The Age Of The Earth

( Originally Published 1915 )

WE have now to consider the first stage of the existence of the Earth, the origin of which was characterised by the formation of a superficial crust, the first rudimentary state of that solid ground on which we actually live.

This crust or shell, due to the cooling of the exterior layers of the heated and rotating spheroid, had the effect of preventing the rapid cooling of the fused layers beneath, which thus retained their high temperature. Immediately beneath the solid stratum were liquid and gaseous masses in motion, while near the Earth's centre the fused matter, liquid or even gaseous, subjected to the enormous pressure of several millions of atmospheres by reason of the weight of the exterior layers, probably existed in a condition practically equivalent to the solid state, compressed as it was beyond any pressure realisable in our laboratories. The heated nucleus would contain all the chemical elements, since it came from a portion detached from the solar matter, and since the spectroscope proves the existence of all these elements in the Sun. But there would certainly be an excess of iron, for, in the first place, spectrum analysis of the light of the stars teaches us that iron predominates in them from the first phase of their evolution. Secondly, the actual general magnetic state of the Earth at the present time indicates, by its effect on a magnetised needle, the presence of magnetic materials in considerable quantity at the centre of the globe. Furthermore, these materials are found in the lava which flows at intervals from volcanic craters over the surface of the Earth.

De Launay has shown that it is possible, by means of geological considerations, to assign the order of superposition of the most widely occurring chemical elements, at the time when the Earth had ceased to be entirely fluid. The elements may in this way be classified into seven groups, the first of which is represented by hydrogen, and the last of which contains the heavy precious metals. The atomic weights would increase with the depth and therefore the elements would be found in the crust at distances from the centre in inverse proportion to the atomic weights, The atoms, freed from chemical affinity at the high temperature in question, individually obeyed, in the fluid rotating sphere, only the laws of gravitation and centrifugal force.

Above the crust thus formed exists an atmosphere which is, at first, at the temperature of solidification of the rock. The latter was, there-fore, formed by the solidification of the most refractory, that is to say the least volatile, elements. The first minerals appearing at the surface would be combinations of silica with alumina, also lime, magnesia, and a little iron and soda.

The terrestrial crust, which was at first very thin, played an important rôle; it separated the interior incandescent ,nucleus from the layer of gases and vapours which surrounded the Earth. This gaseous envelope or atmosphere, the remaining constituents of which envelop our globe at the present time, originally contained a considerable proportion of carbon dioxide gas, which was emitted continuously by the turbulent fluid interior matter. It contained also light gases, notably hydrogen which was present in very large quantity; spectrum analysis demonstrates its existence in the atmospheres of the distant planets, such as Uranus and Neptune, which are in the process of evolution. The atmosphere also contained hydrocarbons and considerable quantities of oxygen and nitrogen.

When the solid terrestrial crust was definitely formed, it was at a very high temperature, namely that of its solidification. It could not, therefore, retain light gases such as hydrogen and helium, which were dissipated into the solar nebula, and thence passed out Into intersidereal space where they constituted rudimentary nebulae. These gases now exist, in the lower regions of our atmosphere, only in very minute traces; at a height of 100 kilometres [62.5 miles] from the ground the little atmosphere which remains is probably composed approximately of 991/2 % of hydrogen and 1/2 % of helium.

Thus, when the crust was completely formed, there remained as atmospheric constituents a great quantity of nitrogen and also a large pro-portion of carbon dioxide and water vapour, for almost all the oxygen was in combination with hydrogen, forming water, which the high temperature prevented from condensing to liquid form. Water cannot exist in the liquid state above the temperature of 36o° C. [68o° F.], which is called its critical temperature.

As the temperature of the atmosphere gradually fell, the most volatile metals remaining in the form of vapour, such as potassium and sodium, were the first to condense. Then, as the cooling continued, elements which the high temperature had prevented from combining were now able to do so, and thus chlorides, bromides, iodides, etc., were produced. When the temperature had de-creased to below the critical one of 36o° C. [68o° F.], the water vapour began to be precipitated in the liquid state. The original pressure of the atmosphere must have been very considerable, since it contained in the gaseous condition the whole of the water actually existing on the earth at the present time. Now, if the oceans were distributed uniformly over the Earth's surface, they would form a layer of water of more than 3000 metres [1.9 miles] in depth, exercising a pressure of 300 times that of our present atmosphere; and this water as vapour would have exerted an equal pressure in the early atmosphere.

During this period, the solid, but still thin, crust was continually kept in a state of agitation by the bubbling up of the internal mass, the upper layers of which, liquid or gaseous, came into con-tact with and pressed against its inner surface. Under these repeated attacks, the crust gave way in places, and became pierced with craters, fissures, and crevices which allowed the upward pressing fused matter to escape. At the lower temperature of the surface, this matter solidified, thus giving rise to the formation which geologists call Archaean, through which jets of the interior magma burst forth, producing the eruptive rocks on solidification. This, however, is not all. On account of the continuous cooling, due to the thinness of the primitive crust, the latter, not being completely sustained by the interior contracting mass, sank in certain parts when the internal pressure raised up other parts. The outer surface of the solid part of the globe, that is the surface of the litho-sphere, would therefore not be uniform; it became wrinkled and indented, presenting protuberances and hollows.

When the atmosphere cooled to the critical temperature of 36o° C. [68o° F.], and the water vapour consequently began to condense to the liquid state, the latter fell as scalding rain on to the solid surface. This water condensed on the higher portions of the surface and flowed down the declivities, dissolving a greater or less proportion of all the substances distributed over the surface of the terrestrial globe. Thus, the streaming of the water commenced on an extensive scale. The water accumulated in the cavities, the folds, and the hollows of the solidified crust, in accordance with the laws of gravity. In this way the oceans first came into being, and it is probable that, as they resulted from the accumulation of water which was originally hot, and which had bathed the entire surface of the Earth, they would have dissolved in the process everything that could be taken into solution and that, therefore, they would contain, at any rate in traces, all the elements which were to be found in the enveloping crust of the Earth.

We shall now examine the form taken by this shell, the lithosphere, the hollows of which received the waters of the primitive seas, and whose higher emerging portions constituted the original continents.

If the superficial crust had continued to envelop a nucleus which sustained it at every point, that is to say with which it was in perfect contact, this solid stratum would have simply taken the form of the fluid nucleus, slightly flattened on account of the centrifugal force due to the Earth's rotation. The crust would thus have the geometrical form of an ellipsoid of revolution. But its support was imperfect, on account of the slow contraction of the central mass due to cooling, and, therefore, as has been said above, it became folded and wrinkled and covered with inequalities, hollows in some parts, protuberances in others. These suffered frequent changes in the early periods, but such changes became rarer and less widespread as, in the course of time, the Earth evolved towards its actual present condition.

Were these foldings produced quite by chance as might be supposed from a superficial examination? We know that chance does not exist and that what we so designate is only the resultant of a number of forces or conditions of which we are more or less ignorant. Everything in that wonderful machine, the Universe, is regulated by inflexible laws. The foldings of the terrestrial crust were not produced erratically; their formation was in accordance with the law of tetrahedral symmetry. At the time of its origin the crust took a certain form which it would tend to preserve unchanged. In order that it should change as little as possible, when the interior volume came to diminish in consequence of the contraction of the nuclear mass, the crust should have a regular figure corresponding to the minimum content for the given surface. Geometry teaches us that the tetrahedron, a regular solid figure with four triangular faces, a pyramid with a triangular base, satisfies this condition. Many causes would operate against the crust taking this form in its entirety, but it would at any rate indicate by the direction of its folds a tendency to take the tetrahedral shape. This tendency would manifest it-self in the diametrical opposition of the continents, since these represent the emergent apices of the tetrahedron, and a similar opposition in the case of the oceans which correspond to the plane faces of the pyramid. These faces are necessarily below the surface of the seas which thus make up the flattened spheroidal form imposed on the Earth by the combined action of the laws of gravity and centrifugal force. (Fig. 4.) Thus from the simple fact of the contraction of the internal mass we are able to form some idea of how the fundamental division of the surface into land and water came about, a division which remained for a long time one of the mysteries of geographical science.

There is another point to be noted about this tendency to take the tetrahedral form. The edges, or arêtes, of the tetrahedron have also an essential significance; they indicate the general orientation of the emergent land which runs roughly north and south. We will return at a fitting time to this important subject from the point of view of the figure of the Earth.

We are, henceforth, able to distinguish two very distinct things : first, the lithosphere which is formed by the solid shell of the globe, a shell originally spheroidal and later deformed by the foldings and furrows of its surface brought about by the tetrahedral laws, at any rate as regards the essential features; and, second, the hydro-sphere, formed by the water surface, the fluidity of which causes it to be governed by the laws of gravity and of rotation, and which maintains, save for slight local perturbations in the immediate neighbourhood of the continental masses, the flattened ellipsoidal figure which is a necessary result of the laws of mechanics.

We shall have to return in fuller detail to the tetrahedral theory in the course of this work; we shall find its application to the theory of seismic phenomena, those eruptions, which are due to the impulse of the heated internal mass and which constantly agitate and dislocate the crust of the earth.

The condensation of the atmospheric water vapour, which began at a high temperature, subsequently continued and its extent increased as the cooling went on. The temperature fell little by little and when it reached the neighbourhood of 55° C. 131°F.], the conditions required for life were realised. Given a living germ, it could grow, reproduce itself, and evolve, that is to say, organised beings could prosper. Furthermore, since the cooling was not rapid, a state of equilibrium was established between the total heat received from the Sun and from the heated interior of the Earth on the one hand and the loss by radiation on the other hand, in such a way that the conditions of temperature favourable to the existence of living beings were brought about in due course. The torrents of water which streamed over the continents carried the debris washed therefrom into the sea and deposited it at the bottom of the oceanic hollows. Thus the process of sedimentation commenced; the primitive rocks were all of igneous origin, but now other kinds began to be formed by the superposition of successive deposits on these original rocks.

Hence was instituted the history of the early ages of the Earth, its geological history, which is that of the time prior to the appearance of man on the globe.

At the period of which we are speaking, the crust was a rigid shell having a composition analogous to that of granite, and the oceans existed, but frequent changes occurred in the configuration of the continents and seas in consequence of the convulsions of the still weak crust, under the influence of the outward thrust of the interior mass. The primitive atmosphere was rich in water vapour and carbon dioxide and did not yet contain all the oxygen necessary to maintain life, a part of which still remained combined with carbon. Thick clouds floated in it on account of the superabundance of uncondensed water vapour.

The large proportion of carbon dioxide in the atmosphere at this period gave to the latter a remarkable property which the actual atmosphere at the present time does not possess to anything like the same extent. It played the part of a protective screen, keeping in the heat and consequently lessening the rate of the Earth's cooling. Carbon dioxide now constitutes scarcely 1/3000 part of the air, but calculations based on experimental evidence have led Arrhenius to the conclusion that if this small quantity of gas were absent the temperature of the Earth's surface would fall 21C. [37.8° F.]. This would further lead to the condensation of a large part of the aqueous vapour still present. As this also acts as a retaining screen in the same way as the carbon dioxide does, it will be seen that the disappearance of this gas would bring disaster upon the Earth, from the point of view of temperature.

Conversely, it will be readily understood how in the early ages of the Earth's history the protecting mantle, formed by an atmosphere considerably richer in carbon dioxide and water vapour than our present one, enabled the soil to maintain the high temperature that caused the extraordinary development of vegetation characterising that period.

From the epoch of the solidification of the crust up to the present time, the history of the Earth is called Geology. It is outside the scope of the present work to trace it in all its details. M. de Launay has given an authoritative exposition of it in his masterly work The History of the Earth. We will confine ourselves here to an outline of the chief facts.

That part of the original solid crust which was covered by the oceans due to the condensation of the atmospheric water vapour, oceans that were destitute of beaches, constituted the foundation upon which all the later solid formations came to be built up. The first disturbances of the primitive crust, the first foldings that it experienced, produced high lands and depressions, thus fixing the original distribution of the continents and seas. The Archaean rocks, that are invariably met with when the soil is penetrated deeply enough to get below all the superincumbent strata, are the oldest known rocks. During the period of formation of the Archaean rocks eruptions from the central mass into crevices of the thin crust were frequent and the Plutonic rocks were produced by the solidification of the interior material thus pushed up. In fact by the study of the Earth's crust we find only the granitic or Plutonic rocks underlying the crystalline or Archaean rocks.

The great thickness of the Archaean formation, which in certain regions is 10,000 metres [6 1/3 miles] or even more, indicates the enormous duration of this first period of the Earth's history.

The rocks formed were still at a high temperature and the primitive atmospheric condensation brought down scalding liquids, so that the conditions at this time were not suited to animal or vegetable life. It is, therefore, not remarkable that we find no trace of any living thing in these first strata. Possibly elementary life made its appearance at the end of this period, but any such creatures being destitute of hard or bony structures left no trace of their existence on rocks so hard and at the same time so convulsed as those which form the Archaean strata.

In proportion as the atmospheric temperature and therefore that of the first oceans fell and reached the neighbourhood of 60° C. [1400 F.], the terrestrial conditions began to be such as would admit the possibility of life. But how did life make its first appearance in the world? Perhaps wandering cells driven from another world by the pressure of radiation reached the Earth and lived and evolved thereon, having resisted the influence of cold during their long journey through space, the possibility of which resistance has been demonstrated by work in the laboratory at Leyden. Arrhenius is of opinion that they must also have escaped from the destructive action of the ultra-violet rays. Or perhaps life originated on our globe in some other unknown way. The problem is one which is at present unsolved and will doubtless remain so for a long time. What is certain is that the Primary Era, characterised by the appearance of life, vestiges of which remain to us as animal and vegetable fossils in the strata of this period, began after the stage represented by the Archaean rocks. The strata corresponding to this era are classified by geologists into Silurian, Devonian, Carboniferous, and Permian.

As previously explained, the atmosphere, rich in carbon dioxide and water vapour, formed around the earth a protective screen preventing rapid cooling and maintaining an extremely high temperature at the surface of the ground. Consequently vegetable life all through the Primary Era, but particularly in the Carboniferous period, flourished with an extraordinary fertility. The remains which are found in coal-beds show that vegetable species, which are to-day merely small plants, were then veritable trees, forming great forests. There were at first only cryptogams and later also gymnosperms. With regard to animals it can be affirmed that life commenced in the seas. The first beings were invertebrates; it is only at the end of the Primary Era that the first fishes, having vertebrate bony systems, are found. There were neither birds nor mammals. It is however remarkable that the first animals whose remains can be found, the trilobites, show an organisation sufficiently high to indicate that they were products of an already advanced evolution. It is a far cry from elementary cells to trilobites.

The great activity of the primary vegetation has had a decisive influence on the history of the globe. The absorption of carbon dioxide by the abundant vegetation restored free oxygen to the terrestrial atmosphere and produced little by little the quantity actually present. Furthermore, the minerals first formed, combinations of silica with lime, alumina, magnesia, iron, and soda, had gradually been attacked by the carbon dioxide of the primitive atmosphere, largely by the agency of water which contained the gas in solution on account of its prevalence in the air. Lime, magnesia, soda, and iron were thus converted into soluble carbonates and accordingly carried down to the seas by the water-streams and accumulated there. The first living beings assimilated these substances, as their remains, deposited in sedimentary layers, testify. In fact the formation of sedimentary limestone and dolomite required 34,000 times more carbon dioxide than is actually present in the air. Thus large quantities of this gas must have been removed from the atmosphere in addition to what was decomposed by vegetable life.

It can legitimately be stated that almost the whole of the actual free oxygen of the air is due to the vegetation, especially that of the Primary Era.

An era of calm and stability then succeeded. This is called the Secondary Era and geologists subdivide it into the Triassic, Jurassic, and Cretaceous periods. It commenced as the atmosphere, by the gradual diminution of carbon dioxide and the increase of oxygen, became more and more suit-able to the development and evolution of living creatures. Other characteristics were the gradual decrease of temperature, which was still high, and the greater stability of the Earth's crust, strengthened and thickened by successive solidifications.

The fossil remains of life which are met with in these strata, which are always superimposed on Archaean or Primary formations, clearly differentiate this era from the preceding one. As regards the vegetation, the prevalence of cryptogams ceased in the Secondary Era while gymnosperms were everywhere predominant. The first living beings whose traces have been found, the trilobites of the Primary Era, had completely disappeared. In their stead were belemnites, the forerunners of our present cuttlefish, and ammonites which were cephalopods with spiral shells; these were characteristic of the era. In the seas, crinoids, sponges, and corals were abundant and foraminifera and radiolaria developed. Their hard parts accumulated in thick layers and by this sedimentation process covered the bottoms of the seas of that period. Thus the work of the ocean began, a work which still continues without cessation in our present seas; geology is mainly the oceanography of the past, and the study of oceanography forecasts the geology of the future.

But the characteristic feature of the Secondary fauna was the evolution of vertebrate animals; the armoured fish of the Primary period had disappeared, having little by little given place to fish with well-ossified vertebrae. In particular, gigantic reptiles came into being, the colossal size of whose skeletons fills us with astonishment. The ichthyosaurus, the plesiosaurus, and the mosasaurus were the monsters that peopled the seas, while the continents were inhabited by immense creatures, dinosaurs, some of which attained a length of twenty-five metres [82 ft.] and a considerable height.1 When one sees, in the palaeontological galleries of museums, the skeletons of iguanodon, brontosaurus, stegosaurus, and diplodocus, one cannot help being struck by the manifestation of strength that these gigantic animals represent. When standing erect, some of them would have overtopped the roof of an ordinary five-storey house. Finally, the first birds made their appearance in Secondary times, and when the gigantic cold-blooded animals, which have just been mentioned, were predominant, in consequence of their size and strength, much smaller animals, the first warm-blooded mammals, had their origin. The geography of these times was characterised by two continents in the northern hemisphere separated by an ocean in the midst of which was emergent land corresponding to the actual situation of northern Europe. In the southern hemisphere a vast continent stretched over the position of South America and Africa; the South Atlantic Ocean did not exist and land marked the future place of Australia. Volcanic eruptions were less frequent, the Secondary Era being, as above remarked, a period of relative tranquillity.

This state of calm came however to an end and gave place to violent convulsions; the volcanoes manifested great activity, increasing to a state of paroxysm, and at the same time arose the great mountain chains which are actually present on the globe. During this time animal life was gradually perfecting its forms and was producing creatures more and more resembling present ones; and mammals, some of which attained gigantic dimensions, were masters of the land surfaces. As examples, we have the palaeotherium, the hipparion, the colossal dinotherium, the mastodon (the first elephant), the hippopotamus, the rhinoceros, the great deer, ruminant and carnivorous animals. This formed the Tertiary Era, subdivided by geologists into the Eocene, Oligocene, Miocene, and Pliocene periods. Palms abounded at first, but towards the end of the era trees appeared which resembled those of our present forests, while tropical flowers narrowed their habitat to the neighbourhood of the equator.

The emergent lands approached more and more to the present continental contours. The great tertiary chains of the alpine type which now surround the Mediterranean basin arose as a result of mountain-forming movements of great intensity. After the retreat of the sea which invaded certain parts of France, the shocks recurred and volcanic eruptions became formidable; the Central Plateau became covered with craters which emitted the lavas now visible in Auvergne, and the actual features of the land surface gradually became established.

During this time, the atmosphere continued to lose carbon dioxide and water vapour, and the cooling of the Earth by radiation became greater, so that the temperature fell little by little, still remaining, however, higher than the mean temperatures now observed in the same regions. The nature of the remains of vegetation shows that the mean temperature of France was more than 25° C. [77° F.], that is to say the climate of that country was similar to that which characterises the equatorial regions at the present time. It was only at the end of the Tertiary Era that the temperature was lowered and glaciers appeared on the highest mountains and commenced their extension towards the lower regions. When this occurred, the fauna and flora of the warm climate gradually receded towards the tropics, abandoning the northern lands where the initial climatic conditions of the Tertiary Era had allowed them to flourish, but whose more rigorous later climate was too cold.

The Earth slowly attained its present aspect. The vegetation had developed into the forms with which we are now familiar; the animals had evolved and had reached a kind of perfection. The environment was thus ready for the existence and development of the creature which came to dominate nature, that is to say Man, and the Quaternary Era began.

The Quaternary strata have a very different character from the preceding ones; exterior agencies predominated in forming them. They cover all the others and are themselves covered only by the soil-cap. They are alluvial deposits, the consequence of enormous precipitations of rain, due to the condensation of water vapour on a large scale, following the great lowering of temperature caused by the almost complete absorption of carbon dioxide. These abundant precipitations, which probably constitute the origin of the story of the Deluge which exists among all peoples, led to great rivers. Snowfalls, prevented from melting by the fall of temperature, caused an enormous extension of the glaciers, which at that time covered the whole of Central Europe and all North America. The ground is covered with erratic blocks, indisputable evidence of the existence of glaciers in the first part of the Quaternary Era, to which geologists have given the name of Pleistocene epoch. This glaciation consequently led to the migrations of animals, because of the great climatic variations which resulted from it.

These rivers deposited the Quaternary strata, in which may be found precious stones, gold, and platinum. Above the Pleistocene deposits are the different recent strata composed of clay, fine sand, and silt which are utilised for cultivation.

At the end of the Quaternary Era, the volcanoes of Auvergne again became active, and distinct evidence of this relatively recent renewed activity may be seen at the present time on the Puys chain. Later on, the glaciers retreated, and the present climatic conditions established themselves by degrees.

The great mammals, the mammoth, rhinoceros, cave bear, and great elk have since disappeared; so also has the megatherium of South America. Diminutive specimens of the wingless birds from which ostriches and cassowaries are derived still exist in New Zealand and are called kiwis, but they are rare.

It is, also, in this latter country that the least civilised natives are found, natives who approach the nearest to a purely natural condition and who are able to afford us some idea of primitive man.

Man made his appearance on the Earth in the Quaternary Era. His presence is proved by the remains of human bones, which have as yet only been found in Quaternary strata, never in any of the preceding formations, and also by the remains of objects unmistakably his handiwork. In the earliest stage, during the prehistoric period, are found only implements formed of hard stone; flints rudely hewn by chipping. This is known as the Palaeolithic period, that of chipped stone, which gave place to that of polished stone, the Neolithic period. Subsequently to the latter, metals began to be worked, first of all bronze, in the bronze age, and secondly iron, in the iron age.

The history of mankind commenced from this time.

Such are the stages passed through by the Earth in the course of its early years, during its infancy and youth, before arriving at the state of maturity to which it has now attained. One very import-ant question suggests itself to the mind : How many years has that slow evolution taken? Or in other words, what actually is the age of the Earth? It is very difficult to give an answer, at any rate a precise one, but in default of an exact knowledge of the length of time the Earth has been in existence, we may get some idea of the magnitude of the period which has elapsed since the crust solidified and enclosed the heated nucleus, the result of its stellar origin.

This evaluation may be approached in different ways. We may for example ask ourselves what time must have elapsed in order for the oceans to have acquired their actual salinity, by the accumulation of material that the streams brought down in solution from the solid crust over which they flowed. Joly has attempted this estimation. He calculated how much salt the rivers annually carry to the ocean and by comparison of this quantity with the amount that sea-water actually contains he arrived at the conclusion that at least a hundred million years must have been necessary for the present salinity to have been acquired in this way. It will not serve a useful purpose to describe how this estimation was attained. At the beginning of the aqueous condensation, the water flowed at a high temperature over the lands then formed and so it dissolved much more of saline substances than can the cold water of the rivers which at the present time flow into the sea. For this reason, however ingenious the above estimate may be, it furnishes us with very uncertain data as to the Earth's age.

The phenomenon of sedimentation enables us to arrive at a much more probable evaluation, which Sir Archibald Geikie has made. If the total thickness of the sedimentary deposits forming the stratification of our globe be estimated at about 30,000 metres [19 miles], and if it be assumed, as the work of geologists has shown that between three thousand and twenty thousand years are required for a layer one metre [39.37 in.] thick to be laid down, it follows that, in round numbers, the time necessary for the depositing of all the known strata is between a hundred and a thousand millions of years. This, moreover, takes no account of Pre-Cambrian formations which have existed perhaps as long again.

The discovery of the phenomena of radioactivity made by the French physicist Henri Becquerel, and the important researches, which these discoveries have led to, have given modern geophysicists another basis of estimation. It is known that the emanation of radioactive substances, such as radium, thorium, or even uranium, becomes transformed into helium. The English physicist Rutherford, to whom we owe the discovery of the emanation, has determined by experiment how much a given weight of uranium or thorium loses as helium in the course of a year. Also, Sir William Ramsay has studied the minerals from which uranium and thorium can be extracted and has determined the proportion of helium therein contained. From his results, Rutherford states that at least four hundred million years must have been required for these minerals to be formed in their present state. It will be seen that this result is in harmony with the result deduced from sedimentation, being at any rate of the same order of magnitude.

The phenomena of radioactivity which have also been brought into requisition to explain the constancy of the emission of heat from the Sun, have enabled us, in recent years, to estimate the age of the Earth with more and more precision. English physicists, in particular, have done not-able work in this direction. Starting from the quantity of helium contained in minerals the following duration periods have been assigned : three million years to the Greensand, six million to the basaltic rocks of Auvergne, fifty-four million to certain Norwegian rocks, two hundred and eighty-six million to some of the rocks of Ceylon, three hundred and twenty million to the blue earth of Kimberley, and six hundred million to the Archaean formation of Ontario. Thus figures similar to those of Joly, Geikie, and Rutherford are reached. A study of the Swedish rock-masses leads to still greater figures, the age indicated for them being a thousand or thirteen hundred million years. Some American formations give results of thirteen and fourteen hundred million, and, in conclusion, specimens of rock from the neighbourhood of Colombo in Ceylon have had assigned to them an age of more than sixteen hundred million years.

Thus, the maximum result of estimations based on the duration of sedimentation, viz., one thou-sand million years, is surpassed. We shall find that the figures in question are confirmed by quite different considerations, of an essentially geographical character. For geographers have also contributed knowledge of the Earth's age. They have studied the folds of which we have spoken previously and which constitute our mountain chains. These foldings were caused by the fact that, owing to the cooling and contraction of the nucleus on which it originally rested, the crust was no longer sustained from below and consequently contracted, becoming shrivelled as in the case of the skin of a fruit when it dries and becomes smaller. If the surface area of the mountain chains be measured in square kilometres, not in projection, as upon maps, but in reality on their sides, it is found that this total area is about one hundred and fiftieth of the entire surface of the globe. The corresponding decrease in the length of the Earth's radius can be deduced; it is a little less than 1/100 part of its value, and this contraction would correspond to a lowering of temperature of more than 300° C. [572° F.]. To produce this fall nearly two thousand millions of years must have elapsed.

As a result of all that has been said, we may consider it probable that the actual age of the Earth lies between one thousand and two thou-sand million years. It is both interesting and very remarkable that estimations based on such different methods give results that are sensibly concordant.

Finally, before concluding this account of the Earth's history, we have to ask ourselves whether the terrestrial crust on solidification was of a uniform thickness surrounding the fluid nucleus, or whether the foldings produced during the first movements of the shell had an influence on the thickness of the solid stratum.

Evidently the crust did not solidify as a whole at one time. It would pass through stages similar to those that can be observed in baths of molten metal. When solidification begins, solid crusts or scoriae form in places and float on the surface of the rest of the liquid mass. Certain astronomers have put forward the rather daring theory that the spots on the Sun are simply the first scoriae so formed, indicating the beginning of a partial solidification. On this theory, matter thrown to a distance by the solar eruptions consequently cools and falls back on to the liquid surface, on which it floats as icebergs float on water.

It is very probable that this occurred in the case of the Earth during the formation of its crust; solid pieces or plates, separate from each other and floating in the main fluid mass, were first formed. Lippmann has suggested the following ingenious hypothesis. Since, as he says, the crust resembles a kind of irregular mosaic formed of floating fragments in juxtaposition to each other, each portion must be sustained from below by a sufficient upward thrust exerted by the fluid mass in which it floats. If, therefore, a given piece carries a considerable mountain mass the weight of the load is much greater than in the case of a piece above which there is a sea, particularly as the density of the solid mass is so much higher.

We have, if not a precise value, at any rate some idea of the order of magnitude of the age of the Earth. Geologists estimate the duration of the respective eras as follows: 75 per cent. for the Primary Era, 19 per cent. for the Secondary Era, and 6 per cent. for the Tertiary Era.