“NATURE vibrates with rhythms, climatic and dystrophic, those finding stratigraphic expression ranging In period from the rapid oscillation of surface waters, recorded in ripple-mark, to those long-deferred stirrings of the deep imprisoned titans which have divided earth history into periods and eras. The flight of time is measured by the weaving of composite rhythms— day and night, calm and storm, summer and winter, birth and death—such as these are sensed in the brief life of man. But the career of the Earth recedes into a remoteness against which these lesses cycles are as unavailing for the measurement of that abyss of time as would be for human history the beating of an insect's wing. We must seek out, then, the nature of those longer rhythms whose very existence was unknown until man by the light of science sought to understand the Earth. The larger of these must he measured in terms of the smaller, and the smaller must be measured in terms of years. Sedimentation is controlled by them, and the stratigraphic series constitutes a record, written on tablets of stone, of these lesser and greater waves which have pulsed through geologic time.” The measurement of geologic time is of much importance outside of the realms of geology and biology. Closely related to the history of sedimentary rocks are the age of the Earth as an astronomical body and the evolution of the planetary system. We shall see from the discussion of the following pages that the nature and speed of stellar development are- also involved in this problem, and even the fundamental physical problem of the source of the radiant energy of stars. Thus the recent important studies of the age of the habitable Earth by geologists and paleontologists—Holmes, Schuchert, Matthews, Barrell, and others—have high significance in general problems of. cosmogony. The progress of science frequently demands and utilizes close co-operation of Its many branches. We may study the stars, indeed, with tne aid of fossils in terrestrial rocks, and acquire knowledge of atomic structure from the climates of Preeam- brian times. I have begun this paper with the introiludory paragraph of a remarkably comprehensive and important memoir on the duration of geologic time by Professor Joseph Barrell of Yale University. In the growth of our concepts of the age of the Earth, his discussion Is likely to ^rk an epoch because of its consistent carefulness, its great expansion of geologic time beyond the commonly accepted limits, and its decided rebellion against the stringent limitations set by Kelvin anil later physicists. Excepting a few others, such as Arthur Holmes' of England, geologists have heretofore hesitated to correlate directly the radioactive evidence of the age of rocks with the records of stratigraphy. THE time scale the history of the earth. Before reviewing some of the more salient points in Barrell's revision of geologic time, a chronological table of earth history may be given, incorporating his final estimates. I have adapted the tabular data from numerous sources'. The scheme of eras and periods follows traditional lines rather than the more logical arrangement of eras based upon organic evolution and of periods based upon modern views of the relative importance of the major disturbances that punctuate geologic history. It should be noted that the birth of the various mountain systems usually extended over more than a single period; the time indicated in the last column is that of greatest activity or of maximum uplift. Numbers of the third and fourth columns, referring to the total time elapsed since the beginning of the corresponding period, are taken from Barrell's memoir. He states that the column designating the minimum values is “regarded as the more probable, but it is desirable to give maximum and minimum estimates in order to prevent a single column of figures conveying the idea of a precision or certainty which is not yet attained.” The divergence of the two columns shows the order of uncertainty given to the results. by the summation of the estimated probable errors of all the various factors which enter the determination of the relative and absolute ages. Some of the points in geologic history are determined with much greater accuracy than others; the beginning of the Cambrian is far from certain, while the age of TIME SCALE IN MILLIONS OF YEARS MINIMUM MAXIMUM CHRONOLOGICAL NCTES Psycwroic (Age of mental life) Cmoooic (Age of mammals modern floras) and .If csoooic (Age of reptiles) Palcocoic (Age of amphibians, fishes, and higher invertebrates) ProrrocoY. Archeozoic.. Primordial. Cosmologic. Recent Pleistocene.... Pliocene Miocene Oligocene Eocene Paleocene Cretaceous.... Commanchian. J urassic Triasic Permian Penrsylvanian Mis,ippian Devonian Silurian Ordovician Cambrian Latc Early Palco-Laurentian, etc. 7 19 35 55 95 120 155 190 215 250 300 35° 300 480 550 (1200) (1500) I.5 f 9 23 39 65 5 150 195 240 280 33° 370 420 460 590 7OO Dominance of man Periodic glaciation; primitive man Himalayas; man-apes Modern Alps; three-toed horses Pyrenees; Apennines Mt. Wilson'; modern mammals; four-toed horses Primitive mammals; five-toed horses? Andes; Rocky Mountains Rise of flowering plants Sierra Nevada; flying reptiles; first birds Rise of dinosaurs Appalachians; Ural Mountains Paleozoic Alps; primitive insects Early Coal Measures Earliest known land floras Age of fishes Rise of invertebrates Dominance of trilobites Oldest known fossils Primitive marine invertebrates Unicellular life? (3000?) Origin of Earth '.Arnold and Strong, Some Crystalline Rocks of the San Gabriel Mountains, California, Bulletin of the Geological Society of A Face of the Larth, vol. (Oxford), 1009, p.425. tMrica 16, 183, 1005; Sues, Devonian and Carboniferous rocks is fairly definite. The numbers for Precambrian times, which I have added parenthetically to the table, are just short of being hypothetical, in that a considerable uncertainty arises in estimating the actual beginning of a division. The ages of some of the rocks of these periods are accurately known; but the precise geologic positions within the periods are in general not as yet determined. Barrell remarks (p. 752) that “surprising as it may seem, the date known with the greatest precision lies far back in Precambrian time. From Norway, Texas, Quebec, and German East Africa uranium minerals associated with granites give an age which approximates 1,120,00,00 years.” The great masses of Precambrian rocks have long been held to represent at least as great an interval of time as the whole of subsequent history. It now appears that the oldest known rocks, the granite-gneisses of the Laurentian system in Canada for example, were in existence nearly a billion and a half years ago. “Beyond these most ancient milestones lies the Primordial era, whose stratigraphic record has been destroyed by engulfment in magmas from below and by repeated cycles of erosion from above. As to its length, there is no indication other than that the oldest known rocks which “mark the beginning of the following era contain sediments testifying to an earth surface on which air and water played their parts, much as in later times. Crust, ocean, and atmosphere had by the opening of the Archeozoic already attained a condition of stability.” The last entry in the table, that for the origin of the Earth is certainly hypothetical, but it is of interest as an independent determination that is not out of harmony with geologic evidence. It is given by Jeffreys' as a rough theoretical value of the age of the planetary system, being derived, on the basis of the tidal evolution theory, from a consideration of the present orbital elements. rhythms and the measurement of geologic timk. Professor Barrell's discussion is primarily an analysis of geologiC evidence, considering especially the influences of “the fundamental factor of composite rhythms.” It is in the recognition of these age-long action and the accumulation of solutes in the sea, tlw rhythms—the pulsatory deviations from strict up i- formity in all geologic processes—that he reaches conclusions differing widely from the usual results. Chamberlin, Holmes, Schuchert, and a few others also recognize that the average rate of erosion, sedimentation, and crustal movement in the remote past cannot be closely equateil with the rates in recent and present times. We now live in an epoch of great continental uplift and all processes are conspicuously accelerated. Chamberlin writes:6 “Because of the relatively high gradients, the wash of elastic material from the slopes and its deposition ill the basins, as well as the transfer of salts to the sea, are today more rapid than in average times. We seem to be at, or near, one of the great extremes of intensification of the processes of solution and degra- dation. And so, whether conclusions are based upon degradation and elastic deposition, or upon solvent action and the accumulation of solutes in the sea, the present rates are high rates.” Admitting the present high rates of denudation and deposition and their irregularly rhythmic nature, with the resulting breaks in the sedimentary records, Barrell concludes that a fair interpretation of present stratigraphic evidence demands a duration of time perhaps ten or fifteen times longer than that. required by a strict uniformitarian treatment. The hypothesis of compound rhythms is applied by Barrell to all phases of geologic action. Home oscillations are extremely short and indefinite, others are long and more surely recorded in the rocks. The compounding and balancing of the effects of various pulsations give rise to crescendoes and diminuendoes in the resultant flow of geologic events. The sharp oscillations of decades, centuries, and thousands of years may be classed as solar climatic rhythms; the long, slow, and massive movements as diastrophic climatic changes. A cycle of some forty million years appears to mark the crustal and climatic disturbances which terminate the periods; possibly a cycle of two hundred million years separates the eras, if a proposed delimitation of the greater divisions be accepted. The present great continental elevation was matched in the Proterozoic, indicating the most far-reaching rhythm of all geologic time. This doctrine of rhythms is taken by Professor Barrell even to the utmost limits of. cosmogony when he subscribes to the argument that “the apparent running down of the visible universe must be but one phase of a recurrent cosmic cycle philosophically necessary in infinite time, or else the running down would have been completed in previous enternlty” (p. 904). Before Barrell's work the principal geologic methods of measuring time wen''based upon: (a) erosion and sedimentation, (b) chemical denudation and the sodium in the sea, (c) the thermal gradient of the crust. He has carefully examined the postulates underlying these methods, which have generally given estimates of time.very much smaller than those gi ven by considerations of rhythms in the sedimentary series and by the measurements based on radioactive processes. Thus the viewpoint of his treatment is strictly geologic, but a full review of the evidence furnished by radioactivity is included, derived mainly from the publications of Holmes and the experimental work of Boltwood and Strutt. The new scale of geologic time is constructed, therefore, by the dovetailing of two lines of evidence': PERIODFirst, the thickness and character of the sediments give stratigraphic ratios of the lengths of the several periods. The ratios are subject to considerable uncertainty; yet, when derived with a knowledge of the variables Involved, they give some measure of the relative durations. Second, the quantities of helium and lead In radioactive minerals give minimum and maximum measurements of age. These ages are open to some uncertainty owing to the loss of helium or the presence of original lead, and the stratigraphic positions of the rocks holding the minerals are also in most cases not closely known. Nevertheless, the radioactive minerals give measures of absolute age which are of the right order of magnitude and in proper sequence, as shown by the geologic data. The adjustment of these two lines of evidence serves as the basis for a scale of geologic time expressed in years. The result is comparable to the first crude measurements of the distances of the stars in space. The progress of research will continually refine the determinations and lead to a higher order of precision; at present the important conclusion is that time, since the beginning of the Cambrian, is from ten to fifteen times longer than has been generally accepted hy geologists. RADIOACTIVITY AND the age Of ROCKS. The method of measuring the age of the rocks that include radioactive minerals is so generally known that little explanation need here be given. From the time radioactivity attains equilibrium in a thorium or uranium mineral, the end products accumulate within it at a uniform rate. These products are not removed from a dense crystalline rock unless the mineral is subjected to passing solvents, which would then surely record their effects by the alteration of the mineral itself. An atom of uranium (atomic weight 238) will ulitmately give rise as stable products to eight atoms of helium (atomic weight 4) and one atom of isotoplc lead (atomic weight 206). If the quantity of these can be measured and compared with the quantity of uranium in the same material, data are obtained for measuring the age of the mineral and with it the age of the rock-formation of which it is a part. If a mineral contains a percentage, Pb, of accumulated lead of radioactive origin, and a percentage of uranium, U, then the age of the mineral is given by: Pb Age = — X 7,500 million years U + 0.575 Ph The numerical values in this formula are based upon an unpublished discussion by Professor Holtwood, who considers that tlie half-value periods of radium and uranium.ire known within two per cent. The rate of decay apparently is not affected by the nature of chemical combination or physical state. Laboratory experiments have duplicated the conditions that exist in the outer crust of the Earth, Temperatures ranging from that of liquid air up to 2,500° C., and pressures up to 160 tons per square inch have been found not to influence the rate of disintegraflon of the products of radium. It is highly probable but not as yet actually demonstrated that uranium is similarly unaffected. Thus the atoms of uranium break up with a uniform rate whether they are In elemental form or combined in a salt; whether they are in solid, liquid, or gas. Nothing is known to support a hypothesis that there is a change in stahlllty of unstable atoms by a process of aging. The possibility that uranium is affected by ranges of temperature and pressure that do not affect its less stable derivatives can be and, to some extent, has been tested by geologic evidence. Uranium minerals from the same well-established stratigraphic position, but in different localities, and, therefore, for millions of years under widely different physical conditions, yield accordant results for the age of the rock-formation. Another valuable test is that of proper sequence—invariably reliable analytical work shows that the older the rock-formation, the greater the lead and helium ratios. Believing he has found discord ance in such cases, Becker is inclined to discount the whole radioactive method; but Barrell's critical examination of the same data shows that the errors in the adopted stratigraphic positions and in the use of the radioactive analyses wholly account for the supposed discrepancies. The summation of all the evidence therefore makes clear that the properties of radioactive elements afford a remarkable and reliable means of dating the principal incidents in the ancient history of the Earth. the earth's loss of heat. In the earlier history of speculations concerning geologic time, those students who were not too.much hampered by conventional interpretations of the first chapter of Genesis felt no need of placing a limit to the age of the Earth. “No vestige of a beginning, no prospect o( an end” is frequently quoted from Hutton. For the deposition of the known sediments according to uniformitarian principles, Lyell saw the necessity of hundreds of mlllions of years; and Darwin believed that the transformation of species and the high development of animal life required similar lapses of time. In propounding the contraction theory of solar energy, Helmholtz in 1856 noted that the past history of the Sun would be limited to some twenty million years. A few years later Lord Kelvin attacked tlie problem from the standpoint of the flow of internal terrestrial heat, obtaining similar numerical results. Kelvin invaded the domain of geology (thus the geologists are wont to put it) hoping to reform its specula- li nns and bring them into accordance with the doctrines of the conservation and degradation of energy. His mighty prestige and his unimpeachable mathematics silenced the protests of biologists and stratigraphers alike, and for several decades he and contemporary physicists allowed but ten or twenty mlllion of years for the past duration of the habitable Earth. It is a result of observation that at least near the surface of the Earth the temperature of the rocks increases on the average about l' C. for every 100 feet in depth. This of course indicates that the Earth is losing heat, and, supposing an original temperature of t he whole Earth equal to that of molten rock, the Fourier theory of heat conduction permits a calculation of the total duration of inhabitable surface temperatures. Such reasoning naturally assumes that the only source is the primal high temperature, and that the Earth is simply cooling off from its- molten beginning. Hence, at some past time the surface was too hot for life and, at a future time, all depending on the observed temperature gradient, it will be too cold. That was all there was to the problem and its solution, according to the physical theory, and the recent Imposing Ages of Ice were for a time considered as evidence of the approaching, inevitable and eternal frigid ity. The physical argument was adopted, in general, and attempts were made to compress the geologic record Into a narow interval of time; many geologists, however, would not accept the dictum. For instance, in 1 S92 Sir Archibald Geike8 objected with the following somewhat prophetic statement: “That there must be some flaw In the physical argument I can, for my own part, hardly doubt, though I (In not pretend to be able to say where it is to be found. Some assumption, it seems to me, has been made, or some consideration has been left out of sight, which will eventually be seen to vitiate the conclusions, and which when duly taken into account will allow time enough for any reasonable interpretation of the geological record.” As a sort of a compromise between the inexorable physics and their own feelings as to the duration of geologic time, a value of a hundred million years came to be pretty generally accepted by geologists with no insistence on certainty, for the age of the oldest known sedimentary rocks. Professor Harker' points out that Kelvin also had n t one time an inkling of the true state of affairs; he recognized that, while the Earth is certainly losing heat, “it is possible that no cooling may result from this loss of heat, but only an exhaustion of potential energy, which in this case could scarcely be other than chemical affinity between substances forming part of the Earth's crust.” This, however, Kelvin dismissed as “extremely improbable,” and proceeded on the assumption that primal heat is the only form of energy to be reckoned with. Kelvin's surmise as to potential energies and chemical affinities was an interesting prophecy; for, as we now know, the solution of the dilemma was the discovery, less than twenty years ago, of radioactivity in terrestrial rocks, which provides enormous stores of thermal energy. The discovery was, in fact, more than a solution; the percentage of radium in surface rocks is one hundred times too much to just counteract the observed loss of heat. The explanation of this condition must be that the radioactive minerals are confined to surface formations only, as otherwise the Earth would be heating up with “geologic rapidity.” “The most probable depth of the radioactive layer may therefore be placed at 30 miles,” Holmes concludes (p. ]35), after considering other values, “and the basal temperature. in this case would be about 750° C, which would be more in agreement with the requirements of volcanic action.” This approximate result is independent of the observed temperature gradient, Involving only the character of terrestrial rocks, “for there can be no doubt that the radium and thorium content decreases with depth for the same reason that the type of rock varies with depth.*** There is a rough proportionality between the acidity or percentage of silica of a rock and its radium content. The more basic rocks are much poorer in radium, and as would be expected, In thorium also. Now we have good reason to suppose that the more deep-seated rocks of the Earth's crust are of basic and ultra-basic composition, and that below the 30-mile crustal zone they are exclusively ultra-basic, perhaps similar in composition to the material of stony meteorites.***Within the stony zone, which extends down several hundred miles, lies the heavy core of the Earth, probably of metallic composition, like the Iron meteorites. If we may judge from the latter, this nucleus is entirely free from radium.”10 With no prospect of ever knowing accurately the amount and distribution of radioactive elements in the Earth's crust, there is no longer any solid basis for calculating age from the temperature gradient of the Earth. The observed loss of heat is not an Indication of previous thermal conditions. The Earth may be growing hotter; it may be cooling more rapidly than it would do if in radio-thermal equilibrium; or It may be in thermal equilibrium (the most probable condition, according to Holmes), gaining as much heat as it loses and eventually cooling only as the slow decay of the radio-elements permits. We are thus justified, It appears, in believing In a greatly prolonged future for the inhabitants of the Earth. In terrestrial climates during geologic times we recognize no evidence of sensible secular change. Atmospheric temperatures are, of course, almost wholly dependent on the radiation of the Sun. The secular progression of solar energies, however, as we shall observe in following paragraphs, is apparently so small, when judged by our base line of a thousand million years, that we may consider the radiation as essentially constant. There is probably much better justification, therefore, in expecting the termination of terrestrial life in catastrophe, whether earthly or comic, rather than in climatic senility. To be sure, there is strong evidence that we now live in an ln- terglacial epoch of the quickening ice pulsations of the Pleistocene, and we may indeed reasonably expect another advance of the oscillating ice sheets In the near (geologically-speaking) future; but in former times there have been equally extensive invasions of ice, followed by unknown millions of years of genial climates. In the past the fluctuations of land forms and of meteorological conditions have profoundly though slowly affected conditions for organic existence; but later than Proterozoic times there has been no complete interruption of plant and animal life, and as far as can be seen or intelligently predicted, the future, for similar ones, promises conditions no less favorable. radioactivity and solar radiation. The discovery of radium rescued the problem of the age of the inhabitable Earth as far as its own interior cooling is concerned, from the narrow limitations set by the thermal gradient. But radioactivity will not suffice to account for the radiation of the Sun during geologic ages. We admit no potent source of energy back of terrestrial life other than solar radiation; and we have seen that, in supplying radiant energy at the required rate, the Helmholtzian contraction, which is the only very powerful origin of stellar energy now definitely recognized by astronomers, would suffice only for a few million years. Hence the geologie time scale is again embarrassed by physical theory. This circumstance, however, does not continue to divert faith from the testimony of the rocks, for, remembering the case of the thermal gradient, we look to a temporary ignorance of the properties of matter in accounting for this presumed discrepancy. It is readily shown that the normal decomposition of known radioactive elements might be amply sufficient in the sun (as on the Earth's crust) to account for the duration of solar radiation; but in the case of the Sun this device is woefully deficient as to amount. If the Sun were composed entirely of uranium, In equilibrium with its disintegration products, the heat generated by known. radioactivity would not be a third of the actual output. “The Importance of radio-thermal phenomena is not felt until cooling has progressed to a more advanced stage, as exemplified by the Earth, when the heat lost is balanced against that set free by atomic disintegration.” Jeans considers that even all known electrical properties of matter, radioactive. (Concluded on page 42).
This article was originally published with the title "The Age of the Earth"