retain its shape when the foot is withdrawn. Then in some manner the impression must be protected while the rock-making process goes on. When such protection is obtained it is usually in the form of more mud and sand, deposited over the surface which received the impression. At a later time the covering may be separated from the lower part of the deposit, which serves as a mold, and if the separation be accomplished successfully a natural cast of the foot will be obtained as well as the mold in which it was produced. Since conditions for perfect work are not always present in a laboratory of this kind, it is not surprising that fossil footprints are very rare considering the number and variety of tracks left by wandering animals.

Impressions of leaves are explained in much the same way except that the leaf remains under its protective covering until it decays. Similar impressions may be obtained from the bodies of delicate invertebrate animals but they are seldom preserved because of the softness of the tissues. The smaller fishes provide much better material for the production of fossils according to this method. While the fish is being flattened by the weight of surrounding sediments, scales, fins, and soft bones retain their positions and provide the necessary resistance to leave an impression of the body form when the flesh is gone.

The larger and more spectacular fossils, such as skeletons, skulls, and detached bones are nearly always of the replacement type. Replacement of plant and animal substances by mineral matter is a slow process and in younger fossils the change is rarely completed, some of the original material being present in a partially altered condition or not modified at all. Since air does not often carry the necessary materials and provide other essential conditions, replacement may be regarded as something which happens underground or in water. It is perhaps best explained in connection with limestones, because calcite or “lime” is frequently the replacing substance although other minerals, especially quartz, may serve the purpose.

Besides converting bony or woody objects into rock substance, mineral replacements may assist in the production and preservation of fossils in another manner. It often results in the filing of cavities with some rock-making substance which retards destruction through crushing or other injury. In many cases, so-called fossil shells are not shells at all; instead, they are merely a stony filling which was once surrounded by shell substance. In other instances the original shell remains as it was during the life of its former occupant, preservation of the shell being due largely to the substitution of a mineral filler for the soft animal tissues once present.

Limestone comes into existence through a more elaborate process than that which produces sandstone and shales. It is one of the three types of common rocks, known collectively as the sedimentaries, in which fossils are found. It differs from sandstones and shales, however, in that much of its substance has been dissolved in water instead of being transported in the form of finely ground rock particles. Lime occurs in many varieties of rock which are exposed to the wear and tear of the elements throughout the world. Slowly but more or less continuously it is taken from this source by ground and surface waters coming in contact with it. Particularly active is carbonated water, moving underground through pores and crevices.

This underground circulation of mineral matter in a dissolved condition explains the occurrence of fossils in land areas which have not necessarily been submerged during any great length of time, for it is well known that plant and animal remains are not invariably washed into lakes or seas, and that all sedimentary deposits have not been built up in large bodies of water. Here we are dealing with what is known as the continental type of sedimentation and such fossils as dinosaurs, mastodons, three-toed horses, and other former inhabitants of land areas.

In order to become properly fossilized, certain conditions are absolutely necessary, and only a small percentage of the once-living multitude secures the required treatment. There must be present, soon after death, some protection from the activities of the carnivorous birds and beasts that would separate and scatter the parts of a carcass, also from the smaller gnawing animals that would continue the destruction, and finally from wind, sun, rain, frost, and bacterial and chemical activities which in the course of only a few years would remove everything but possibly a few scraps of tooth enamel, which is the hardest of animal tissues.

A slight covering of earth substance in any form serves to check the disintegration, and this may be acquired in several ways. Animals that perish in bogs or quicksands are soon covered over; in many localities wind-blown dust and sand do the work; and flooded river valleys provide an abundance of mud for the necessary burial of others. Even underground, the decay of soft tissues is too rapid to permit of replacement by mineral substance in a manner that would reproduce form and texture. Skin and flesh are almost invariably lost, although in a few instances the thick scaly hides of dinosaurs are known to have produced natural molds and casts by the method explained in connection with footprints and other impressions.

With regard to the more durable tissues found in the teeth, bones, and shells of animals, or the woody parts of plants, the case is different. These parts become firmly imbedded in the ground, but moisture still has access, and it begins to work immediately; for all water moving underground finds soluble substances which it picks up and carries with it wherever it goes, and much of the load consists of mineral matter which may be unloaded again when the necessary conditions are found.

Mineral-laden waters will drop one kind of substance to take up another which dissolves more readily, and this happens sooner or later when a buried bone or log is encountered. Complications of various sorts enter into the process, but the final outcome frequently is a complete change from one chemical composition to another which is more enduring, the transformation being brought about so gradually and thoroughly that in many fossils the inner structure of the original tissue is as accurately reproduced as the fine detail of surface features.

Converted into stone, however, the result is still far from permanent. While yet underground the fossil is subjected to distortion and breakage due to earth movements which bend and dislocate the rock deposits. What causes these upheavals and depressions of the earth’s surface remains the subject of much discussion, but that they have occurred on a large scale and continue to occur is clearly evident. At higher altitudes the surface rocks and fossils are exposed to a larger variety of destructive activities than at lower levels where protective coverings are more likely to be provided and retained. Once stripped of that protection there is little chance for a fossil to survive. Beyond a doubt there are many thousands of tons of prehistoric remains damaged or destroyed each year, by weather and stream erosion.

FLORAS AND FAUNAS

As the various types of sediments continue to accumulate on land and in water they produce deposits of sandstones, claystones, and limestones which in time may acquire great thickness and cover wide areas of sea floor, or continental surface. Usually there is more or less mixing of sediments resulting in sandy limestones, limy clays, and other combinations. Quite commonly, however, the types remain fairly pure but become arranged in layers which alternate from one kind of material to another. At all times the character of the deposit will depend upon the nature of the rocks which supply the materials, and any fossils that may be produced will consist of such plants and animals as live and die during the time the rock is in the making.

Some of the rock layers will be rich in plant and animal remains, others quite barren, the difference being due partly to conditions influencing the life of the region. In addition, the character and amount of rock-making materials at the time may be favorable or unfavorable to the preservation of fossils. Seas, lakes, and valleys may at any time be drained, or enlarged and deepened, by changes in the elevation of underlying rocks. The amount and variety of mineral substances dissolved in the waters of a region not only affect the character of rock deposits but also the plants and animals living in the water. Some of these chemical solutions provide cementing materials which bind together the grains of sands and mud; others have a detrimental effect upon cementing material previously deposited, and so construction and destruction go on continuously, more or less hand in hand, to produce complicated and often puzzling results.

A little more salt, or a little less of it, may change completely the variety of life inhabiting a body of water. A slight change in the depth of the water often accomplishes the same thing, for plants and animals are so delicately adjusted to their environments that conditions fatal to one race of creatures may provide the exact life requirement of another. This is a matter of practical knowledge which is being used today in the cultivation of plants and animals for market purposes. It is being demonstrated continuously, also, upon living subjects in experimental laboratories throughout the world; and, in a bigger way, the facts are observable wherever life is considered in relation to habitat. That anything so obvious should be regarded as guesswork or theorizing, or opposed to truth, when applied to former inhabitants of the earth, is somewhat surprising. And, it may be added, the cultural worth of fossil study comes to a focus on this very point, for men and women are now meddling, consciously or unconsciously, wisely or unwisely, with an all-important environment about which they have learned very little—one called, among other things, “civilization.”

For any portion of the world a complete-list of the different kinds of plant inhabitants comprises the flora of that region, and a like summary for the animal life is known as the fauna of the district. It is generally understood that different species of both plants and animals inhabit different regions of the earth, but outside of professional circles it is only beginning to be recognized that changes in floras and faunas occur from time to time, that slight differences may be noted in the course of observations extending over a period of only a few years, and that everything in a fauna or flora eventually may be displaced by new forms.

It is, however, a convenient practice to use these terms in connection with time periods, rock beds, and types of environment, as well as geographical areas. Thus we have such phrases as a “Cretaceous fauna” (attaching the name of a geologic period), a “Benton fauna” (with reference to the fossils of a rock formation), a “marine flora” (using the name of an environment), an “Arctic flora” (which applies to a definite portion of the earth surface and its plant inhabitants).

Faunas include animals which many persons do not recognize as such. Sponges, corals, insects, worms, crabs, oysters, and a host of other boneless creatures are grouped together as invertebrate animals, while another group includes the fishes, amphibians (toads, frogs, and salamanders of today), reptiles (crocodiles, lizards, snakes, and turtles being well known varieties), birds, and mammals. This second lot, provided with backbones and skeletons, comprise the great division of vertebrate animals.

Floras also include types which are commonly seen but not popularly identified as plants. The algae are perhaps best known as seaweeds, water-silk, and pond scums; fungi as toadstools and moulds. Both groups are large and of important rank in the vegetable kingdom; only the algae, however, are recognized as important fossil producers. Better known types of plants are the mosses, ferns, evergreens, grasses, and the more conspicuous flower-bearing forms, from weed size to tree size.

Many rocks owe their character to the work of large colonies of plants or animals, for the living organisms are frequently the active agency which takes dissolved mineral substance from the solvent liquid and gets it back into solid form. The liquid is, of course, the water in which the creatures live, while the mineral substance often becomes a commodity required by a plant or animal in its mode of living. Mollusks have a way of using lime in the production of shells, and many a bed of limestone consists almost entirely of this by-product of molluscan life. Tiny coral polyps build complicated and beautiful structures from the same mineral substance. Either intact or in broken condition, these structures contribute in a large way to the making of limestones. Algae, among the lowliest of plants, have done extensive work along similar lines, and numerous invertebrate animals could be named as important factors in the production of rocks. Many of the shells and other fabrications retain their peculiar patterns long after the extermination of their makers, and a highly informative part of the fossil record is provided in this manner. It is also by far the larger portion of the record, for the earlier ages of prehistoric time failed to produce a vertebrate animal of any kind, while the invertebrate record dates back to pre-Cambrian time.

FORMATIONS

If in some part of North America there had been steady accumulation of sedimentary materials under constantly favorable conditions since the beginning of Cambrian time, the result would have been a deposit of sandstones, claystones, and limestones measuring nearly fifty miles from bottom to top. These figures are based on actual production in North America where extensive measurements have been made in many localities. When other parts of the world are as thoroughly investigated and older deposits included in the calculations, the total thickness of such beds will probably be more than one hundred miles.

No single pile of rocks offering a complete cross section of the geological record has ever been produced, but portions of the section are exposed to view on all the continents. In order to carry on desirable investigations and make comparisons, it has been necessary to divide this great composite section into small units which may be named in some way and placed definitely with relation to lower and higher, or older and younger, layers. To serve this purpose there has been developed the idea of rock formations, and here we have a word which is not defined readily, even for the use of those who are familiar with it. Nevertheless it is used so commonly that some understanding of its meaning becomes desirable.

A formation may be regarded as an extensive rock mass, variable, in thickness and other proportions, as well as in composition, but representing a period of time during which there was no great change in the character of plant and animal life, and no serious interruption in the depositing of the rock-making materials. Occasionally the lower and upper limits of a formation are well defined and readily located. Frequently, however, the transition is gradual, one formation merging into another with no apparent mark of separation. In such event the original description serves to establish more or less definitely the boundaries of a formation.

Descriptions are published whenever a worker believes he has discovered a significant part of the great section which has not previously been named. The usual practice is to apply a name taken from the locality in which the beds were investigated, and in this manner the names of formations become associated with towns, rivers, counties, mountains, states and other geographical features. The locality which supplies the name is then regarded as the “type locality” for the formation, but wherever these same beds may be traced or otherwise identified the one formation name applies.

The “Dakota formation,” to use a convenient illustration, is mentioned in scores of reports bearing on the geology of Colorado, Iowa, Kansas, Nebraska, New Mexico, Texas, Utah, and Wyoming, as well as the Dakotas. On the geological map of Colorado it appears on both sides of the Rockies, scattered in strips and patches from north to south boundary lines. The beds are easily located in the foothills district west of Denver because of their tendency to produce the prominent ridges known as hogbacks.

Many formations are exposed over much less territory, some have even greater extent. Thickness may vary from a few inches to thousands of feet, and no two exposures will be exactly alike though some similarity necessarily prevails throughout. “Exposures” are simply portions of the beds which are not concealed by loose rock, soil and vegetation, or overlying formations. Canyon walls, steep cliffs and mountain slopes, gullies, and badlands provide a large variety of natural exposures. In such places rocks and fossils may be studied to best advantage.

Since a formation may contain a variety of beds, including sandstones, shales, limestones, and all sorts of mixtures, there is sometimes need of subdividing it; but formations are the smallest units commonly shown on geological maps. They are actual rocks which fit into a historical scheme of things and may be regarded aptly as the pages of a book which nature has done in stone.

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.