ONE of the most interesting problems for persons who devote themselves to the study of geology is this. Can we estimate, at least approximately, the age of the globe, we mean the time that has elapsed since the formation of the terrestrial nebula up to our day ? A number of physicists and geologists have undertaken this task, but the discordance - II the results obtained plainly says that the methods employed offered no certainty. The majority have found improbable figures— these being hundreds of millions and sometimes thousands of millions of years. The most moderate of these calculations, due to Sir William Thomson, is one hundred million years—a number that appears to us still too exaggerated. The causes of this disagreement and want of success must, we think, be attributed to our ignorance as to the numerical value of the coefficients which enter the putting of the problem into equation when we approach it squarely. Such are the temperature and volume possessed II the beginning by the terrestrial nebula, its density, i ts degree of heat conductivity, the thermic state of space at that epoch, etc. As direct methods have given negative results only, or at least very doubtful ones, let us have recourse to indirect methods. Until better ones are offered, we propose the following. Let us say, II the first place, that, instead of calculating the age of the earth in a lump, as has hitherto been done, it is better to divide it up and calculate each part separately. The journey of the earth through space and time, in fact, comprises three distinct stadia, which we shall call the igneous or nebulo-stellar stadium, the stadium of solar illumination or of life, and the stadium of darkness, cold, and death. We shall pass these successively in review and indicate the characters proper to each. the nebulo-stellar stadium. This began at the moment at which the terrestrial nebula detached itself from the solar, and ended in the formation of the crystalline crust of the globe. This stadium, the shortest of the three, has, as its characteristic, the transformation of the igneous star into an extinguished one, we mean planet, in consequence of the incrusting of the surface. This hardening was a natural consequence of the gradual lowering ' of the temperature caused by the radiation of the terrestrial nebula in space. It is possible, we believe, to estimate the duration of this stadium approximately by the aid of considerations drawn from solar radiation. As one square foot of the terrestrial surface receives, according to physicists, four one-hundredths of a heat unit per second from the photosphere, we find by calculation that the solar mass annually loses, in consequence of its radiation in space, one heat unit per pound. From this we conclude that the great luminary will be extinguished in a dozen million years. On another hand, thermo dynamics teaches us that the photosphere already counts an existence of a dozen million years, and this carries the total cycle of solar radiation up to about twenty-five million years. As the earth, in mass and volume, is incomparably less than the star of day, it has necessarily cooled much more quickly. We think we shall remain within the bounds of truth in estimating the duration of its cool- iug at a hundredth part of that of the 8un, say 2.50,000 years. Let us double this figure to take into account the nebular phase which preceded the stellar one, and we find about half a million years as the duration of the first stadium traversed by the earth in its evolution through the ages. the stadium of light and life. The second stadium, which comprises the present epoch, and will continue for a long time to come, will embrace the entire cycle of geological formations. It begins with the Cambrian, and will end at the extinction of the sun, when the cold, congealing the last seas, stops the formation of oceanic vapor, and, consequently, sedimentary action. Its distinctive feature is the advent of the organic kingdom, which marks the adult age of the planet. Like the preceding stadium, it has cooling for its agent. This great factor of celestial mechanics, which we find everywhere, will serve as a guide and clew for ascertaining the events that will signalize the end of this period. The most important will be the retreat of the seas, which will profoundly modify the relief of the terrestrial surface. This retreat, due to the infiltration of water into the earth in measure as the latter cools, will give the globe the physiognomy of Mars. As that planet is more distant from the sun than ours, and of less size, it has cooled more quickly. Consequently, the infiltration of the liquid element has been more rapid than with us. So upon the disk of Mars we observe only small Mediterraneans that occupy no more than about a half of the planet's surface, while the ocean still covers three- quarters of our globe. The water will continue to retire until the day when the cold due to the extinction of the photosphere will congeal our last seas. The consequence of this recoil will be a gradual increase of the islands and continents, an evaporation of the more and more limited maritime basins, less and less abundant atmospheric precipitations, an increasing sterility of the earth, and, finally, the decline and death of plants and animals. Thus, deprived of all life, the earth will no longer be anything but an inert mass, lost in space. The duration of the stadium just described may, we think, be calculated by the aid of two methods, each of which controls the other. According to the estimates of geologists who have measured in various parts of the globethe thickness of the sedimentary deposits, we may fix the total thickness of the same at about sixteen miles. If we know the time that has elapsed during the formation of a stratum 1,000 ft. thick, a simple multiplication will allow us to find the age of the sedimentary crust of the globe. If the calculations that we have already given * on the subject of the periodicity of great circumpolar winters are exact, we can fix the duration of the quaternary epoch at about seventy thousand years. The mean thickness of the quaternary formation being estimated at 650 feet, we conclude therefrom that it will take three hundred and fifty thousand years to produce a stratum about half a mile in thickness, and this carries the duration of the sedimentary stadium traversed up to our day to thirteen million three hundred thousand years. This figure should be increased a little, since, the stratification of the deposits having in the course of ages been interrupted here and there, in consequence of the oscillations of the earth, a number of layers have not been deposited upon the subjacent formations until after a lapse of time difficult to estimate precisely. We do not think that we get far away from the truth in estimating the addition that it is pioper to make at about two millions, which would raise the duration of the organic stadium traversed up to the present to fifteen million three hundred thousand years. Such is the result given by the first method. The second rests upon observations drawn from the eye of the trilobite. This crustacean, as we know, inaugurates the paleozoic age, for it is found in the upper strata of the Cambrian. The mode of structure of the visual organ indicates that the latter was formed under the influence of a light that was more intense than that of our day. Hence we conclude that the photosphere of the sun. was at that epoch formed, and had as yet lost nothing of its energy. We have said that, according to the calculations of thermodynamics, it has already existed a. dozen million years (a figure that -agrees pretty well: with the one that the preceding method gave us), and that the career that remains to it to finish is estimated at two-thirds of this figure, say a total of twenty-five million years. Such will be the cycle of the organic stadium. the stadium of darkness, cold, and death”. The third and last stadium will have for a starting point the end of solar illumination, of sedimentation, and of the living world, and will terminate in a terrible catastrophe—the fall of the earth upon the extinguished globe of the sun. A new era, that of darkness, cold, silence, and death, will open for our planet. Our abode will no longer be anything but a frozen tomb, circulating noiselessly around another tom b likewise frozen—the extinct sun. An extraordinary, but not unforeseen, event will interrupt the monotony of this silent travel and will, for a few seconds, render heat and light to the dark globe; we mean the fearful cataclysm caused by the fall of our globe. Here we change guiding thread and leave aside the cooling, which has nothing more to give us, and substitute for it another factor of celestial me-- ch anics—gravitation. The secular accumulation of the lunar motion has for a long time been demonstrated, and astronomers of the last century justly occupied themselves with this frightful eventuality. It results, in fact, from Kepler's third law that a star which accelerates its travel shortens its orbit at the same time, so that, in the long run, it is sure to fall upon the body around which it gravitates. Such is the case of the moon with respect to the earth. Laplace reassured his contemporaries for an instant by demonstrating through mathematical analysis that the motion of our satellite is connected with variations in the eccentricity of the terrestrial orbit, and that the present accumulation will stop some day in order to become retrograde. But the calculations of the great geometrician render account of but half the value of this motion. A pro- foiinder study of thE! gearing of the cosmical machine has revealed the existence of a new agent that was unknown in Laplace's time. We mean the one hundred and forty or one hundred and fifty thousand millions of meteorites that annually traverse our atmosphere and cover the earth with their debris. We pass in silence those captured by the moon, although their number is not to be disregarded. However small these corpuscles be supposed, their dust, continuously falling upon the two stars, will eventually, in the course of ages, perceptibly increase their mass. According to the great Newtonian law, we know that two bodies attract each other in direct ratio of their masses, and in inverse ratio of the square of the distances that separate them. The planet and satellite will therefore move toward each other until they join. The smaller of the two globes will be crushed in clashing against the larger, the latter will return to incandescence through the conversion of the lost motion into heat, and the astronomers of neighboring planets will see a new star shining in the sky. The light will last but a few days or few weeks, and darkness, resuming its sway, will soon prevail over the transitory illumination. Starting from this moment, the globe will resume its silent course through space, having (gained by the catastrophe but an insignificent increase in mass and volume. How will it end its career ? Here we shall appeal to celes- 1EW FROM LAKE KLAMATH, LOOKING- NORTH TOWARD CRATER LAKE. tial mechanics, and the law of gravitation will give us the key to the enigma. As the earth, with respect to the sun, is what the moon is with respect to the earth, the course of the latter is naturally moulded after that of the satellite, and will end in the same catastrophe. If we reckon the meteorites that annually fall into the terrestrial atmosphere to num ber thousands of millions, it is by myriads of thousands of millions that it is necessary to number those that are engulfed in the solar atmosphere. The two stars are approaching each other, then (in an imperceptible manner, it is true, since it has hitherto escaped the attention of astronomers, but appreciable in the course of ages), as the continuous fall of corpuscles is increasing their mass, and consequently their attractive force. Hence the collision is certain. The earth will end its career by becoming crushed like a meteor on the surface of the extinct sun, which the violence of the impact will restore to incandescence for a few instants. The last act of the tellurian drama will be the appearance of a temporary star in the firmament. In the present state of our knowledge, it would be puerile to try to estimate the duration of the stadium that has just been described and that may be defined, as we have said, the age of darkness or eternal night, of cold and of death. Such a calculation cannot be undertaken until the day when we know precisely the secular acceleration of the motion of the earth around its focus of attraction. All that it is permitted us to advance is that, according to every probability, the duration of this stadium will be much longer than that of the preceding, and we think that we can, at the lowest, estimate it at one hundred million years, perhaps more. Upon the whole, the present age of the earth appears to be about sixteen million years. This is but a small part of its existence, and everything leads to the belief that its total evolution through the immensity of space will exceed a million of centuries.