In the High Heavens/Chapter 14

by Robert Stawell Ball

CHAPTER XIV.

THE ORIGIN OF METEORITES.

AVING thus given grounds for believing that meteorites have come neither from the comets nor from the moon, neither from the planets great nor the planets small, nor yet from the stars or the other orbs which may revolve at stellar distances, I have now to explain the source from which, according to evidence, the meteorites do seem to have been derived. I am, however, quite conscious that the question is by no means free from difficulty. The descent of a meteorite, whatever be its source, is one of the most astonishing facts in nature. The improbability that such an occurrence should take place is indeed great. But we know that it does take place, and the only course to follow in the search for the origin of these bodies is to bring under review all conceivable sources, and then to adopt that one which on the whole appears to have most in its favour.

It seems perfectly demonstrable that one suggested source of meteorites is much less improbable than any of those which have been discussed in the last chapter. We are to advocate a prosaic solution of the origin of these mysterious wanderers which reach us from the sky by affirming that they have originally been fragments torn from the earth. But that such is indeed the case I shall endeavour to demonstrate. I have no doubt to some extent discussed this matter in previous writings. I have found, however, that the force of the argument is often imperfectly understood, and as the evidence that can be adduced seems to be strengthening in proportion as our knowledge of these bodies accumulates, I have thought it desirable to take this opportunity of treating the question fully with the help of additional information which recent investigations have placed at our disposal.

The notion that terrestrial volcanoes should have been the agent by which meteorites have been driven off into space seems at first to be refuted by the circumstance that we have no volcanoes which possess at present anything like the intensity of explosive energy which would be required. There is, I believe, sufficient testimony to prove that certain bombs projected from Krakatoa in 1883 fell to earth at a distance of fifty miles from their source. It is easy to demonstrate that a body which performed a flight so tremendous must have been expelled from the crater with an initial velocity considerably in excess of any speed that has been attained by a missile in artillery practice. This statement offers a remarkable illustration of volcanic energy. To appreciate it fully we should reflect on the dimensions of a volcanic crater, and on the free opportunity it affords for the escape of the explosive gases, in contrast with the conditions found in the discharge of a missile from a cannon, where all the force of the explosion can be concentrated on the projectile.

It must, however, be admitted that there is no observation, so far as I know, of any volcanic explosion in which a velocity has been imparted to the fragments discharged exceeding twice that which can be produced by artillery. And yet, as we have shown, we should require a velocity many times as great as that which our mightiest cannons can generate, if the missile so discharged were to be carried free from the earth altogether. No doubt the inadequacy of our present volcanoes is a very great difficulty in the acceptance of the terrestrial theory of meteorites. But the improbability that meteorites should have so sprung appears to me to be very much less than the improbabilities which accompany the supposition that the origin of these bodies can be explained in any of the other ways which have been proposed. It seems therefore necessary to submit this notion of the terrestrial origin to an extremely close scrutiny, and see how far the objections that may be urged against it can be overcome.

I am quite aware that able geologists have maintained that there is no stratigraphical evidence to show that volcanoes of early geological times were more potent than those of the present day. But in so far as such evidence may be wanting I think we can only attribute it to that imperfection of the geological record which has been so often invoked to account for the absence of direct testimony in support of conclusions which there are good theoretical reasons for entertaining. It would seem that the present case is one in which the absence of direct testimony need not greatly concern us. The laws of cooling are not to be impugned, and those laws of cooling declare with unfaltering logic that the thermal conditions to which volcanoes owe their origin must have been such as to make volcanoes far more potent in primeval times while the earth was still young than they are at present.

Whether there be geological evidence bearing on the question, or whether such evidence be wanting, I do not think that the result of the laws of heat will be disputed by any one who is aware of the physical necessity which those laws impose. We can indeed cite analogical evidence from other bodies in the universe in support of our contention as to the superior efficiency of the early terrestrial volcanoes over their degenerate successors of the present day. I have already had occasion to refer to the striking if well-known fact that the lunar volcanoes are now quite extinct. The exhaustion of their primitive power is due to the circumstance that the moon is small enough to have cooled rapidly and thus to have lost almost all its internal heat. It therefore no longer retains the energy necessary for the production of volcanic outbreaks.

Here we have an instance which proves that the uniformitarian hypothesis is not satisfactory so far as lunar geology is concerned. If there were a geologist at present on the moon he would be constrained to admit that his globe was once the seat of volcanic activity of the most widespread character, and of a vehemence and potency the like of which could not be paralleled on any known globe. But while the evidence of this fact was all round him, he would be constrained to admit that his volcanoes had now lapsed into a state of permanent quiescence. So far from uniformity in volcanic matters being the characteristic of lunar geology, we find the volcanic activity of that globe gradually declining through a succession of ages until at last the present stage of absolute inertness has been reached. Uniformitarianism could not be the creed of any lunar geologist; nor can there be a reasonable doubt that in volcanic matters our earth is progressing along a like course to that which the moon has taken.

Some of our volcanoes still retain activity, because our globe is so large that it has not yet parted with all its initial fiery vigour, but day by day the internal heat is being dissipated. We are tending from a period when the volcanoes were perhaps as important a feature on this earth as they once were on the moon; we are tending towards a period when the volcanoes on this earth shall have become as silent and as extinct as are those craters on the moon, which make our satellite so interesting a telescopic picture. The analogy of the moon will at least justify my contention that the activity of the volcanoes now on our earth is not to be regarded as adequately representing the much more terrific vehemence of the internal heat which must have devastated our globe in anterior stages of its history.

We have next to consider how the volcanic energy of these early fiery mountains on the earth can have produced sufficient explosive power to project missiles into space with a speed at least as great as that critical value to which I have so often referred. It is unfortunate that there is no globe in our solar system which can be seen to be passing at present through the same phase as that through which our earth must have passed in the early times referred to. It would have been so very instructive to have made a telescopic examination of some other world of about the same size as the earth and in about the same stage in its evolution as this globe was before the deposition of the sedimentary rocks. But there is no such globe. Jupiter does not present the conditions, for we are not able to see its surface through the encompassing clouds. Mars will not answer our purpose, for it seems to have long passed the stage in which volcanic energy was a prominent manifestation on its surface. We cannot examine the details on Venus sufficiently to know how far it might supply the information we seek, and the same may be said of the other planets. They give us but little aid.

Fig. 40.—Solar Protuberance. April 6, 1892.

There is, indeed, only one globe in the system which can be cited, and that is the sun. Of course, there is but very slight resemblance between the present condition of the sun and the condition of our earth at the early period referred to. The sun is so highly heated that it probably does not contain any solid matter whatever, and therefore is very unlike what our earth must have been at the time of which we speak when its exterior parts had already become solid, if not actually cold. But there is one matter as to which the sun will render valuable evidence pertinent to the question at issue.

Fig. 41.—Solar Protuberance. April 15, 1892.

Remember that our main difficulty relates to the greatness of the velocity with which missiles must have been expelled by explosions that have taken place in the interior of the globe. We find a difficulty in believing that the nascent earth could have launched projectiles at the speed of seven miles a second. Let us then see what the sun can do.

It can be shown from observations of the solar prominences that vast masses of vapour are frequently expelled from the interior of the sun by convulsive throes with a speed of three hundred, four hundred, and sometimes nearly a thousand miles a second. Let us ponder on these facts. They are indeed most astounding, and would hardly be credible were they not supported by ample evidence of unimpeachable authenticity. The spectroscope enables the observer actually to witness the ascent of these solar prominences, notwithstanding that they are at a greater distance than ninety millions of miles. Careful measurements demonstrate the startling results which I have already stated. Of course, the present fervour and energy of the sun wholly surpasses that which could have been possessed by the earth at the phase of its history which we are considering. But remember that all we want is an explosive power in the terrestrial volcanic gases sufficient to impart a velocity of seven miles a second, nor need this be denied when we have shown that the actual velocities produced by outbreaks in the sun at this very moment are more than one hundred times as great as that modest allowance for which we ask.

Those who ponder on these facts will, I think, come to the conclusion that, though from our ordinary points of view a speed of seven miles a second seems too monstrous to be admitted, yet that the cosmical forces available at the time of the consolidation of the earth would have been sufficient for the purpose. It must also be remembered that we have no reason to believe there is any substantial difference between the materials in the sun and the materials of which the earth is composed. The gases by which explosions in the sun generate velocities of many hundreds of miles a second, are only the same gases as those which are found on the earth. It therefore appears that by the study of solar phenomena the objection to the terrestrial theory of meteorites on account of the great initial velocity required may be said to have been considerably abated, if not entirely overcome.

There is another objection which has been frequently urged against the hypothesis of the terrestrial origin of meteorites. We have been reminded of the existence of the earth's atmosphere, and attention has been called to the fact that a missile discharged from the lower strata of that atmosphere with an enormous speed would find its velocity greatly reduced by the resistance of the medium through which it had to cleave its way. It has thus been maintained that when the body reached the outer layers of the atmosphere it could not have retained sufficient speed to carry it away from the earth. I freely admit that this is a difficulty. Artillerists know well that the resistance of the air robs their projectiles of energy and greatly reduces the range and the effectiveness of their weapons. Nor am I surprised that this difficulty may have sometimes appeared to be a very formidable one when urged against the doctrine of the terrestrial origin of meteorites. There are, however, certain considerations which will, I think, show that the objections founded on the resistance offered by the air to the passage of projectiles may perhaps be over-estimated.

In the first place, it must be remembered that the analogy of artillery is hardly to the purpose, for the trajectory of the missile, if not quite horizontal, has at all events no great inclination. The consequence is, that the flight of the body is confined to the lower and denser strata of the atmosphere, where of course the resistance of the air operates much more vigorously. A cannon ball fired vertically upwards would meet with much less obstruction to its upward passage after it had passed through the lower portions of the atmosphere, and the amount of that obstruction would be diminished with every increase in the body's altitude. It is therefore plain that if a volcano were projecting missiles in a vertical direction, as the terrestrial theory of meteorites supposes, the resistance of the air would be much less effective than if the missile were projected in any direction largely inclined to the vertical.

There is also another consideration. It is well known that on the top of a mountain the air is less dense than at the foot. Suppose, therefore, the volcanic discharge takes place from the summit of a lofty mountain peak, the missile will then start on its vertical journey exposed to a resistance not very considerable at the commencement and lessening as each additional foot of elevation is attained. Now there is good reason to believe that in those primeval periods, when the meteorites were launched from the earth, the mountains were more elevated than they are at present. Here, again, I fear I shall have to reckon with the uniformitarian geologist. He will probably dispute this contention, and if he does so I confess that I know no direct facts which I can urge in support of my belief But my belief is founded on a reason which I hold to be so secure that, until some facts can be brought forward which demonstrate directly that these primeval mountains were not more lofty than those which we now have, I shall continue to think that our present mountains are but degenerate types of the mountains of antiquity.

The reason is a very simple one. The altitude of mountains depends primarily on the resultant of antagonistic agencies. There are the disturbances from below which push the mountains up, for were our earth solid and cold throughout its interior no fresh upheavals could arise. On the other hand, the disintegrating tendencies of air and water tend to reduce the elevations of the mountain summits. In the early days to which we refer the subterranean agents by which upheavals were produced were far more vehement than they are at present. I do not see any reason to think that there has been any corresponding large difference between the efficiency of the disintegrating energies then and their efficiency now. I therefore see no reason to doubt that in days so remote as those of which we are speaking the mountains may under certain circumstances have had greater elevations than any mountains of which we know. A volcano which has its vent in the summit of one of these exceptionally lofty mountains would discharge its missiles vertically, under conditions where the resistance of the atmosphere would be reduced to a mere fraction of that resistance which the artillerist so greatly abhors.

There are also other considerations which may be brought forward in relation to the question of atmospheric resistance. In a vast volcanic outbreak a prodigious volume of gases accompanies the more solid materials at least in the initial stages of their flight. The whole is projected aloft in such a way that the missiles moving upwards are surrounded by gases or vapours moving perhaps just as fast. In such a case the atmospheric resistance, so far as the missile is concerned, is reduced to a mere nothing.

In considering the objections arising from the resistance of the air to the terrestrial theory of meteorites, observations on the sun are very instructive. It is known that our great luminary is surrounded by a prodigious atmosphere which would expose to missiles projected through it a resistance enormously in excess of that which any bodies projected from the surface of the earth would encounter in their passage through our atmosphere. And yet we find that in some of the great solar eruptions in which metallic vapours are projected outwards, a prodigious elevation is frequently reached by these gaseous masses, notwithstanding the resistance of the medium through which they have to force their way. Indeed, observations have been made which seem to show that the velocities thus attained are frequently sufficient to force the emitted gaseous volumes completely through the encompassing atmospheric envelope, and even then leave them at the exterior of the solar atmosphere with sufficient speed to carry them entirely away from the sun on a voyage through space. Here, then, we see that the explosive forces on one body of our system are amply tufficient, not alone to overcome the retardation due to she resistance of its atmosphere, but to allow the missile to depart from its precincts with a speed which is nearly a hundred times as great as that which would be required to expel a projectile from our earth.

After attentive consideration of the various points which have now been discussed, it will, I think, be admitted that the great initial velocity which the terrestrial theory of meteorites demands can no longer be deemed an insuperable objection to its acceptance. We need not ask that the necessary violence shall have been manifested very often in comparison with the total number of volcanic outbreaks. All we demand is, that during the millions of years in which the crust of the earth has been consolidating there shall have been occasionally outbreaks of sufficient vehemence to have discharged clouds of fragments with such energy that they shall leave the earth, or rather the earth's atmosphere, with a velocity of not less than six miles a second. If this be granted, then the explanation of the origin of the meteorites is complete.

Each little particle after taking leave of the earth to commence its voyage through space, will be acted upon by the attraction of the sun. To a certain extent, of course, the attraction of the earth must also affect the movement of the body, but after a time it will generally happen that the attraction of the sun will become of paramount importance in the control of the movement. This will be obvious when it is remembered that the sun is more than three hundred thousand times as massive as the earth, and that therefore at equal distances the potency of the sun's attraction when compared with that of the earth is to be expressed by the same number. We know that the attraction varies inversely as the square of the distance, so that by the time the missile has reached a point which is about the five-hundredth part of the present distance between the earth and the sun, the attraction of these two bodies on the missile will be equal. In fact, long ere the little object had reached a distance which is as great as that between the moon and the earth, the sun's attraction would have surpassed the attraction of the earth.

Assuming that the body moved in a straight line with a speed of seven miles a second, it is easy to show that after eight or nine hours it would have passed more under the influence of the sun than it was under that of the globe from which it had taken its rise. It is, therefore, obvious that ere long the object would be so far affiliated to the sun that it would have practically renounced all connection with the earth, which would now mainly operate as a perturbing agent in its movement. The laws of dynamics assure us that under these circumstances the missile will continue to move in what is practically an orbit around the sun. The size of this orbit will depend upon the particular circumstances of the case.

Let us suppose that the initial velocity does not exceed a certain definite amount. Then it is plain that the figure in which the object is constrained to move must be an ellipse, the motion being performed in accordance with Kepler's laws, around the sun placed in the focus. The plane in which the little body moves as well as the time which the journey requires depends on the initial conditions as to speed and direction under which the projection took place. It is all-important for our present purpose to know that the orbit must cross the point from which the meteorite originally took its departure. Perhaps it would be more correct to say that it must pass in the vicinity of that point, for the attraction of the earth will no doubt slightly modify the initial circumstances of the movement, and indeed there are other perturbations which ought not to be overlooked. We may, however, conclude that in its voyages through space the meteorite pursues a track which generally or frequently crosses the earth's track. This conclusion is a most essential point, and in it lies the fundamental argument in favour of the terrestrial origin of meteorites.

Have we not seen the improbability that a meteorite projected from a minor planet, such as Ceres, shall fulfil the necessary condition of crossing the earth's track, for there are so many chances against it? Have we not also seen that a meteorite projected from a globe lying in the stellar regions would almost certainly fail even to cross the orbit of our earth, inasmuch as the probability of its doing so would be only one against many thousands of millions? When, however, we locate the volcanoes which are presumed to be the origin of meteorites on the earth, these improbabilities disappear altogether. Instead of the chances against the orbits of these bodies crossing the earth's track being millions to one, or thousands to one, the fact that they must generally or frequently do so has become a demonstrable certainty. Indeed, they would always do so were it not possible in certain cases for the existence of disturbances, such as would be produced by the passage of a meteorite near the moon, to derange the path so far as to warp the little body into a direction which passed slightly within or slightly without the earth's track, instead of directly through it. There is also the effect of planetary perturbation both on the orbit of the meteorite and on that of the earth to be borne in mind. But the probabilities of the situation have been entirely transformed when we consider the terrestrial theory of meteorites. The probabilities are now that the missile does fulfil the necessary condition of crossing the earth's track, while on the other theories the probabilities ranged themselves against this fundamental requirement. Every missile projected from a mighty primeval volcano on this earth with the requisite speed will be freely revolving around the sun in an elliptic track, and frequently crossing the earth's path.

We have learned from Tschermak the fundamental fact that in all probability meteorites owe their origin to some volcanic source. Adopting this as a fundamental position, I have given the astronomical grounds which appear to demonstrate conclusively that these volcanoes must have been on our own earth. I am, however, at once met by the objection, that the mineral substances found in meteorites are not those which mineralogists generally speaking recognise as the products of terrestrial volcanoes at the present day. I fully admit this, indeed it is well known that in many cases a meteorite can be shown by chemical analysis alone to be one of those bodies which has fallen from the sky. Not only the actual minerals which are present, but the form in which they are associated, are generally speaking so characteristic that the meteoritic character of a stone can often be unhesitatingly pronounced upon even though the fact of its fall is entirely unknown. This is a very great difficulty, and I should be inclined to regard it as an insuperable one were it not for a particular circumstance on which I must now dwell. No doubt it is quite true that the character of a mineral mass will, generally speaking, enable the experienced mineralogist to pronounce decisively whether the object is meteoritic or whether it is not. There is, however, one notable instance in which the ordinary rules for the diagnosis of a meteorite would have certainly proved fallacious. I allude here to the famous mass of so-called meteoric iron discovered by Nordenskjöld, at Ovifak, on the coast of Greenland. Great masses of metallic iron amounting collectively to many tons were found at the place which we have named. The occurrence of iron in a metallic state is a circumstance so unusual that specimens of this body were examined with particular interest. The iron was found to be mixed with nickel, thus producing the remarkable alloy generally recognised as characteristic of meteorites. This suggested that these Ovifak irons might have tumbled down from the sky, like other masses of iron-nickel alloy which were known to have done so. The conclusion was indeed quite a natural one when it was observed that the metal in question differed in no perceptible respect from that in undoubted meteorites.

Close examination on the actual sea-shore where these masses were found has, however, made it clear that the famous Ovifak irons have come, not from the heavens above, but from the earth beneath. There are a large number of pieces which lie imbedded in basalt, and as the surrounding rock has weathered away the lumps of iron have become exposed. It is also plain from the complete manner in which the irons are incorporated with the once molten lava that it would be absurd to attribute to them a meteoritic origin. Had lumps of iron-nickel alloy been lying simply on the basalt beneath, then of course they might have been concluded to be meteoritic with just the same logic as that by which a celestial origin was attributed to the great iron mass found by Pallas in Siberia. But it is perfectly plain that the Ovifak iron has once been surrounded by molten lava, for sections of some specimens exhibit in the most striking manner the way in which the iron graduates into the adjacent minerals.

A meteoritic origin of these masses would be only possible on a wholly preposterous hypothesis. To account for them on such a theory we should have to suppose that at a time when molten lava was poured forth from a volcano or was welling upwards from the earth's interior to the surface, it so happened that an unparalleled fall of tremendous iron meteorites plumped down from the sky just into that particular spot which the lava occupied. We need not entertain a supposition so widely improbable. It is perfectly plain that the Ovifak irons have not come down from above. We must adopt the other alternative that they have come out with the basalt from the interior of the earth.

This is a conclusion of an extremely instructive character. We may note that it derives confirmation from the discovery of Andrews that metallic iron, though no doubt in minute particles, was a constituent in some specimens of basalt obtained in the north of Ireland. Other circumstances corroborate the notion that iron is extremely abundant in the interior of the earth. The phenomena of terrestrial magnetism indicate that this element must be an important constituent in the earth's interior. The argument derived from the density of the earth also deserves notice. It has been established that the mean density of our globe is upwards of five times that of water. But even the heaviest rocks which lie on the earth's surface have a density scarcely three times that of water. Of course it must be admitted that the materials in the centre of the earth are exposed to prodigious pressure, but yet there is nothing we know as to the nature of granite, whether solid or molten, which would lead us to believe that any amount of pressure could elevate its density from three to five. The only way of accounting for the high density that the earth exhibits as a whole, is by the assumption that there must be vast metallic masses in the interior. Considering the well-known abundance of iron in the earth's crust it can hardly be doubted that this is the element which gives its high specific gravity to our globe.

Spectroscopic evidence as to the presence of iron elsewhere demonstrates its abundance. Iron exists in profusion in the sun and in many of the stars, so that taking all these matters into account it seems highly probable that this element abounds in the deep interior of our earth. This being granted there is now no difficulty in accounting for the presence of the great Ovifak masses. As the molten lava issued from some volcanic vent it swept forth mighty fragments of that iron whose abundance in the earth's interior is shown on other grounds to be so extremely probable.

There is also one other instance which may be adduced in which specimens of this peculiar iron-nickel alloy have been shown to be of terrestrial origin. There is a remarkable extinct volcano known as Coon Butte in Mexico. It is one of many others which indicate that the country which contains them was at one time intensely agitated by volcanic forces. All the evidences of volcanic activity are around, the congealed streams of lava are still to be seen, as well as the craters from which that lava has emerged. The circumstance in connection with this region which is important for our present purpose is the presence of a multitude of masses of metallic iron, which are strewn around. These irons so closely resemble undoubted meteorites both in appearance and in composition that it was perhaps not unnatural that they should have been deemed to have had a celestial origin, and the fact that these so-called visitors from the sky were strewed in and around the volcanic vents has been commented on as a remarkable coincidence.

With the knowledge obtained from the Ovifak iron, we can at once place what is doubtless the true interpretation on the interesting phenomena at Coon Butte. It is perfectly certain that these iron masses never came down from the sky. We do not doubt their resemblance to meteoritic bodies; in fact that there is such a resemblance is a part of our argument, but what we desire to point out is that they have been simply extruded from the earth itself in the course of a volcanic eruption. Their existence in their present situation forms an independent line of testimony to show that so far from the iron-nickel alloy being only found in genuine meteorites, it is distinctly a terrestrial substance appearing abundantly in certain volcanic localities. I would therefore look at Coon Butte as an illustration on a feeble and degenerate scale of one of the mighty volcanoes of ancient days.

I do not think, indeed suggest, that this particular volcano was one of those from which meteorites have been launched. Probably at the comparatively modern date when Coon Butte itself was active, its energy was of a far less potent description than that which would have been required for driving the missiles free from the earth. But it requires no great effort of the imagination to think of a primeval volcano charged with materials similar to those which now lie around Coon Butte and testify to its former activity. If it had projected those materials aloft with the necessary speed, they might well have been the meteorites which sometimes fall down here to-day.

In conclusion I will just recapitulate the main points of the argument on which we have been engaged in this chapter and the last. I have started from the doctrine put forward by Tschermak, and not, so far as I know, successfully impugned by any other mineralogist, that meteorites have had a volcanic origin on some large celestial body. I have examined the different globes that might possibly be presumed to be the source of the meteorites. I have shown that though the moon may once have been the parent of certain missiles, it cannot be reasonably held to be the source of the meteorites which now fall, inasmuch as the lunar volcanoes are now extinct, and the ejects of the volcanoes of a past epoch must, if they escaped at all, have fallen long ago or shortly after their ejection. I have then brought under review certain of the planets belonging to our system. I have shown how it would be extremely improbable for any missile projected from one of the smaller of these bodies ever to tumble on the earth. I have also shown that the orbits of the small planet are so situated that a very high velocity of projection would be necessary, in order to convey a missile from a volcano there to a resting-place here. I have shown the extreme unlikelihood that any missile discharged from a volcano, on a globe lying somewhere in the stellar distances, could ever reach this earth. I have shown this improbability to be so great, that even though there might be hundreds of millions of presumable globes throughout space, we cannot entertain the supposition that any missile starting from one of them could ever fall down here. I have shown that meteorites cannot be reasonably associated with comets, notwithstanding the undoubted alliance which exists between comets and shooting-stars.

In the search for the body which must be credited with the parentage of the meteorite I have come at last to our own earth. It is true that difficulties have been urged against the view that meteorites have been derived from the globe on which we live. I have endeavoured to remove these difficulties, and I believe I have shown that they are, at all events, far less considerable than those which are experienced when we endeavour to attribute the meteorites to any other source that has been alleged. The objections that are felt to the view of the terrestrial origin of the objects are threefold. They are, firstly, the excessive initial velocity which would be required; secondly, the interference which the resistance of the air exposes to the passage of a meteorite from this globe to outer space; and, thirdly, the circumstance that meteorites do not resemble the more familiar substances recognised as belonging to terrestrial volcanoes. I do not think I am over sanguine in the belief that, serious as these difficulties may seem, they can be overcome.

In the first place, with regard to the initial velocity, I have pointed out that velocities, even one hundred times as great as that which we require, are at this moment occasionally imparted by explosive outbursts of the sun. Why, then, may not eruptions possessing one-hundredth part of the power of those which we see in the sun have taken place in our earth in primeval days, shortly after it had attained a solid exterior, but while its internal fervour was more vigorous than it is at present? I have pointed out, that though a great column of gases and vapours, projected vertically from a volcano, may be exposed to tremendous atmospheric resistance at its exterior, it may yet contain missiles in its interior whose movements are but little interfered with. In illustration of this I cited the well-known fact, that notwithstanding the resistance of the solar atmosphere, metallic vapours are often projected through it, with prodigious velocity, to an altitude of scores of thousands of miles. Lastly, I have dealt with the argument that may be derived from the fact that the minerals at present most abundant on our earth are not those which come down in meteorites. It has, however, been demonstrated, both at Ovifak and at Coon Butte, that the iron-nickel alloy, which is above all other substances most characteristic of meteoritic masses, is a terrestrial mineral. It has been found in the two places I have named, under such circumstances as to prove that it must have been extruded from the earth's interior. It therefore seems that there can be no doctrine, with regard to the source of meteorites, which has the same probability in its favour as that which assigns to them an origin in volcanoes in a primeval condition of the earth.