• IIP ID iillllfc 1 1 II I ; n ! ' '! P!I ! !' ! i I HH I jji |i l|!'.;> i lili It: ' II! i i iiih'H'Wi'H' ''p l>?Mpp i il.'.lli! • I It! Hit 1 ! i i 1 I III llii 1 i ill i ii-ii l-'- P*i:il:: ||>! ijjHjIj'ij;!:; iiijj if I&i Ml i mm 1 1{ i| ji] Hi i;i lill ! h i'l '• !i ! f -t il. r: .'it: i M i:!i:|i: ' i n !«»>!!( II a/ ru nj n- O O m a THE EVOLUTION OF THE EARTH AND ITS INHABITANTS -o u o c ciS rt p >- •M o 1 The Evolution of the Earth AND ITS INHABITANTS Series of JTectures 'Delivered before the Yale (Chapter of the Sigma Xi during the ^Academic Tear 1916-1917 Joseph Qharles Schuchert jTorande J^oss Woodruff T(ichard Swann jTull Ellsworth Huntington NEW HAVEN YALE UNIVERSITY PRESS J^ondon : Humphrey <3tfilford : Oxford University 'Press MDCCCCXIX i COPYRIGHT, I9l8, BY YALE UNIVERSITY PRESS First published, July, 1918. Second printing, March, 191 g. PREFACE DURING the collegiate year 1916-1917, the conduct of the Yale Chapter of the honorary scientific society of the Sigma Xi was entrusted to my guidance, and in order to render the meet- ings of the chapter as profitable as possible, a symposium was proposed on the geological and biological evidences for the evolution of our planet and the earth-borne life. I therefore asked such of my colleagues as were authorities on the several subjects 'involved to prepare addresses to be delivered before the chapter and later to be published in the form of the pres- ent book. The course of lectures was as follows : THE EVOLUTION OF THE EARTH AND ITS INHABITANTS Lecture I. The Origin of the Earth, November 23, 1916, Professor Joseph Barrell. Lecture II. The Earth's Changing Surface and Climate, December 13, 1916, Professor Charles Schuchert. Lecture III. The Origin of Life, January 24, 1917, Pro- fessor Lorande Loss Woodruf. Lecture IV. The Pulse of Life, February 15, 1917, Professor Richard Swann Lull. Lecture V. Climate and Civilization, April 20, 1917, Doctor Ellsworth Huntington. The scope of the combined essays is of necessity very broad, ranging as it does from a conception of the universe to the trend of modern civilization. Thus the first chapter deals not alone with the genesis of the earth but of the parent solar system, and, the earth having been established, its history is traced until the time of its becoming a fit environment for the abode of life. The second lecture deals with the changing lines vi PREFACE of demarcation between land and sea, the rise and growth of continents, the formation and severance of land-bridges, and the climatic changes which are recorded for geologic time. The physical environment once established, Professor Wood- ruff tells what we know and do not know of the origin of life. This is largely an academic discussion of the several theories which have been advanced to account for the evolution of life- less into living matter, for from the nature of the problem evidences of direct observation are not available. The lecture on the pulse of life attempts to link up cause and effect; to find those forces which are responsible for the more or less rhythmic accelerations of evolution shown by the fossil record. The main cause is found to be climatic change, which in turn has as a chief controlling factor earth shrinkage and the con- sequent warping of the crust discussed in the second lecture. The pulse of life applies not alone to the evolution of animals and plants, but also to mankind. How climatic changes have influenced the growth of civilization and the formation of racial characteristics of mentality is set forth in the last lecture, that by Doctor Huntington. In so far as possible, these essays are the fruits of the original research of their several authors, which in certain instances are set forth here for the first time. The treatment of the entire subject and the marshaling of the facts thus assembled are entirely new. I am deeply grateful to my colleagues, not only for their having accepted the tasks thus laid upon them, which in several instances implied new and extensive research, but also for the success with which the lectures were presented, as attested by the society. RICHARD SWANN LULL, President, Yale Chapter, Sigma XI. 1916-1917. Yale University, December I, / 200 to 400 0 to 34 42 [Jupiter. . 86,500 1.33 483 1 19 Saturn 73,000 0.72 886 2 30 Uranus 31,900 1.22 1,782 0 46 Neptune 34,800 1.11 2,792 1 47 1 Inclination of sun's equator to earth's orbit. 2 Mean distance of moon from earth. Notable Planetary Relations. That the orderly nature of this system implies some mode of evolution was seen by the framers of the nebular hypothesis. The more notable of these relations, following the summary by Young, are : 1. The orbits are all nearly circular. 2. They are nearly in one plane (excepting the cases of some of the little planetoids). 3. The revolution of all is in the same direction. 4. There is a curiously regular progression of distances from the sun (expressed by Bode's law, which, however, breaks down at Neptune). See foregoing table. 6 EVOLUTION OF THE EARTH 5. There is a rough progression of density, increasing both ways from Saturn, the least dense of all the planets in the system. As regards the planets themselves, we have : 6. The plane of the planets' rotation, the plane of their equators, roughly coinciding with those of the orbits (probably excepting Uranus). 7. The direction of the rotation of the planets about their polar axes the same as that of their revolution in their orbits (excepting probably Uranus and Neptune). 8. The plane of orbital revolution of the satellites of each planet coinciding nearly with that of the planet's rotation, its equatorial plane. 9. The direction of the satellites' revolution in their orbits also coinciding with that of the planet's rotation about its axis, with exceptions in the case of the ninth satellite of Saturn and probably the seventh of Jupiter. 10. The largest planets rotating most swiftly. The sun, a member of the stellar system. The sun, as has been stated, is but a star and a member of the stellar system. What are the orders of magnitude in number, in size, in dis- tance, in speed, in duration among these countless orbs, and how do these relations enter into the problem of the origin of the earth as one of that retinue of planets which attend upon the sun? The luminous stars of our system are estimated to be more than a hundred million in number. The number of the dark stars is unknown. Giving no ray of light to reveal their exist- ence, they may for all we know be as numerous or more numerous than those in the radiant stages of their existence. The few stars whose sizes are known range in diameter from somewhat below a million to upward of ten million miles and more. Many of the stars are in reality double or multiple stars, consisting of companions so close that the two or more appear as one star to the naked eye, or even under the highest power AND ITS INHABITANTS 7 of the telescope, the evidence of their composite nature being revealed only through the analysis of their light by the spectro- scope. These double stars revolve swiftly about each other, but such internal motions must be sharply distinguished in thought from the streaming or drifting of the stars as parts of the great stellar system. Relatively to the sun they are found to move through space with speeds averaging between 10 and 30 miles per second, but ranging from less than 10 to more than 200 miles per second. They do not move, however, singly and in closed orbits, but rather in broadly scattered groups whose paths are almost straight lines. These courses of the stars must slowly curve under the aggregate attraction of the millions of stars, but can never return into themselves. The paths of groups of stars intersect other groups and are to some extent interwoven among themselves. These groups have been found to be integrated into two greater groups inter- meshed among each other and forming two great star streams whose average motions are in opposite directions. With the passage of millions of years, the stars thus continually enter into new relations and build new configurations in the skies: a myriad host of stellar fireflies, the living and the dead, streaming through space hundreds of millions of miles per year. Although the stars are so great in number, their distances from each other average tens of millions of millions of miles, those in our part of the stellar system averaging between sixty and eighty trillions. The star nearest to the sun, a Centauri, happens, however, to be at a lesser distance of about twenty- six trillions of miles. To bring down the dimensions of the universe to finite comprehension, we must divide the scale of nature by a thousand million. Then the earth would be represented by a pebble half an inch in diameter, circling once a year about a sun 4.5 feet in diameter, at a distance of 500 feet. The nearest 8 EVOLUTION OF THE EARTH star, a Centauri, would on this same scale be seen as two spheres revolving about each other at a distance apart equal to 2 miles, and each comparable in size to the sun. This double star would be situated at a distance of about 25,000 miles from our planetary system with its sun, but the other stars in this part of the stellar system would be separated from each other on the average by more than twice this dis- tance. The Galaxy, or Milky Way, is the cloud-like zone of faint stars which extends as a belt around the sky. The stars in it appear faint and close together because of their remote- ness. They seem to constitute the outer zone of our stellar system, and its dimensions are only vaguely known. On this diminutive scale the Milky Way might be found to be encom- passed by a circle of a hundred million miles diameter, or it might be more or less. The nebula. All hypotheses of earth origin derive the planets and the sun from an antecedent nebulous or meteoritic state. The cloudy patches of light known as nebulas, which are revealed especially by stellar photography, are, however, of several very different natures and it is a vital question as to which, if any, of these types, could have given birth to our planetary system. First are the irregular nebulae, diffuse clouds of luminous matter, pervading whole groups of stars as in Orion and the Pleiades, shown in Plate I, A, denser about certain stars, but nevertheless enormously attenuated. This kind of nebulosity is associated with certain regions of the Milky Way. From the characteristics of their spectra, the stars in such nebulae are regarded as young stars and the nebulous matter may represent the remains of an antecedent stage. The planetary nebulae are a distinct type, comparatively few in number, and also found associated with the Milky Way. They show in the telescope faint, greenish, circular discs from which they derive their name rather than from any known u -o V o u: bx O rt £ & CM C/2 .2 'S < O _o ^ u Z _i Ok AND ITS INHABITANTS 9 relationships to planetary systems. They seem to be related in their origin to New Stars and these in turn are thought to be produced by stars sweeping through clouds of meteoric or gaseous matter and attaining temporarily, from the swift impacts, an enormous brilliancy. The impact is so super- ficial, however, that the extreme brilliance is usually lost in a few days or weeks, and the star subsides through a stage like a planetary nebula into a peculiar type of star known as the Wolf-Rayet stars. The origin of the true plane- tary nebulae has not, however, been observed, as they appear to possess a longer life than those which have originated in the past few centuries from new stars. The ring nebulae are few and special, having the form of a vortex ring. The stellar nebulae form another small group which look in the telescope like hazy stars. By far the greatest number of the nebulae are classified as spiral nebulae, more than 120,000 of which have been made known by photography in connection with the greater tele- scopes. Their actual number must of course be far greater. These objects, unlike the other forms of nebulae, avoid the Milky Way, and are scattered over regions where the stars are more widely spaced. They are very remote and may be entirely beyond the stellar system. This implies enormous magnitude. It seems probable that in general they possess high internal velocities, which implies in turn enormous masses to generate such velocities. These nebulae possess spectra similar to those of stars rather than, like the other types of nebulas, spectra of diffuse clouds of gas. Some astronomers look upon them, therefore, as possibly systems of stars rather than true nebulae ; systems so remote as to give the appearance of faint cloud-like spirals, even when viewed under the highest powers of the telescope. A typical spiral nebula is shown in Plate I, B. io EVOLUTION OF THE EARTH HYPOTHESIS OF PLANETS DERIVED FROM A PRIMAL NEBULA The original hypothesis of Kant and Laplace. In 1754 and 1755 Immanuel Kant, the philosopher, published the most remarkable papers which had appeared up to that time upon the evolution of the solar system. He conceived matter to have been originally diffused and cold. From a position of rest it began to converge under the influence of gravitation and gave rise to the sun. In some manner he held that the matter in converging acquired a movement of rotation. Certain nuclei grew up independently from the center and gave rise to the planets and satellites. In 1785 he developed the idea that the contraction of the sun's mass would develop its heat, a view elaborated by Helmholtz in 1854 and generally held by astronomers at the present time. Thus Kant sought, and with a large measure of success, to evolve the present state of the universe from the simplest condition by means of mechanical laws alone. In 1796 Laplace, one of the most eminent of French astronomers, published a general work on astronomy, and in a short note at the end of the appendix proposed a theory of the origin of the solar system which shortly became widely known as the nebular hypothesis. He was evidently unaware of Kant's work published forty-one years previously. Laplace is most noted for his mathematical work on celestial mechanics, yet he did not develop his hypothesis along such lines and apparently did not attach much importance to it. Neverthe- less, it became the dominant idea in cosmic evolution through- out the next century. Laplace postulated an original nebula as a very hot, gaseous mass extending beyond the orbit of the farthest planet and possessing a uniform rotation throughout, as if it were a solid body. Its size was the result of a balance between expansion from its heat and contraction from its gravitation. As it lost AND ITS INHABITANTS n heat it contracted and, with the same energy of rotation that it possessed before, necessarily revolved on its axis in a shorter time. At last a stage was reached where, in the equatorial belt, centrifugal force balanced gravitation and the matter subjected to this balance of forces could sink in no further. It is thought to have existed as a ring, left behind by the condensing mass. The ring, however, was unstable; it broke up and gathered into one body. During the further shrinking of the main mass other rings were in turn abandoned. Each gathered into a subordinate nebula, passed through an inde- pendent evolution, and the whole gave rise to the system of planets and their satellites. Modifications of the nebular hypothesis. During the first half of the nineteenth century the nebular hypothesis was accepted by astronomers almost without question, but during the second half many serious dynamical objections were de- veloped and a process of modification began, until now not much remains of the original conception of Laplace. A rather full statement of the hypothesis and the objections to it has been given recently by Campbell.2 A briefer summary and a citation of but a few of the modifications in the general concept must here suffice. George Darwin, Lockyer, Faye, Fouche, Poincare, and others have taken part in this work, and in the opinion of these mathematicians and astronomers the framework of the result- ing structure is still sound, though subject of course to further modification as knowledge increases. It was shown that the original nebula need not have been hot, but, as perceived by Kant, would develop heat from its self-condensation. A loose swarm of cold meteorites would suffice as well as an original gas for the initial state. The mass could never have revolved as a unit body, as if it were a solid. On the contrary, the inner 2 Campbell, W. W., "The Evolution of the Stars and the Formation of the Earth." Scientific Monthly, vol. 1, 1915, pp. 189-194. 12 EVOLUTION OF THE EARTH parts would be condensed and revolve fast while the outer parts were still diffuse and revolved slowly. The mode sug- gested by Laplace for the separation of the rings is also dynamically very unsatisfactory. Moulton has shown that the growth of the planets and the development of rotation in the same direction as their orbital motion could be much better attained from an initial state in which the component particles revolved in the same plane but independently and in highly elliptic orbits about the central nucleus. This is a wide de- parture from the idea of a circular ring revolving as a unit body. Still more fundamental objections, emphasized by Cham- berlin and Moulton, are found in certain of the existing dynamical relations of the solar system. It would be expected that in condensation the central mass would continually aban- don matter from its equatorial zone, the inner planets would presumably have possessed the greater masses, and the final sun would now show a high speed of rotation, giving an equa- torial diameter far greater than the polar. Such expectations are contrary to the facts. The sun revolves so slowly on its axis, once in twenty-five days, that it has no measurable equa- torial bulge. In other words, centrifugal force is negligible in the sun. Furthermore, the equatorial plane of the sun, instead of lying precisely in the mean plane of the planets' orbits, is inclined seven degrees to such a mean plane. A hypothesis to gain scientific credence must emerge success- ful from the test of observed facts and mathematical theory. The nebular hypothesis has not done so. It is on the defensive and has lost standing during the past generation. Neverthe- less, it would be premature to abandon it entirely. It has the advantage of simplicity in that satellites, planets, and sun are explained as the products of a single process, convergence in a rotating nebula. But nature is often found to be complex in her operations, so that this advantage is of doubtful weight. AND ITS INHABITANTS 13 HYPOTHESIS OF PLANETS DERIVED FROM A SMALL SECONDARY NEBULA Distinctive features of the planetesimal hypothesis. The planetesimal or spiral nebula hypothesis of Chamberlin and Moulton postulates the sun already in existence from the in- gathering of a primal nebula. It was at some later stage dis- rupted through the tidal forces produced by the close approach and passage of another star. The result was a secondary nebula, but one essentially unlike the primary. The secondary nebula was developed in a plane and initially possessed a spiral form with the sun at its center. All of its parts moved with freedom and independence in elliptic orbits, a point of difference from the Laplacian hypothesis. The nebula con- tained only a minute fraction of the solar matter, but was endowed by the passing star with a great rotational energy, so that, although so insignificant in mass, the planetary matter dominates enormously over the sun in the moment of momen- tum of the whole system. Thus the planetesimal hypothesis is a bold and frank abandonment of the terms of the original or nebular theory. It is too early as yet to predict what will be the ultimate fate of this hypothesis of a secondary, and, in a measure, an accidental origin of the planets, but, as expounded by its originators, it must be regarded as dynamically more satisfactory than the present form of the hypothesis of primary origin. The essential features will, therefore, be presented as the more probable preliminary steps in the genesis of the earth. Forces of tidal disruption. The power of stars to disrupt each other without coming into actual contact, merely through relatively close approach, must be understood, as it is the basis of the planetesimal hypothesis. The sun and moon raise terrestrial tides by virtue of the pull of gravity and thus modify that spheroidal form of the earth which is given by its own gravity and the centrifugal i4 EVOLUTION OF THE EARTH force of its revolution. Suppose, in other examples of heavenly bodies, the tidal pull to be many times stronger, the self-gravitative cohesion to be many times weaker. A limit will be reached at which the body may be pulled to pieces. This phenomenon, which has been actually observed in the case of comets passing close to the sun, has been called tidal dis- ruption. FIG. 1. — Diagram to illustrate tidal forces. Let M and N in Figure i be two bodies passing each other in space, and consider the action of the larger on the smaller. According to Newton's law, the bodies attract each other directly as their masses and inversely as the square of their distances, causing them to swing toward each other while passing by, but soon losing influence as they separate in their journeys through space. Consider three particles, a, b, c, on the line of attraction, taking them as separate parts of the smaller body. But a is nearer to M than is b, and b is nearer than c. Therefore if we represent the relative attractions by lines, these lines will correspond to the distances which the particles would move in a given time if free to obey the attrac- AND ITS INHABITANTS 15 tion of the other body. The line at a is longer than that at b and the latter is longer than that at c. If N was not bound together by its own gravity or rigidity, a, b, and c would therefore drift apart and fan out while passing M. Consider that rigidity is negligible, as in a fluid globe; then, if a minus b, or b minus c are quantities which become greater than the self-gravitative force of N holding together a and c, the unity of the body becomes destroyed. The problem, however, is not quite so simple, since the influence of all other points in N must also be considered. As the nearer part of the body is pulled from the center, and as the center is pulled from the farther side, there will, further, be two simultaneous tides of approximately equal height, but on opposite sides of the distorted body. They tend to be always on the line joining the two bodies. Thus, on the earth there are two tides on opposite sides, but the revolu- tion of the earth on its axis, like a car wheel under two oppo- site brake-shoes, gives an apparent effect, to one on the surface of the earth, of a revolution of the tidal wave. As a result of the equal tides at opposite ends of a diameter there are, on any part of the ocean, two high tides in twenty-four hours. Mode of tidal disruption in stars. A star is characterized by its enormous size and mass and by the possession of a gaseous constitution. The diameter and density are dependent upon the balance at every point between the tremendous ex- pansive forces of internal heat and the equally great compres- sive forces due to its own gravity. If it contracts, then its surface and each component shell below comes nearer to the center, the effect of gravity upon any shell accordingly increases inversely as the square of the new radius, and a higher internal temperature becomes necessary to balance the higher gravita- tive force. From this there results the paradox known as Lane's law, — that so long as a body maintains a gaseous 1 6 EVOLUTION OF THE EARTH constitution its temperature must rise as it contracts, even though at the same time it is radiating heat. The temperature of the interior must furthermore be higher than the tempera- ture of the surface, because of the greater compression with depth, as is illustrated in the different strata of the terrestrial atmosphere. In those convective or slow boiling movements which are necessary in the sun and other stars in order that they should be able to maintain their surface radiation, there is then a constant liberation of energy from the depths and a system of balanced motion which if disturbed could lead in any star to an explosive blowing out of material from it on an enormous scale. The tide-generating force varies directly with the mass of the disturbing body and also with the radius of the body dis- turbed. It varies approximately inversely with the cube of the distance between the centers. The deforming force is, furthermore, greatest in the interior because the tidal forces acting on the zone at right angles to the line of attraction have a component which tends to squeeze in the points d, d' of Figure i toward the center. The gravitative control is accordingly weakened along the line a, b, c, and is strengthened in the directions at right angles. Now apply this principle to the gaseous balanced nature of a star, and it is seen that the expansion in the line a, b, c is no longer exactly balanced by the gravitative compression, and the unbalancing is greatest in the center, where also is the region of highest compression and highest temperature. The effect is as if one squeezed a syringe bulb with orifices for exit at both ends, a bulb, however, like an air rifle, filled with gas compressed to an explosive degree. Tidal disruption of the ancestral sun. The sun is occa- sionally observed to shoot out streams of gas, known as solar prominences, to heights of nearly 300,000 miles, and at velocities ranging up to 300 miles per second. Such phe- AND ITS INHABITANTS 17 nomena Indicate the enormous elastic and explosive energy resident in the sun's interior, an expansive potency held in restraint by the equally prodigious power of the sun's gravity. Supposing then that the ancestral sun was subjected to tidal disruption by the approach of another and possibly much more massive star, it remains to be seen how the nebula resulting from tidal disruption can become the embryo of an orderly planetary system. If the matter were shot out from great depths in the sun by its normal expansive forces plus the tidal forces, the velocity of departure might rise high above the observed velocities of 300 miles per second. If 400 miles or more, it would be above the "critical velocity" of the sun. The gravitative attraction of the latter could then never reclaim that matter, because the decrease in the outward velocity due to the solar attraction would never bring the velocity down to zero, and could therefore never reverse the motion of the escaping matter and bring it back to the sun. It is doubtful if the sun could have drawn back to itself material expelled with a velocity of even 300 miles per second, for the passing star, by lowering the gravitative power of the sun on the line passing through the two, would temporarily decrease on that line the critical velocity. In other words, it would help to drag matter away from the sun, even though that matter could not catch up to the passing star, but would be left wandering in interstellar space, forming possibly come- tary and meteoric material for other systems. But some, or possibly all, of the matter of the exploded sun may have had lesser velocities of escape and would consequently remain within its gravitative control. In so far as it was not deflected sideways by some extraneous force, it would fall back on the surface of the sun as the water of a geyser falls back into its pool. But the gravitative pull of the passing star would serve as such an extraneous force, analogous to the wind which blows part of the geyser water, as it rises and falls, to one 1 8 EVOLUTION OF THE EARTH side of the basin. The matter shot out toward the passing star would be attracted sideways after it as the star receded into space. On falling back toward the sun it would consequently pass to one side and elliptical orbits of the separate particles would become established. The material shot out in the reverse direction, from the opposite side of the sun, would meet much the same conditions except that the sidewise pull of the passing star would be less on it than on the sun. It is seen that the lateral or deflecting force acting on both arms of the nebula would be due to the difference between the pull of the passing star on each arm and on the central body. The initial spiral arms do not then represent the path along which the material was shot out, but mark the rotation around the central body or sun, both of the axis of expulsion during the passing of the star and of the matter after it is expelled, as shown in Figure 2. FIG. 2. — Origin of a spiral nebula according to Chamberlin and Moulton. The spiral nebula would be developed in a plane. That plane is established by the hyperbolic orbit of the passing star with the sun at the focus of the orbit. The new system would thus show in its nature features imposed by both its parents. From such a nebulous fiery birth Moulton especially has shown how, in accordance with the laws of celestial mechanics, a planetary system could result. The matter which has converged into the planets would be that residue of the solar tidal disruption which did not pass beyond gravitative control and did not fall back into the body AND ITS INHABITANTS 19 of the sun. This residue is only a very small fraction of the sun's mass. It would appear probable, however, that the solar disruption was very great in order to give an axial revolution to the reaggregated matter forming the present sun, so that its equator should be, as observed, only seven degrees from the mean plane of all of the planetary orbits. The present revolution of the sun is probably due then to the whirl pro- duced during tidal disruption and not to an axial rotation belonging to the sun before the event took place. This brings us to the final stage in the evolution of the planets according to the planetesimal hypothesis. In the arms of the spiral nebula were knots or nuclei of matter constituting the cores of the planets. Four small knots, the earth-moon knot being a double one, represented the beginnings of the four small inner planets (see table, page 5). In the zone of the planetoids there was, however, no dominating nucleus, and they have therefore remained to this day largely in the plan- etesimal state. Four greater nuclei beyond were the begin- nings of the major planets. Smaller nuclei associated with the larger marked the presence of satellites. The orbits of the planetary nuclei and of the scattered planetesimal swarm were highly eccentric, having the form of a tangle of ellipses of all forms and sizes but lying in nearly a common plane and with a common direction of revolution about the central body. Collisions would inevitably occur at the crossing of the paths in the course of numberless revolu- tions and the nuclei would have sufficient mass and conse- quent gravitative power to retain the matter colliding with them. In this way, each planet would in the course of time clear up an orbital zone, and these zones, because of the eccen- tricities of the orbits of the component particles, overlapped each other with the exception of a region between Mars and Jupiter. But Moulton has shown that in such planetary growth by accretion, an axial revolution would arise in the 20 EVOLUTION OF THE EARTH same direction as the orbital revolution, and that the incor- poration of all the planetesimals would cause the eccentrici- ties to cancel out, giving to the whole mass a nearly circular instead of a highly elliptical orbit. This would lead us to believe that the original nucleus was but a small part of the completed planet, so that its original ellipticity of orbit was submerged beneath the average influence of the added masses. Outstanding difficulties of the planetesimal hypothesis. The disruption or spiral nebula hypothesis explains the features of the solar system more successfully than the older nebular hypothesis has thus far been able to do, but there are difficul- ties still remaining, though these may perhaps be the result of special conditions. The most striking departure of the real system from that expectation deduced from the hypothesis is found in the rota- tion of the sun. The passing of a star able to drag matter from the sun to the distance of the planet Neptune would be expected to lead to an enormous tidal distortion of the sun's mass. This great tidal wave would involve a lifting and revolution about the sun, tending to give it a certain energy of rotation. A very little stronger action and the sun would in fact have been literally pulled to pieces and its matter scattered beyond its gravitative control. It is possible that it may in this way have lost a part of its mass. Considerable quantities of the expelled matter should have fallen back obliquely in the sun and tended further to increase its velocity of rotation. The path of the approaching star could have had no relation to the previous equatorial plane of the sun. The probabilities would consequently be that the final rotation would be a resultant between the older and newer forces and lie in an intermediate plane at a considerable angle to the plane of the planets' orbits. Now, as a matter of fact, the sun, as has been previously noted, revolves but once in twenty- five days and its equator is inclined but seven degrees to the AND ITS INHABITANTS 21 mean plane of the planets' orbits. To explain this, Chamber- lin supposes that the sun had originally a rotation in a plane not greatly different from that in which the passing star ap- proached, but rotated in the opposite direction. The whirl given to the solar mass by the tidal disruption is assumed to have been a little greater than its initial rotation, but, being in the opposite direction, the resultant was at a slow speed and yet nearly in the plane of the planetary orbits.3 Campbell points out that the chances are highly against such a special arrangement. If in a number of solar systems such an arrange- ment prevailed, it would constitute a conclusive proof against the hypothesis, but in the one example the exceptional com- bination may have occurred and it cannot be urged as a disproof.4 Turning to another aspect of the hypothesis, the innumer- able spiral nebulas of the heavens, although good illustrations of the initial hypothetical form of the solar planetary system, do not appear to be stages in a similar evolution in the way that Chamberlin and Moulton have conceived them to be. They are, as previously stated, of a much vaster order of magnitude, they avoid the region where the stars are clustered, are at remote stellar distances, and by their very number show a notable duration of their form. On the other hand, the postulated originally spiral form of the solar nebula would have been evanescent. Within a century from the time of origin all except the outer nuclei would have completed many revolutions about the sun. But the different periodic times of the nuclei would in a few revolutions have caused the initial spiral form to disappear. It would become wound up and further blended together owing to the high ellipticities of the constituent orbits. 3 Chamberlin, T. C., "The Origin of the Earth," 1916, pp. 130-132. 4 Campbell, W. W., "The Evolution of the Stars and the Formation of the Earth." Scientific Monthly, vol. 1, 1915, p. 241. 22 EVOLUTION OF THE EARTH The temporary dispersal of the solar mass would lead to an enormous increase in its radiant energy. The planetesimal matter would as a consequence of its dispersion and increased radiating surface cool with great rapidity, except in the nuclei of masses, and cease to be self-luminous, the smaller particles almost immediately becoming cold except as they were heated by the larger and profoundly disturbed solar mass. The new stars which have been observed are not regarded as of this nature since they are not expanded into a spiral. In fact, as previously mentioned, no examples are known which serve as illustrations, in the terms of this hypothesis, of the birth stage of planetary systems. This lack of examples may be connected with the small scale as well as the temporary character of such a nebula. The whole solar system, extend- ing to the orbit of Neptune, would subtend slightly less than half a minute of arc as seen from the nearest star. The average star is hundreds of times farther away and at the greater distances a nebula of this order would not betray its existence by its form but only by the temporary great increase in radiance at the time of its birth. Chances of close approach. Whether the chance is great or small of a planetary system being generated from any particular star by tidal disruption cannot be used as an argu- ment for or against this hypothesis unless it were known what proportion of the stars possessed planetary systems. But that knowledge is hopelessly concealed from us in the depths of space, since such a system as ours after its temporary initial brilliance would, as shown, be invisible in the most powerful telescopes even if it existed about the nearest star. It is not known how near an approach would be necessary to generate such a solar system; but the chances of close approach must be very small for any particular star. The motion at any instant of any individual star is the result of its motion inherited from the past plus the attractions of all the AND ITS INHABITANTS 23 matter in the universe pulling upon it in the present. If two bodies without previous motion were to be attracted toward each other, and were able to ignore the gravitative pull of all other bodies, they would move in a straight line toward each other's centers with ever increasing velocity until colli- sion would result. But the least inherited motion in any other direction, or the least deflecting pull upon one of them more than upon the other by other bodies would prevent to that degree a central collision, or in almost all cases any collision whatever. Now the velocities of the stars through space at great distances from each other are so great that individual stars can have almost no attractive influence upon each other. They must move in nearly straight lines past each other unless they happen to pass within a thousandth part of their average distance. It is seen then that the chances of close approach depend primarily upon the acci- dental crossing of their paths and only secondarily upon their mutually attracting each other. For this reason the chance of actual collisions may be regarded as negligible, even con- sidering the vast number of stars. Approach sufficiently near to generate strong tidal forces would, however, have occurred during their long lifetimes as radiant bodies to a considerable number out of the hundred million or more of stars which are known to exist in the stellar system, but for any one individual star, where the spacing is of the magni- tude existing in our part of the stellar system, the chance of such approach even in a billion years would be very small. In fact, it has been estimated as only one chance in 1,800 in that time. It is possible, however, that this happened once to our star, that is, to our sun of that time, in the distant past and from that disruptive tidal force was born our system of planets. If such an event was in fact a necessary antecedent condition, fortunate indeed has been our planetary fate, for not only did this happen so early in the sun's career 24 EVOLUTION OF THE EARTH that its radiant energy has been able to endure through all the ages needed for organic evolution, but the spacing of the stars is so wide and the chance of approach so rare that no other of them has since advanced sufficiently near to throw this system into disorder, or to disrupt and sweep away the earth and its sister planets as a wasted effort, and start the re-creation of a new heaven and a new earth. HYPOTHESIS OF EARTH-GROWTH BY SLOW ACCRETION OF PLANETESIMALS Under the terms of either nebular or planetesimal hypoth- esis a scattered state of the planetary material is implied as a stage antecedent to the origin of the planets. Was this growth of the planets geologically slow or rapid? Did it take tens or hundreds of millions of years, or was it on the contrary largely accomplished in tens or hundreds of thou- sands of years? Was the material largely in dust-like or molecular form, or was it to a large extent in nuclei of con- siderable size? From these different postulates very diver- gent consequences may be traced in the formative stages of the earth; and finally the present nature of the earth itself may speak in favor of one or the other of these views. Chamberlin, who has been the chief writer on this sub- ject, adopts the hypothesis that the stages of earth-growth were very prolonged, even geologically speaking, and that the accretion was dominantly of dust-like or molecular par- ticles. According to him the building up of the planets fol- lowed three stages: first, the direct condensation of the nuclear knots of the spirals into liquid or solid cores; second, the less direct collection of the outer, or orbital and satelli- tesimal matter; third, the still slower gathering up of the planetesimal material scattered over the zone between adjacent planets. This third factor, in Chamberlin's view, is AND ITS INHABITANTS 25 regarded as very important and he believes this diffused matter contributed much of the earth substance, very slowly and in a dust-like form. This is one of the critical points in the details of the theory upon which turns much of the development of his following argument. Chamberlin conceives the earth to have been built up as a solid body, not to have been fluid or viscous at any time later than the early nuclear stage and to have begun to hold an ocean by the time it contained 30 or 40 per cent of the present mass. Such liquid rock as was generated by compression or radioactivity during earth-growth is re- garded as having been kneaded and squeezed to the surface, where it solidified approximately as fast as it was formed. In earth-growth, the denser planetesimal dust, he argues, tended to be somewhat segregated into the primitive ocean basins and served to maintain in them, as the earth was built outward, a greater density than in the elevated zones between, establishing thus a relation between density and elevation. It seems a debatable question if such a large proportion of the added material was necessarily dust-like and capable of being weathered, sorted, and distributed by the primitive atmosphere and ocean. In fact, from this beginning of earth-growth the preponderance of the evidence appears to the writer to be against those sub-hypotheses which Cham- berlin has followed. This evidence, its bearings and con- clusions, will form the following parts of this article. It will be of ultimate value to both lines of argument that each may be weighed against the other. HYPOTHESIS OF EARTH-GROWTH BY RAPID INFALL OF PLANETOIDS Preliminary statement. Alternative views quite different from those which have been presented under the previous 26 EVOLUTION OF THE EARTH heading will now be discussed. It appears to the writer that the chemical character of the igneous rocks, the limited depth of density variations in the crust, the limited amount of salt in the sea, the rotation periods of the moon and planets, — all point to a molten condition of the earth at the completion of its growth. In the limited space available the more technical aspects of the arguments must, however, be omitted. The questions raised by this conclusion are : What mode of growth would have favored a molten state and how far did this precede the beginning of the geologic record, as given by the oldest rocks exposed at the surface of the globe? Up to this point the method of alternative hypothesis has been pursued, and from the standpoint of scientific pro- cedure it should be continued to the end. The limitations of space in a single essay, however, forbid. This subject, which for complete analysis should be developed in a volume, must be compressed into a few pages. The judicial style must often be abandoned for the declarative. Descriptions of some things which no eye has ever seen will be given graphically as though viewed by a witness. This change in method necessitated by limitations of space is, however, least objectionable in the closing parts of the subject, since the foundation hypotheses have been already presented and the argument leads from them toward the established facts of the geologic record. Significance of the planetoids. The belt of asteroids, better called planetoids, appears to have remained more nearly in its original state than have other parts of the solar system. The lack of aggregation into a planet may be due in part to the absence of any dominating center. More than eight hun- dred of these bodies have now been discovered and listed and countless others must be so small that they will largely remain unknown. The diameters range from a maximum of 485 AND ITS INHABITANTS 27 miles in increasing numbers down to 15 to 20 miles, the limit of telescopic visibility. At some diameter below the limit of visibility in the tele- scope, although the number may be increasingly great, the summation of their masses must begin to fall off, since other- wise the combined bulk would produce a perceptible glow in the sky. Furthermore, Leverrier demonstrated from the limited perturbations of Mars in its orbit that the whole amount of matter distributed between the orbits of Mars and Jupiter cannot exceed about one-fourth of the mass of the earth. It may be less. In fact, later calculations limit it to less than one-hundredth of the mass of the earth. The rate of increase in numbers in the smaller visible sizes suggests in connection with the limitation in aggregate mass that a con- siderable part, perhaps a larger part of the matter, is not in dust-like or molecular form but is in fragments of appreciable size ranging up to some miles in diameter. These masses, owing to their small diameters and weak gravitative force, would possess almost no power to grow by accretion. They must retain almost the original state of the nebula, or better, the meteoritic swarm, and are perhaps as likely to have suf- fered occasional shattering and scattering by impact as to have grown from a lower order of size. Their evidence favors the hypothesis that the scattered matter which was added to the nucleus to form the earth was largely of such size that the individual planetoids would have plowed through a primordial atmosphere and ocean, if such existed, and have penetrated beneath the surface of the liquid or solid body below. The energy of impact from dust-like material would be absorbed at the surface and, as heat, quickly radiated into space. The accretion of dust would favor the growth of an earth solid throughout. Larger masses would, on the other hand, carry the energy of impact into the earth. They would not strike with the high velocities of the meteors which collide with the 28 EVOLUTION OF THE EARTH earth, since the different planetoids were traveling in the same general direction, but nevertheless a state of incandescence and liquidity would be likely to result from the sizes of the masses involved. If in addition the infall of masses was sufficiently rapid to bury the heat of previous infalls before it could be dissipated by conduction to the surface, a general heating and liquefaction of the earth would tend to take place, both from the increased compression of the deeper nucleus and the effects of impact at higher levels. The fact that the planets have cleared up the zones about them, whereas the planetoids have remained permanently in a scattered state, is an argument for holding that the existence of dominating nuclei determined the growth of the planets. It is likely that the nuclei were of various sizes, were clustered to various degrees, and many of them united by their impact. A somewhat limited number and considerable size of the units as well as their grouping would be in accord with the lack of relation of the amount of eccentricity and inclination of orbit to the masses of the several planets. Indications of primordial tidal retardation. The moon keeps the same face turned always toward the earth. Con- sequently, from a point in outer space, it would be seen to rotate on its axis in exactly the same time that it completed its orbital revolution. Mercury is known also to keep the same face turned always toward the sun and the same relation is probably true of Venus. The other planets revolve many time on their axes during the period of revolution, the earth, for example, 366 times. The exact correspondence in the moon, Venus, and Mercury between their times of axial rota- tion and orbital revolution points to some causal relation be- tween the two periods. That relation is one of tidal forces. The moon distorts slightly the earth's figure, but as the tidal forces due to the moon are weak and the earth is very rigid, this distortion in figure is expressed mostly by the rise and AND ITS INHABITANTS 29 fall of the earth's fluid envelope. In so far as the body of the earth yields, it is an elastic yielding which involves no measurable expenditure of energy. The oceanic tidal waves tend to continually face the moon and the earth revolves be- neath them, like a wheel revolving between two opposite brake-shoes. This generates tidal friction which tends to slow down the axial rotation of the earth. There is no question as to the correctness of this theory, but there is a very serious question whether the forces are not so weak as to be without any geologic consequences, at least under the present rigid condition of the earth's interior. Some of the latest work has been given to measuring directly the retardative influence of the tides, if such exists. Mac- Millan has made an estimate of the loss of energy by friction of the oceanic tides. He used the formulae employed by en- gineers for the loss of head due to friction and viscosity, and applied them to the ocean. His conclusion is that the day would be lengthened by one second in about 500,000 years. Even if this figure be in error tenfold or a hundred-fold it is still in great contrast with the conclusion of Adams in the middle of the last century, that the earth was losing time at the rate of 22 seconds per century, a figure raised to 23.4 seconds by Darwin and lowered to 8.3 seconds by Newcomb. Mac- Millan's method brought to bear as a retardative agency prac- tically all the friction of the tides, irrespective of their positions or directions of motions, and seems to show that the water tides do not have and have never had an appreciable effect on the earth's rotation.5 Tidal retardation if it has ever been an important factor in planetary history must then be chiefly due to a body tide. In so far as there is a mere elastic yielding of the body no energy 5 MacMillan, W. D., "On the Loss of Energy by Friction of the Tides." In "The Tidal and Other Problems." Carnegie Institution of Washington, Pub. No. 107, 1909, pp. 71-75. 30 EVOLUTION OF THE EARTH is consumed, but a viscous drag will produce retardation. Recent measurements of the rigidity of the earth under tidal stresses, both by the horizontal pendulum and by the water level in a horizontal pipe, show that the earth as a whole is more rigid than steel and that under the exceedingly small tidal stresses the yielding is essentially elastic. The estimates of viscosity are so small that they are within the limits of error of the measurements. The smallness of the tidal strains in the earth may be appreciated by citing Darwin's calculations. According to this investigator, the tides raised by the moon upon the earth generate a stress-difference of 16 grams per square centimeter at the poles, 48 grams at the equator, and 128 grams at the center of the earth. Thus the earth is stressed by the lunar tidal forces even at the center to only about one part in fifteen thousand of the strength which granite has at the surface of the earth. The tidal force exerted by the earth on the moon is about twenty-two times as great as the lunar tidal force on the earth, and reaches about one part in six hundred or seven hundred of the strength of granite. If the moon were once nearer the earth, the tidal stress-difference was much greater, varying inversely with the cube of the distance. Tidal retardation must have acted efficiently upon the moon, nevertheless, until the moon was at its present distance and the stresses reduced to their present magnitude, in order to have reduced its period of rotation to the same value as its final orbital period about the earth. The action must have been that of a viscous body tide since the moon has never been able to hold to itself an ocean envelope. The tidal force exerted by the earth upon the body of the moon consequently must have produced a notable viscous yielding and continued to do this in spite of increasing distance of the moon and increasing rigidity. The far greater mass of the earth prevented such large effects of tidal retardation from being felt, but its period of AND ITS INHABITANTS 31 revolution compared to those of Mars and the outer planets suggests that the tidal forces of the moon and sun have pro- duced a notable slowing down of the earth also. The largest planet, Jupiter, 86,500 miles in diameter, revolves the most rapidly, completing one revolution in 9 hours 55 minutes; Saturn, the next largest, revolves in 10 hours 14 minutes. Uranus also shows by the pronounced polar flattening of its disc that it revolves in some similar short period. Mars, with a diameter approximately half of that of the earth and a twentieth of that of Jupiter, revolves in 24 hours 37 minutes. These planets can never have suffered largely from tidal retardation and a rough rule appears to prevail that the larger the planet the more rapidly it rotates. Judging from its mass, the earth may consequently have originally rotated in a period of between 15 and 20 hours. This argument is only of sug- gestive value, but it is in accord with other lines of argument. If the moon passed through a viscous state sufficiently pro- longed for it to respond completely to tidal control in its rotation period, the presumption is clearly that the earth, a larger body and better able to retain its heat, also passed through a similar stage of viscosity. The present rigid and elastic condition of the earth appears then to be a secondary feature and the present ineffectiveness of the tides cannot be safely used as an argument against the strong indications of a primordial tidal retardation. Significance of the oceanic salt. Sodium derived from the weathering of igneous rocks has been stored through all geo- logical time in the ocean as sodium chloride. The ocean has grown more salty since it first gathered on the earth, yet it is so far undersaturated that sea water must be nine-tenths evaporated before sodium chloride begins to be precipitated. Furthermore, the indications are that it never was saturated, even though in primordial times the sea water may have been less in volume. Concentration to a degree which eliminates 32 EVOLUTION OF THE EARTH part of the sodium chloride raises the percentage content of bromine, magnesium chloride, and magnesium sulphate, so that sodium becomes subordinate to magnesium and the ratio of bromine to chlorine is increased Subsequent dilution would not change this ratio and the introduction of new salts could never bring it back to the original composition. The evidence from the sea itself is substantiated by the testimony of the sedimentary rocks. The amount stored as impregna- tions or as salt deposits in the sediments is quantitatively negligible, either as compared to the volume of the sediments or the mass of the oceanic salts. Salt deposits, furthermore, so far as known, began to be present only in the Paleozoic, in the later half of geologic time, the great masses of earlier sedimentary strata being barren of them. It has been calculated that the total sodium in the ocean would be derived from the weathering and erosion over all the earth of a mantle of igneous rock of average composition only 2,300 feet thick, corresponding to 6,500 feet as the average thickness of erosion if restricted to the area of the con- tinental platforms, including the lands and extending out to a depth of 600 feet below sea-level. Daly has noted the significance of these facts upon the hypotheses of earth-growth.6 Chamberlin supposes an ocean to have existed for long geologic ages upon the surface of the earth during its growth from a body about half of its present diameter and one-eighth of its present volume. The planetesimal material, he holds, was weathered and sorted into lighter and heavier portions, leading to the development of lighter protuberant and heavy depressed areas. The limited quantity of salt in the sea, however, is distinctly against such a hypothesis of oceanic antiquity and continental build- ing. The amount of erosion in evidence where the older rocks are exposed as crystalline masses and from the great «Daly, R. A., "Igneous Rocks and Their Origin," 1914, pp. 159, 163, 164. AND ITS INHABITANTS 33 uplifted tracts of later times is apparently about sufficient to account for all the salt in the sea. In fact, the estimates of erosion through known geologic time based on the nature of rock exposures and the thickness of sediments have fully equaled or exceeded the amount given by the quantity of salt in solution. Weathering, erosion, and the accumulation of salt had therefore played no considerable part previous to the time recorded by the oldest rocks. The earlier physical con- ditions must have been very different from those which later prevailed. FAVORED HYPOTHESIS OF AN EARTH INITIALLY MOLTEN Indications of a primordial molten state. The indications of primordial tidal retardation and the limited amount of salts in the sea both point to the conclusion that the earth was molten at the completion of its growth. A molten state suggests a rapid earth-growth due to an original clustering of the matter whose convergence built up the planet. Larger nuclei hun- dreds of miles in diameter and smaller ones comparable to the planetoids moved in elliptic and nearly intersecting orbits. Mutual perturbations kept modifying these orbits and provid- ing new chances for collisions, union, and growth. Such colli- sions led to a development of energy of impact sufficient to produce in the growing earth a molten state, at least in the outer portions. The earth kept growing at the same time by sweeping up large quantities of finer material, but a molten state suggests that the greater growth was due to the infall of larger nuclei. Finally, but one outstanding nucleus, the moon, was left beside the earth, and the earth-moon system attained a condition of stability and completed growth. If the composition of the earth as a whole is similar to that of the meteorites, those samples of matter which come to us from the heavens, the most abundant material in the deep body of the earth is metallic iron. Now the blast furnace 34 EVOLUTION OF THE EARTH makes familiar the fact that slag is insoluble in iron and, being lighter, gathers in the upper part of the crucible, like cream upon milk. The slag is similar in composition to basaltic igneous rocks. The density of the deep interior sug- gests that it is layered like the crucible of the blast furnace and that the silicate rocks form an envelope some hundreds of miles thick, grading down into a great metallic core. The silicate envelope ultimately differentiated further, resulting in a rise of the more siliceous and lighter fraction into an outer layer, perhaps 50 to 75 miles in thickness. This in turn crys- tallized into a primordial, universal, granitic crust above a thicker basaltic shell below. The primordial atmosphere. Granting the conclusions in regard to the initial fluid state of the earth, let the primordial atmospheric conditions be pictured. A gaseous envelope exist- ing in equilibrium with rock magma would be dominantly water-vapor, followed in order of importance by carbon dioxide and carbon monoxide, chlorine and hydrochloric acid, with some nitrogen, but no free oxygen. The present atmosphere of nitrogen and oxygen penetrates by solution into the liquid envelope of water. The primitive atmosphere of water-gas penetrated in the same manner by solution into the ocean of molten rock. But such an admixture of molten rock and water-gas is known to have a remarkable effect upon the melting point of silicate magmas. Under dry fusion the exalted temperature of 1300° to 1500° C., a dazzling white heat, is necessary to make granite thinly molten. But if the fluid rock has dissolved an abundance of water- vapor the mixture stays fluid until it has cooled below a tem- perature of 800° C. The surface of the earth when molten was perhaps no hotter than this. At such a comparatively low temperature and even at somewhat higher temperatures there would be but little dissociation of water into its com- ponent gases, and the earth would be capable of holding to AND ITS INHABITANTS 35 itself, even in its molten stage, an envelope of water in the form of a deep and heavy atmosphere of water-gas. Such an envelope, including also an abundance of carbon dioxide and carbon monoxide, formed an effective thermal blanket, pre- venting a precipitous chilling and freezing at the surface of the ocean of molten rock. The effectiveness of the blanket depended upon the pecu- liarity of both water-gas and carbon dioxide in being opaque to the slow vibrations of dark heat, absorbing these near the bottom of the primitive atmosphere and reradiating them from higher levels as long, slow heat waves. Strong convec- tion currents carried up these heated gases from the super- heated base to the higher levels of the atmosphere. There the chilling condensed the water-vapor into a thick and universal canopy of cloud, boiling up like thunder-heads from below, shedding continuously a downpour of acid rain, rain dissipated again into vapor as fast as the drops fell into the deeper and hotter strata of the atmosphere. The intensity of the vertical convection maintained a high electric tension. Incessant flashes of lightning linked as with living, fiery tentacles the cloudy heavens to the lurid molten earth. Tremendous re- verberations of thunder, unsensed by mortal ears, shook the atmosphere in the worldwide primeval storm. The sunlight of the Primordial Era illumined and was reflected from the outer side of the mantle of cloud. The planet shone brilliantly by this reflected light, having an ex- ternal appearance similar to that which Jupiter and Saturn still possess. Above the zone of cloud the carbon dioxide and other gases, with very minor amounts of water-vapor, ex- tended with diminishing density as an upper transparent envelope. During the more rapid growth-stages the molecular and dust-like matter swept up by the earth settled like a never ceasing cloud of volcanic ash. The planetesimals of sand and 3,6 EVOLUTION OF THE EARTH gravel size were swept up by the earth many millions of times more abundantly than are meteors at the present time. Those meeting the earth with the higher velocities were consumed by impact. Over the hemisphere of night the otherwise invisible atmosphere above the cloud canopy scintillated with incessant flashes of light and trails of luminous dust. Bodies of larger size gave in their dissolution a still more brilliant display and penetrated to greater depths. At longer intervals, with titanic rush and roar, a greater projectile, tens or even hundreds of miles in diameter, cleaved through the canopy of cloud, leaving a tumultuous maelstrom behind, drove almost unchecked through the deep and dense atmosphere below, and, with worldwide commotion, was engulfed, with development of fervid heat, within the molten sea. THE PASSAGE OF THE MOLTEN INTO THE RIGID EARTH Density stratification by fractional crystallization. The central parts of the earth were compressed during the growth stages by the increasing load above. This compression developed heat, but also raised the fusion point and made for a greater rigidity. It is not known, consequently, whether during earth-growth the center tended toward a liquid or solid state. The outer part, however, with a thickness of perhaps the outer quarter of the radius, comprising about one- half of the volume of the sphere, seems to have passed into a truly molten condition. The heavy atmosphere and canopy of cloud prevented a rapid radiation from the molten surface, probably sufficient during the highly liquid stage to prevent a crusting over of frozen rock. The method of solidification approached nearer to that which occurs in a large reservoir of magma intruded into the crust than to the freezing of a modern lava stream in contact with the air. AND ITS INHABITANTS 37 At last the rapid generation of heat by impact lessened, and the fluid sphere, seething with slow convection currents, began to cool. Certain compounds in the mutual solution of rock elements became insoluble and fractional crystallization was initiated. The heavy basic crystals were the first to form: crystals of metallic sulphides, magnetite, hornblende or pyroxene, and olivine. These crystals, because of their high specific gravity, tended to work downward in the convective movement. At first they were dissolved again in the abyss, but as time went on they remained undissolved and accumu- lated in the deeper parts of the fluid zone. The remaining magma was more siliceous, of lighter gravity, and in crystalli- zation gave to the upper shell a higher proportion of feldspar and quartz. The original crust of the earth was consequently a granite. The process of fractional crystallization may, however, not be a sufficient explanation. An immiscibility of the complex mineral solutions may have developed upon the lowering of the temperature. In one way or the other or by a combination of several causes a density stratification is sug- gested by a number of lines of evidence as existing in the earth. The conclusion, then, is more than a mere inference from theories of crystallization. The gathering of the ocean waters. At last the ocean of molten rock had shallowed, crystallization went forward in separate basins, convection became hindered, the surface froze as in a lava caldron. Then rain, ever descending from the shield of perpetual cloud, but never heretofore reaching the bottom of the atmosphere, at last began to splash on the hot surface of the earth. The raindrops at first were dis- sipated by contact and sent flying back as scattered molecules of gas. But, owing to the low conductivity of rocks, the tran- sition stage was very brief, and perhaps even in a few thousand years from the time when the crustal congelation of the earth had taken place a permanent ocean of acid water began to 3 8 EVOLUTION OF THE EARTH rest upon the surface. For a while the balance swung, as one section or another of the crust was broken through and lavas would pour out abundantly. Rapidly, however, from the geologic standpoint, as the surface cooled, the atmosphere of water-vapor condensed in a never ceasing deluge until an ocean, probably universal in its extent, had gathered to a mean depth of several thousand feet. The remaining atmosphere was comparatively rare and cold. Carbon dioxide became the dominant gas, and water-vapor subordinate. Solar heat began to play the principal part in warming the equatorial zone. A system of planetary winds developed in accordance with the new order of nature. The cloud canopy became thin and broken, resolving itself into climatic belts. Sunlight for the first time began to pierce the lower atmosphere and illumine from without the surface of the earth. During the earlier time, when the water could exist only as gas in the atmosphere, the great pressure of this envelope had kept much, perhaps most, of the gases in the molten rocks. With the great fall in atmospheric pressure which accompanied the gathering of the ocean, magmas which broke through the higher levels of the crust into the regions of this decreased pressure were able to give off great volumes of gases which in the earlier stage had been repressed. These gases, freed for the first time, are termed juvenile and from this time forward juvenile waters were added to the ocean. In the first ages following the solidification of the earth the additions were large in volume, but igneous, action continues to bring new magmas to the surface recurrently from age to age. These give off the gases which have been repressed in them since the origin of the earth. Thus, in intermittent and lessened rate, the surface waters have increased through geologic time. As Suess has said, the body of the earth has given forth its oceans. AND ITS INHABITANTS 39 THE ORIGIN OF OCEAN BASINS The relations of crustal density to ocean basins. The fluid earth had a surface as level as the ocean, and the process of solidification which has been outlined does not account for those marked variations in density and in surface form which are expressed by the outer crust of the solid earth being divided into continents standing high above the ocean floors. A sketch of the formative period is therefore not complete unless the processes are briefly discussed which are thought to have shaped the earth's surface, giving rise to the existence of lands even before the period of the oldest known rocks. Reasons will be given below for holding that the ocean basins have been formed by subsidence of broad areas of the crust, owing to the weight of magmas of high specific gravity rising widely and in enormous volume from a deep core of greater density into these portions of an originally lighter crust. This regional subsidence was especially characteristic of primordial times, but the process did not wholly cease then; since certain lines of evidence suggest that some ocean basins have been extended in later geologic ages, breaking into once wider continental platforms. The resultant increase in the volume of the ocean basins has led to a drawing off of the ocean waters from the continental areas, and a marked diminu- tion of the shallow seas of earlier ages. The cause of the continued generation of new bodies of molten rock in the sub-crustal shell, adequate to account for the observed results of later geologic time, is thought to lie in the slow accumulation of heat from radioactivity in these depths below the crust. This is discussed in the subsequent topic on the rise of basic magmas. Some very thick bodies of intrusive rock are observed to be more dense and basic below, lighter and more siliceous above. The lower part is a gabbro, whereas the upper may 40 EVOLUTION OF THE EARTH be a granite. The separation has taken place after the in- trusion of the magma, the denser material sinking, the lighter rising. There are indications that the process goes forward on a larger scale also, a scale so large that the dark and heavy base is never seen, erosion of later ages being restricted en- tirely to the granite zone. Such a splitting in composition is indicated in that the earlier intrusions in a period of igneous activity are intermediate or basic and the later products are vast bodies of granite. The greater density in the earth's interior suggests a primal density stratification on even a larger scale, which has been discussed under a previous topic. But in the outer shell, 50 to 75 miles thick, the density is far from being uniform. In recent years it has been proved by means of precise geodetic measurements on the local in- tensity of gravity and deflections of the vertical that the crust beneath the continents is notably less dense than that beneath the oceans. The most of this difference in density exists prob- ably within the outer 50 miles. The continents stand high, consequently, for the same reason that an iceberg rises above the surface of the sea : it is the position of equilibrium. At a certain depth the downward pressures given by the thicker continental and the thinner oceanic crust are the same and a condition of hydrostatic equilibrium prevails in the sub-crustal shell. This condition of equal pressures at a certain depth is called isostasy. It is not inconsistent with a solid and rigid condi- tion of the earth, but does mean that at a greater depth, apparently from 50 to 300 miles or more, hot but solid rock can slowly yield and flow by recrystallization. The process is physically the same as that by which a glacier flows under the slight stimulus of an almost level surface slope. The condi- tion for such ready recrystallization is found in temperatures which are close to those of fusion. At such temperatures molecules under strain pass readily from the solid to the liquid X> ••-^ w r3 13 C s- u — AND ITS INHABITANTS 41 state, shift into positions which ease the strain, and again enter into a crystalline solid condition. The proof that such a pro- cess exists in the earth is based on several lines of evidence. First, evidence of a broad isostatic equilibrium notwith- standing the agencies of mountain folding, of erosion, and of sedimentation, all of which work through geologic time tend- ing to destroy those relations of elevation which are needed to maintain isostasy, giving equal pressures by broad crustal areas of unlike density upon the yielding zone below. Second, the evidence of increasing temperature with depth, giving temperatures close to those of fusion at depths below 40 to 50 miles. Third, the evidence from tides and earthquakes that the earth as a whole is more rigid than steel and cannot possess a fluid shell beneath the crust. Fourth, the physical principle that at temperatures close to fusion a crystalline substance is incapable of supporting permanent shearing stresses, but yields slowly by recrystalliza- tion, notwithstanding the fact that under short stresses the same substance may be as rigid as steel. The conclusion to which this argument leads is that an outer crust or litho sphere, the rock sphere, 50 to 75 miles thick and very strong, is marked by broad variations in density amount- ing to as much as 5 per cent, and more local variations up to 10 per cent, which correspond to the broader relief of the earth's surface. Below this lies a thick, hot, basic, rigid yet weak shell which the writer has named the asthenosphere,1 the sphere of weakness. The problem of the origin of the ocean basins and of continental platforms resolves itself con- sequently into the origin of the density differences in the lithosphere and the maintenance of the heated and weak con- dition in the asthenosphere. 7 Barrell, Joseph, "The Strength of the Earth's Crust." Jour. Geology, vols. 22, 23, 1914, 1915. 42 EVOLUTION OF THE EARTH Rise of basic magmas from the as theno sphere. The series of radioactive elements slowly break down into elements of lower atomic weight and give off in the process enormous quan- tities of energy. Uranium, in degenerating through radium to the stable element lead, develops more than a million times the heat given by the combustion of an equal weight of coal, but the disintegration of the element and the liberation of its heat are so slow that the whole duration of geologic time has not sufficed to eliminate uranium from the crust of the earth. Therefore it has acted as a permanent generator of heat in the rocks which contain it. Uranium and thorium, the parents of the radioactive series, are widely though sparsely diffused through the lithosphere. It has been calculated that, if they extend in their surface amount to a depth of 40 miles, they must supply heat to the surface as fast as it is lost by radiation into space. The earth therefore appears not to be growing colder, though losing heat. The small content of radioactive elements in the basaltic shell below the granitic crust of the continents would then supply that slow increment of heat which is necessary to generate new molten rocks. The granitic shell above, though somewhat richer in radioactive elements, is sufficiently near the surface to lose its excess heat by conduction. The excess heat generated in the asthenosphere is, on the contrary, so deeply buried that it cannot escape in that manner but must slowly transform some of the solid rock into liquid form. Reservoirs gather, until their mass, combined with their de- creased density in the fluid form, enables them to work their way through the crust above and demonstrate their existence in igneous activity at the surface of the earth. The magma which thus comes from the greatest depth and in greatest volume would, because of the initial density stratification, produce a notable increase in the density of the outer crust. AND ITS INHABITANTS 43 In order to reestablish isostatic equilibrium such a region must subside. Most of the igneous rock of later geologic ages which has been intruded into the outer continental crust clearly has not increased the density sufficiently to produce a foundering and would appear therefore either to have come from somewhat higher levels or to have risen in lesser quantity. In some regions, however, as in that of the Lake Superior basin, large masses of basic magma do seem to have overweighted the crust in an early geologic period and produced a tendency to settle as a basin. The same effect may have taken place to even a larger degree in some regions of notable subsidence, as in the Mediterranean basins. In the earliest times, following the solidification of the earth, the forms and relations of the ocean basins suggest that dense molten matter from the depths of the earth broke into or through the outer crust, on a gigantic scale, eruption following eruption until the widespread floods had weighted down broad areas and caused their subsidence into ocean basins. As seen in the lava plains of the moon, such an action, once started at a certain point, is conceived to have gone forward with widening radius, leading to the origin of the many rudely circular outlines characteristic of the ocean basins. The process left great angular segments of the original lighter crust as continental platforms standing in relief between the coales- cent basins. The waters gathered into the basins and the continents emerged. THE REIGN OF SURFACE PROCESSES AND BEGINNING OF THE ARCHEAN It is possible that shallow ocean basins began to form nearly as fast as the waters gathered, tending to maintain some land areas above the level of the primordial sea. Or the lands may have emerged later, as the ocean basins spread and deep- 44 EVOLUTION OF THE EARTH ened. With the separation of the lands from the seas, ero- sion began, carbon dioxide was abstracted from the atmosphere to make carbonates, and a further cause of atmospheric depletion was initiated. Thinner, rarer, and colder grew the gaseous envelope, until an oscillating balance was established between the supplies of new gases from the uprising molten rocks and the losses involved in the weathering of their solid forms. Nitrogen was at first relatively small in quantity and oxygen not present in more than a trace. An evolution in atmospheric composition had still to go forward through following ages to transform it into a gaseous medium for the support of the higher life. But even in the early periods following the gathering of the oceans and the emergence of the lands, the sun warmed the atmosphere and earth. An environ- ment suitable for the lowest organisms had arisen and the earliest forms of life may not have been long in coming into existence. The reign of the surface processes had begun, but, at age-long intervals, the still youthful energies of the interior broke forth. Magmas in great volume ascended, now seen as the most ancient granite-gneisses. In the great crustal over- turning of these earliest revolutions the foundation rocks ap- pear to have been everywhere destroyed. The oldest rocks pre- served are mashed and crystallized sediments and lava sheets resting as fragments of a cover on the reservoirs of younger magma. Such sediments, altered and intruded, are the oldest Archean rocks: It is not known how close they stand in point of time to the formative processes whose description has been attempted. With these oldest rocks, the dimly known, heroic and mythical eon of the earth is closed and the first historic eon opens as the remote and long enduring Archean division of geologic time. CHAPTER II THE EARTH'S CHANGING SURFACE AND CLI- MATE DURING GEOLOGIC TIME CHARLES SCHUCHERT PROFESSOR OF PALEONTOLOGY IN YALE UNIVERSITY UNIFORMITY OF NATURE. The previous lecturer in this course had to seek for the probable origin of the earth in far-off space among the stars, examining them through the great telescopes of our time, through the light-sensing chemical plate of the photographic camera, and through the even more wonderful spectroscope. With this knowledge in hand, postulate upon postulate has been tried out through that talismanic study, mathematics, and so through astronomy and the science of numbers there is revealed an earth evolution still hazy, to be further established through geodesy, mechanics, and chemistry before the geologist comes to be its interpreter. Then, hand in hand with the geologist, the paleontologist, or student of ancient life, reveals the organic hordes that have gone on and through whose fossil remains is unraveled the history of the earth. In all of this we see the brotherhood of the sciences and the fundamental postulate of the uniformity of nature. The geological time-table. As some geologic terms must be used in this lecture, it is desirable to define them here. The geologic history of the earth is now divided as follows : Present time. Psychozoic era. Age of man or Age of reason. Includes the present or "Recent time," and the time during which man attained his highest civilization, estimated to be probably less than 30,000 years. 46 EVOLUTION OF THE EARTH Geologic time. Cenozoic era. Age of mammal dominance. Glacial or Pleistocene time. Last great ice age. Late Cenozoic or Pliocene and Miocene time. Trans- formation of apes into man. Early Cenozoic or Oligocene and Eocene time. Rise of higher mammals. Mesozoic era. Age of reptile dominance. Cretaceous period. Rise of archaic or primitive mammals. Comanchian period. Rise of flowering plants and higher insects. Jurassic period. Rise of birds and flying reptiles. Triassic period. Rise of dinosaurs. Paleozoic era. Age of fish dominance. Permian period. Rise of reptiles. Another great ice age. Pennsylvanian period. Rise of insects and first time of marked coal accumulation. Mississippian period. Rise of marine fishes. Devonian period. First known amphibians. Silurian period. First known land floras. Ordovician period. First known fresh-water fishes. Cambrian period. First abundance of marine animals, and dominance of trilobites. Proterozoic era. Age of invertebrate dominance. An early and a late ice age. Archeozoic era. Origin of protoplasm and of simplest life. Cosmic time. Formative era. Birth and growth of the earth out of the spiral nebula of the sun. Beginnings of the atmos- phere and hydrosphere, and of continental platforms and oceanic basins. No known geological record. Origin of the sun and earth. Professor Barrell, in the pre- vious lecture, pointed out that the sun is a star, and one of the smaller among the countless millions seen through the tele- scope. It may have had its origin in a diffuse nebula like Orion, and, according to the disruption hypothesis of Cham- AND ITS INHABITANTS 47 berlin and Moulton, had a long antecedent history before it gave birth to its planetary system. Therefore before there was an earth, the sun had been condensing and moving undis- turbed in space for countless ages, until it came within the influence of another and a greater star. The gravitative influence of this mighty passing intruder acted catastrophically upon the sun and caused its partial disruption. As a conse- quence, from opposite sides of the sun there streamed rapidly outward materials of its own mass that soon arranged them- selves into a two-armed luminous spiral nebula. The mass which was so extruded, and which did not fall back again into the sun, was less than i per cent of the sun's mass, and yet it gave rise to the planetary system which encircles the parent body. From this we also the more readily appreciate the great size of the parent sun, whose volume is 1,300,000 times greater than that of the earth. Growing earth during the Formative era. In the solar nebula there existed eight knots of more or less loosely aggre- gated denser and hotter matter, the nuclei of the future minor and major planets and their satellites, and these in the course of time attracted to themselves most of the surrounding nebu- lous material, usually spoken of as the planetesimals. The earth-knot, it is estimated, may have had an original diameter of about 5,500 miles, and the moon-knot perhaps half its present diameter of 2,162 miles. In the course of cosmic time, the earth and moon, revolving as companions about the sun, gathered into themselves the planetesimals that lay within their spheres of influence. The speaker holds to the postulate of Barrell that the plan- etesimals conceived by Chamberlin to have been of the size of sand and dust were probably for the most part much larger and more like the planetoids in dimensions. From time to time, as one or more of the planetoid-like bodies, singly or in combination, plunged in upon the earth with velocities up to 48 EVOLUTION OF THE EARTH tens of miles per second, their impact with and engulfment by the earth engendered much additional heat. In fact, they liquefied themselves and their earth surroundings as a result of their great impacts. Furthermore, the high internal heat of the earth was accentuated by the condensation accompany- ing the increase of mass from an original earth diameter of 5,500 miles to one initially larger than 8,100 miles. At no time, however, does it appear that the earth was necessarily in an astral condition, shining like a star. The present diameter of the earth is 7,918 miles, but at the close of the growing period it must have been 200 and possibly even 400 miles greater, for it is well known to geologists that throughout geologic time it has been losing volume, due in part to the loss of heat into space, but probably in greater degree to internal molecular rearrangements. It is now known that the earth does not shrink all the time because of these changes, but does so only periodically, and at these times the surface of the planet becomes wrinkled, giving rise to more or less high ranges of mountains. Even though the earth has been dissipating its inherited and derived energy for at least some hundreds of millions of years, and in so doing repeatedly giving birth to new series of mountains, our mundane sphere is still far from having attained the internal stability that will, when achieved, result in a featureless earth, an atmosphere devoid of the carbon dioxide that is the basis of life, and a universal lifeless ocean — the facial expression which the earth will have in its old age. Volcanism during the Formative era. While the earth was internally very hot and viscous, the body-mass was in slow movement, with the heat ever rising toward the surface. These movements simulated a slow boiling process, and the upwellings of the molten matter repeatedly broke through the outer cold shell and engulfed it. As the cold crustal masses descended, they liquefied, and in expanding brought about AND ITS INHABITANTS 49 increased pressures that again found relief in the rising tongues of heated matter. This condition is believed to have continued for a very long time, and to have been particularly active during the last half of the Formative era and the first era of geologic time, an age that may have endured for one-eighth to possibly one-fourth of the earth's entire history. The boiling process not only disseminated the internal heat, but also brought about an irregular sorting out of the heavier and lighter substances, so that the more basic and metalliferous materials as a rule sank to ever lower levels, while the less heavy acidic ones rose as granitic rocks toward the surface of the earth. Finally, at the close of this first era, the Archeo- zoic, the lithosphere or outer rocky and more or less rigid shell may have had a thickness of 50 miles. Through such a rocky mass little of the internal heat rises to the surface, and life would be impossible on the earth if it were not for the constant flow of the sun's radiant warmth, made equable and usable by the atmosphere. These conditions were already present in the Archeozoic era. Origin of continents and oceanic basins. It is well known to geodesists and geologists that the continents are built of lighter materials, essentially of granites, while the greater oceanic areas have the heavier basaltic rocks beneath them, and that the difference in specific gravity amounts to about 3 per cent. We have seen that these differences came about seemingly as a result of the internal boiling process that pre- vailed during the Formative era and the Archeozoic, and so it appears that the higher continental masses and the depressed oceanic basins came into being very early in the history of the earth. The same interrelation of masses is also to be seen in the moon. This is the igneous theory of the origin of oceanic basins and continental platforms of Barrell, and it holds that the differentiation took place after the earth had attained its com- 50 EVOLUTION OF THE EARTH plete growth, in other words during the closing time of the Formative era and during the Archeozoic era. The differ- ences between the densities of the lands and oceans are held by Barrell to be only skin deep — in the outer one-fiftieth of the earth's present diameter, or in the outer shell of about 150 miles in thickness. On the other hand, it is true that the lands throughout geologic time have repeatedly gone down as well as up, but the sum of their movements has been upward, and for this reason geologists speak of them as the positive areas of the earth's surface. In the same way, the lower-lying fields have also risen locally at times, but their movement has in the aggregate been downward, bringing about the increasingly greater oceanic basins, the negative areas, by far the largest of the earth's surface. We shall see later on that the oceanic fields have become larger at the expense of the lands to accommodate the ever increasing volume of water, and it therefore naturally follows that the continents in former geo- logic times were considerably larger than they are now, pos- sibly even 25 per cent larger. But the newer geology no longer holds to the theory that the oceans and lands have repeatedly changed places; quite the contrary, we agree with Dana that the present positions of the land and water areas have been more or less permanent throughout geologic time. Origin of the atmosphere. It was not so long ago that most geologists held with the French astronomer Laplace and with Dana that the earth was originally very hot, and that all of the water of the hydrosphere, the salts of the oceans, and the carbon dioxide of the air and oceans had been parts of the primal gaseous envelope. Not only this, but that the latter also included all the carbon locked up in the rocks, estimated to be 30,000 times greater than the amount now in the atmosphere. Accordingly, as the earth was held to have been cooling throughout its history, it was thought that only AND ITS INHABITANTS 51 recently, geologically speaking, was the present thin condition of the atmosphere brought about. Even though more than sixty years ago a few English geologists who were familiar with the geology of India and Africa began to point out that the earth had passed through a cold period toward the close of the Paleozoic era (see Figs. 4 and 5), this discovery did not at once break down the teach- ings of Laplace and Dana. When, however, still older glacial periods began to be discerned in the stratified rocks and espe- cially in the oldest ones near the beginning of the Proterozoic era (see Figs. 7 and 8) — a record that is remarkably well preserved in Canada — the final straw was at hand to break down the theory that the climates of the earth had passed progressively from an astral through a torrid to a temperate condition, along with a slow clarifying of the atmosphere (see Figs. 3-8). The newer geology holds, with Chamberlin, that the grow- ing earth was originally too small in mass to hold an atmos- phere and accordingly there was a time when our planet had none. As it grew in diameter, the earth was more and more able to hold an atmosphere and a hydrosphere, and during the long growing period there was finally originated an atmos- phere probably not much unlike the present one, which in a roughly approximate way may be said to be composed of four-fifths nitrogen and one-fifth oxygen. It should, however, be noted here that at first the atmosphere had but little oxygen. Since life came to be, using the carbon and freeing most of the oxygen of the carbon dioxide of the atmosphere, the quantity of oxygen has more or less steadily increased, though at all times much of it has been consumed in the oxidation processes of the rocks and minerals. The most constant accessory constituent of air is carbon dioxide, one of the three fundamental materials at the basis of life. In the present atmosphere there are about three vol- 52 EVOLUTION OF THE EARTH umes of this gas to 10,000 of air, and there is as much more in living things as there is in the atmosphere. On the other hand, there is in the oceans of today, according to F. W. Clarke, the geochemist, from 18 to 27 times more carbon dioxide than in the air (Johnston and Williamson say that at 15° C. there is about 70 times more), while the still vaster volumes locked up in the sedimentary rocks and in the fuels and carbonaceous deposits of the earth are computed to be 30,000 times greater than the volume in the present atmos- phere. These facts are brought forward at this time to show that the constituents of the atmosphere have always varied because of the constant loss of carbon dioxide and oxygen to the sedimentary rocks, but that at the same time there has always been a resupply of carbon dioxide through the ever active volcanoes and the mineral springs, and of oxygen through the life activities of plants. Holmes says:1 "Even now, the outer 70 miles of the earth's crust would be competent to supply all the nitrogen of the atmosphere, the water of the oceans and the vast quantity of carbon dioxide represented by limestones and carbonaceous deposits." It has been well said that if volcanism should cease it would not be long before the existence of life would be impossible because of the absence of carbon. We should add here that if there were again as much life as there is at present, all the carbon of the atmosphere would be in the living plants and animals, and, if such a condition were possible, death would come to them all. Therefore life and its abundance at any time are conditioned by the amount of this gas present in the atmosphere. Climates of the past. The quantity of carbon dioxide in the atmosphere is to a certain extent also a climatic regulator, though the greater factor in this matter is water-vapor, which is also the most variable constituent of the atmosphere. With 1 Holmes, A., "The Age of the Earth," 1913, p. 30. o o CO JZ