* lit ie IY \ Te, ~ = | WY os a3 ie aes Se aor eo Pa God LON Wy, Se . ae ar . : ¢' i mee J . a cog Wem Vie Coe ats ht i iL a > Wt av 4,3 nl oe “gy \! % roa Ia 5° LD a “' ie Aan Sa pa" BY Mad sane tenant Sa ay? FY ih "al | it Is yi ae a 7 xe ; me fu a NMihy ited We ad ‘ , ay an sed FOr Ke Ede m VE bo 7 DISCUSSIONS A J . a ON CLIMATE AND COSMOLOGY, BY JAMES CROLL, LL.D., F.B.S., AUTHOR OF 2 Sianthgas AND TIME,’ ‘STELLAR EY OLUTION,’ ETC, LONDON: EDWARD STANFORD, _ 26 & 27, COCKSPUR STREET, CHARING CROSS, 8.W. 1889 oS Pie jee Lie a sige 6 i 2 re Palen * . A y : fs Tie ey + ge oie: ‘ 5.3 : pe ee wh aves sf Fy earl i- pale sf ee e + bn oa / iP 5 me Y : of ad ; i. . mag © ee REPLY 10 cRITIOS. oe rial * fom the temperature of the ground; for the ace of the ground in equatorial regions is kept at n is due to the intensity of the sun’s heat. It thus _ somes a very intricate problem to determine how | ( h the surface of the ground is kept below the ‘keeps the earth below and the air above at or near the freezing-point. When the snow becomes per- petual, as on the summits of high mountains, permanent cold is the result; and however strong the sun’s rays may be, the temperature of both the air and the earth cannot possibly rise much above the freezing-point. “This,” he says, “is illustrated by the often-quoted fact that at 80° N. lat. Captain Scoresby had the pitch melted on the one side of his ship by the heat of the sun, while water was freezing on the other side owing to the coldness of the air.” Doubtless this is perfectly correct; but on page 502 he states that he has pointed out with more precision than has, he believes, hitherto been done, the different effects on climate of water in the liquid and solid states. This is a somewhat doubtful statement ; for in Chapter IV. ‘Climate and Time, in “Phil. Mag.” March, 1870, and in other places will, I think, be — found all that this section contains. In fact the influence of snow and ice as a permanent sowrce of cold is one of the main factors of my theory. The three great factors are (1) the influence of snow and ice, (2) the influence of aqueous vapour, and (3) the influence of ocean-currents. How persistently has it been urged as an objection to my theory that, during the glacial epoch, the great heat of the perihelion summer would more than counterbalance the effect of the aphelion winter. But I have maintained that the summers, notwithstanding the intensity of the sun’s ‘MODIFICATION OF THEORY EXAMINED. 85 a rays, instead of being warmer than at present, would in reality be far colder; for this reason, that the temperature of a snow- and ice-covered country can never rise much above the freezing-point. As an example of this I pointed out that, ‘were it not for ice, the summers of North Greenland would be as warm as those of England (whereas in point of fact they are colder than our winters); and that were India covered with an ice-sheet, its summers would be colder than those of England.’ _ Another point,” he says, “of great importance in connection with this subject is the fact that this permanent storing-up of cold depends entirely on the annual amount of snowfall in proportion to that of the sun- and air-heat, and not on the actual cold of winter, or even on the average cold of the year.” This I have shown at considerable length in Chapter IIL, is one of the most widespread and fundamental errors within the whole range of geological climatology. Perpetual snow, instead of being due “entirely” to the annual amount of snowfall in proportion to the quantity of heat received by the snow, isin most cases not even maily due to this cause. The overlooking of the fact that in the conservation of snow the temperature of the snow itself is one of the main factors has been a fruitful source of error. High Land and Heavy Snowfall vn relation to the Glacial Epoch.—According to Mr. Wallace, “high land Bs and great moisture” are essential to the initiation of a glacial epoch. Undoubtedly high land and great moisture are the most favourable conditions for bringing about a glacial state of things; but I can B hardly agree with him that they are necessary and _ indispensable. As to the second of these conditions, great moisture 86 DISCUSSIONS IN CLIMATOLOGY. is required only in order to produce a great snowfall ; a great snowfall only in order that the snow may become permanent ; and the permanent snow in turn is only in order to-have permanent glaciation. But it has already been shown in Chapter III. that we frequently have permanent snow with a very light snowfall, even where the direct heat of the sun is excessive, as on the summits of lofty mountains. Greenland also has but a very small snowfall, and yet the snow and ice are there perpetual. What is necessary is, that the small amount which falls should not all melt. If this be the case, the ice will accumulate year by year, and a glacial condition will ultimately result. Suppose that the annual precipitation of snow on a continent is equivalent to only 10 inches of ice, and that at the end of each summer one inch remains — unmelted, then, in such case, the ice will continue to accumulate year by year until the quantity annually discharged by the outward motion from the centre of — dispersion equals that annually formed. But in the case of a continent, this condition can be attained only when the sheet at the centre becomes of enor- mous thickness. Whether high land be necessary to a glacial epoch or not, it is evident that a heavy snowfall is not an indispensable condition. As to the first of these conditions, namely High Land, it must be borne in mind that the question is not, Could the causes which are now in operation bring about a glacial condition of things without high land? but, Could those physical agencies brought into — operation during a high state of eccentricity produce a glacial state of things without high land? Mr. Wallace’s answer is that they could not. But I am not satisfied with the grounds on which he bases this i u stan ah peice G5 ore A eh oe oe ee he ae > dc® lar. ot opinion. A condition of the greatest importance, though one not absolutely necessary to the produc- tion of a glacial epoch, as will presently be shown, is the existence of perpetual snow. The question then is, Could not those physical agencies brought into ~ operation during a high state of eccentricity cover low lands with perpetual snow without the aid of high lands?, Mr. Wallace replies, “Perpetual snow nowhere exists on low lands.” Supposing this were true (I have endeavoured to show in the last chapter it is not), still it does not follow that perpetual snow may not have existed on low lands, or that, when the present condition of things changes, it may not yet exist. It is not difficult to conceive how, under certain conditions, the snow-line may in some places have been brought to the sea-level. In arctic, or even in subarctic regions, an excessively heavy snowfall, followed by piercingly cold winds from the north, during the whole of the summer months, would keep the snow at a low temperature, and certainly prevent it from disappearing. Keep the surface of the snow at or below the freezing-point, and melting will not take place, no matter how intense the sun’s rays may be. A strong wind below the freezing-point will cool the surface of the snow more rapidly than the sun can heat it. Another cause which would tend to keep the snow at a low temperature would be that, along with a cold northerly wind, there is usually a great diminution of aqueous vapour, thus allowing the surface of the snow to radiate its heat more freely into stellar space. For, were it not for the aqueous vapour in the atmosphere, as has been shown in Chapters II. and III, the snow-line, even at the - equator, would descend to the sea-level. Perhaps it is owing to the warm southerly winds of 88 DISCUSSIONS IN CLIMATOLOGY. the two midsummer months that Siberia, even with its inconsiderable snowfall, is not at the present day covered with permanent snow and ice. Mr. Wallace ‘mentions that “in Siberia, within and near the Arctic circle, about six feet of snow covers the country all the winter and spring, and is not sensibly diminished by the powerful sun so long as northerly winds keep the air below the freezing-point, and occasional snow- storms occur. But early in June the wind usually changes to southerly, and under its influence the snow all disappears in a few days.” But what would be the consequence were these northerly winds to con- tinue during the whole of June and July? It would probably be that the snow of autumn would begin to fall before that of spring had disappeared. Were this to result, the country would soon become covered with permanent ice. Matters would be still worse if these southerly winds, instead of ceasing, were simply to change from June and July to December and January, for then, in place of producing a melting effect, they would greatly add to the snowfall. The only Continental Ice on the Globe probably on Lowlands.—The only two continents on the globe covered by permanent ice and snow are Greenland and the Antarctic. But are these continents to be regarded as high lands or as low lands? Mr. Wallace maintains that they are high lands. “It is,” he says, “only where there are lofty mountains or elevated plateaus, as in Greenland, &e., that glaciers accompanied by perpetual snow cover the country. The north polar area is free from any accumulation of permanent ice, excepting the high lands of Greenland and Grinnell Land.” And in regard to the Antarctic continent he says, “The much greater quantity of ice at the south pole is undoubtedly due to the presence of a large MODIFICATION OF THEORY EXAMINED. 89 extent of high land.” Were it not for these extensive highlands and lofty mountains, Greenland and the Antarctic regions, according to Mr. Wallace’s theory, would be free from permanent snow and ice. He, however, nowhere, so far as I can find, offers any proof for the conclusion that those regions possess extensive high lands, elevated plateaus, and lofty mountains sufficient to account for these icy mantles. In the last chapter the subject has been discussed at considerable length, and the conclusions arrived at are diametrically the opposite of those advocated by Mr. Wallace, viz., that Greenland, and probably the greater part of the Antarctic regions, consists of land probably not much above sea-level, and that the mass of ice under which they are buried must be due to some other cause than elevation of the land. Permanent Ice may origvnate without Perpetual — Snow.—lIt is not necessary that, in order to have permanent ice, there should be perpetual snow. If snow softens or becomes partially melted and after- wards re-freezes, it is then far more difficult to melt than it was in its original condition. Half-melted snow, when re-frozen, resists the summer sun long . after the loose snow has disappeared. We have a good example of this amongst our Scottish mountains. In many places the frozen half-melted snow is permanent; and were the climate from any cause to become deteriorated its amount would yearly increase. In fact, it might go on increasing till not only all our Highlands but the greater part of the surface of Scotland might be covered with ice, long before the snow-line had descended below the level of the summit of Ben Nevis. 90 DISCUSSIONS IN CLIMATOLOGY. Modification of the Theory Examined. Mr. Wallace’s chief, and, I may say, only real modification of my theory is this. I give it in his own words :— “The alternate phases of precession—causing the winter of each hemisphere to be in aphelion and perthelion each 10,500 years—would produce a complete change of climate only where a country was partially snow-clad; while, whenever a large area became almost wholly buried in snow and ice, as was certainly the case with Northern Europe during the glacial epoch, then the glacial conditions would be continued, and perhaps even intensified, when the sun approached nearest to the earth in winter, instead of there being at the time, as Mr. Croll maintains, an almost perpetual spring.”—P. 503. “When geographical conditions and eccentricity combine to produce a severe glacial epoch, the changing phases of precession have very little, if any, effect on the character of the climate, as mild or glacial, though it may modify the seasons ; but when the eccentricity becomes moderate and the resulting climate less severe, then the changing phases of precession bring about a considerable alteration and even a partial reversal of the climate.”—P. 153. Again—“ It follows that towards the equatorial limits of a glaciated country alternations of climate may occur during a period of high eccentricity, while near the pole, where the — whole country is completely ice-clad, no amelioration may take place. Exactly the same thing will occur inversely with mild Arctic climates.”—P. 154. I have, on the contrary, maintained that the more severe the glacial condition of the one hemisphere, the warmer and the more equable would necessarily be that of the other; for the very same combination of causes which would tend to cool the one hemisphere would necessarily tend to warm the other. The process 4 Sat ue ee See MODIFICATION OF THEORY EXAMINED. 91 to a large extent consists of a transference of heat from the one hemisphere to the other. Consequently the one hemisphere could not be heated without the other being cooled, nor the one cooled without the other being heated. The hotter the one, the colder the other, and the colder the one, the hotter the other. It there- fore follows that the more severe the glacial conditions, the warmer and more equable must be the interglacial warm periods. But, according to Mr. Wallace, there could be no warm interglacial periods, either in temperate or polar regions, except during the com- mencement and towards the close of a glacial epoch. Before, however, proceeding to examine in detail the steps by which he arrives at this modification of my theory, it will be as well that the reader should have a clear and distinct knowledge of what that theory really is, and what it professes to explain. These I shall now briefly state in the most general terms, for misapprehension in regard to the main features of the theory lie at the root of most of the objections which have been urged against it. General Statement of the Theory—Ist. It is not professed that the theory will account for the condition of climate during all past geological ages. It treats mainly of the cause of the Glacial Epochs; and one of its essential elements is that these epochs consist of alternate changes, to a greater or less extent, of cold and warm periods; or, in other words, that glacial epochs must consist of alternate glacial and interglacial periods. The chief, though not the sole, aim of the theory is to account for geological climate in so far as such epochs are concerned. Although it could be satisfactorily shown, for example, and this has certainly not yet been done, that during some past geological age, such as the Miocene, the Eocene, or the Cretaceous, 92 DISCUSSIONS IN CLIMATOLOGY. the climate was throughout uniformly warm or sub- tropical, this would not prove that the theory was wrong, unless it could at the same time be shown that the necessary conditions demanded by the theory did then exist. But instead of this supposed condition of climate during Secondary or Tertiary periods being inconsistent with my theory, the fact is, as we shall see by and by, that this theory affords the only rational explanation of such a state of things which has yet been given. 2nd. The theory is not that a high state of eccentricity will necessarily produce a glacial epoch. No misapprehension has been more widespread or more difficult to remove than this. From the very commencement I have maintained that no amount of eccentricity, however great, could produce a glacial condition of things; that the Glacial Epoch was the result, not of a high state of eccentricity, but of a combination of Physical Agencies, brought into operation by means of this high state*. As an example of this misapprehension, how frequently has the present condition of the planet Mars been adduced as evidence against the theory. The eccentricity of Mars’ orbit is at present greater than that of the Harth’s even when at its superior limit; and its southern winter solstice is not far removed from aphelion. It is therefore maintained that, if my theory of the cause of the glacial epoch be correct, the southern hemisphere of Mars ought to be under a glacial condition, and the northern enjoying a perpetual spring—and this, as is well known, is not the case. Here it is assumed that, according to the theory, eccentricity alone ought to * For this reason I prefer to term the theory the Physical Theory rather than the Eccentricity Theory, as it has been called by some writers. : MODIFICATION OF THEORY EXAMINED. 93 produce a glacial epoch, irrespective of the necessary physical conditions. We know with certainty that those physical conditions which, according to the theory, were the direct cause of the glacial epoch on our globe, cannot possibly exist on the planet Mars. * Just take one example: either the properties of water on the planet Mars or the conditions of its atmosphere must be totally different from that of our earth; for were our earth removed to Mars’ distance from the sun, our seas would soon become solid ice, and we could have neither snow nor rain, ocean-currents, nor any of the necessary conditions for secular change of climate. This is doubtless not the present state of Mars; but the reason of this can only be that the physical and meteorological conditions of the planet must be wholly different from those of the earth. When we reflect that a very slight change in the properties of aqueous vapour, or in the condition of our atmosphere, would effectually prevent the pos- sibility of a glacial epoch occurring on our earth, notwithstanding a high state of eccentricity, we need not wonder that the planet Mars is not in a state of glaciation. But the eccentricity of Mars, though high, ‘is still far from its superior limit, and the planet may yet, for any thing which we know to the contrary, pass through a glacial epoch. 8rd. Another prevailing misapprehension is the supposition that the theory does not recognize the necessity for geographical conditions. In reading “Tsland Life” one might be apt to suppose that one of the chief points of difference between Mr. Wallace and myself is that he regards geographical distribution of sea and land as an important factor in a theory of geological climate, whereas I entirely ignore this * See ‘Climate and Time,’ p. 79. 94 DISCUSSIONS IN CLIMATOLOGY. condition. Nothing could be further from the truth — ~than such a supposition. I can boldly affirm that the necessity for geographical conditions is as truly a part of my theory as of Mr. Wallace’s modification thereof. One of the most important agencies, according to my view, is the enormous amount of heat conveyed from equatorial to temperate and polar regions by means of ocean-currents, and the deflection of this heat, during a high state of eccentricity, from the one hemisphere to the other. But all this depends on ocean-currents flowing from equatorial to polar regions; and the existence of these currents in turn depends, to a large extent, on the contour of the continents and the particular distribution of sea and land. Take, as one example, the Gulf-stream, a current which played so important a part in the phenomena of the glacial epoch. A very slight change in geographical con- ditions, such as the opening of communication between - the Gulf of Mexico and the Pacific, would have greatly diminished, if not entirely destroyed, that stream. Or, as I showed on a former occasion, a change in the form or contour of the north-east corner of the South- American continent would have deflected the great equatorial current, the feeder of the Gulf-stream, into the Southern Ocean and away from the Carribean Sea. One of the main causes of the extreme condition of things in North-western Europe, as well as in eastern parts of America, during the glacial epoch, was a large withdrawal of the warm waters of the Gulf-stream ; and this was to a great extent due, as I stated in my very first paper on the subject *, to the position of Cape St. Roque, which deflected the equatorial current into the Southern Ocean. That a * “Phil. Mag,” for August, 1864. MODIFICATION OF THEORY EXAMINED. 95 geographical distribution of land and water permitting of the existence and deflection of those heat-bearing currents is one of the main factors in my theory is what must be obvious to every reader of ‘Climate and Time.’ The difference between Mr. Wallace and myself is - this:—I maintain that with the present distribution of land and water, without calling in the aid of any other geographical conditions than now obtain, those physical agencies detailed in ‘Climate and Time’ are perfectly sufficient to account for all the phenomena of the glacial epoch, including those intercalated warm periods, during which Greenland would probably be free from ice and the Arctic regions enjoying a mild climate; while Mr. Wallace, on the other hand, maintains that without assuming some change in the geographical conditions of our globe those physical agencies will not account for that state of things, at least in so far as the disappearance of the ice in Arctic regions is concerned. To narrow the field of inquiry, and bring more prominently before the mind the real question at issue, I shall state the main points on which Mr. Wallace -and I appear to agree. Points of agreement—1. Mr. Wallace agrees. with me that a high state of eccentricity could never directly produce a glacial condition of climate; that the glacial epoch was the direct result, not of a high state of eccentricity, but of a combination of physical agencies brought into operation by means of this high state. 2. He agrees W ith me also in regard to what these physical agencies really were; for the agencies to which he ae im: his: t ielead Life” are almost identically those which I have advanced in ‘Climate and Time’ and elsewhere. 96 DISCUSSIONS IN CLIMATOLOGY. 3. Mr. Wallace agrees with me in regard to the - mutual reactions of the physical agents. He maintains with me that these physical agencies not only all lead to one result—the accumulation of snow and ice—but that their efficiency in bringing about this result is strengthened by their mutual reactions on one another. At pp. 137-139 he gives a variety of examples of these mutual reactions, and says that they “produce a maximum of effect which, without their aid, would be altogether unattainable.” 4, As has already been shown, we both agree as to - the necessity of certain geographical conditions for the production of the glacial epoch. For although © that epoch was mainly brought about by the physical agencies, yet these agencies could not have produced the required effect unless the necessary geographical conditions had been supplied, these being necessary for their effective operation. 5. Mr. Wallace admits, of course, that the necessary geographical conditions existed during the glacial epoch; for, unless this had been the case, no glacial epoch could have occurred. Therefore, all that was required to produce glaciation was an amount of eccentricity sufficient to set the physical agencies into — operation. Be it observed, it did not require, in addt- tion to the physical agencies, some changes in the geographical conditions, or some new conditions; for the geographical conditions being existent, all that was then required to bring about dhe olacial epoch was the operation of the physical agencies. The overlooking of this fact has led to much confusion. For example 210,000 years ago, with winter in aphelion, “the pro- blem to be solved,” says Mr. Wallace, “is, whether the — snow that fell in winter would accumulate to such an extent that it would not be melted in summer, and so Se ee ea r ‘ oe on. as , ‘cae : Fe i‘ | MODIFICATION OF THEORY EXAMINED. 97 _ go on increasing year by year till it covered the whole ee Scotland, Ireland, and Wales, and much of England. Dr. Croll and Dr. Geikie answer without hesitation E that it would. Sir Charles Lyell maintained that it would only do so when geographical conditions were favourable.” * Here we have a complete misappre- ‘ ein of the relation between Sir Charles Lyell’s _ views and mine; for I would certainly maintain (and, I _ presume, Dr. Geikic also) as emphatically as Sir Charles 4 could do, “that it would only do so when geographical _ conditions were favourable.” For undoubtedly, accord- ing to the theory advocated in ‘Climate and Time,’ no @ glacial epoch could result without geographical condi- _ tions suitable for the operation of physical agencies; and this is virtually what Sir Charles maintains. The glacial epoch resulted during the last period of high 4 eccentricity because the ceographical conditions suit- i able for the effective operation of the physical causes _ then existed. 6. It is assumed in ‘Climate and Time’ that, with the exception of those resulting from oscillations of sea-level, afterwards to be considered, the general distribution of sea and land, and other geographical nditions, were the same during the glacial epoch as they are at present.f Consequently, in accounting for ‘the glacial epoch I had only to consider the effects ae __* *Tsland Life,” p. 136. ‘Gi Prof. J. Geikie, however, believes that during early Postglacial times a considerable change in the physical geography of the North _ seas took place (see ‘‘ Prehistoric Europe,” chap. xxi.). In order to account for the floras of Greenland, Iceland, and the Farje Islands, thinks a land connection must have existed between these places d Scandinavia. For reasons which will be stated in a future apter I am somewhat doubtful on this point. There is, I think, important agent overlooked in the question of the distribution of Arctic flora and fauna. Prof. Geikie, however, does not believe that the climatic condition of that period was in any way due to this change. u 98 DISCUSSIONS IN CLIMATOLOGY. resulting from those physical agencies called into | operation by an increase of eccentricity. To have speculated on hypothetical geographical conditions different from those which now obtain, and on the influence which these may have had in bringing about the glacial epoch, would have been on my part per- fectly absurd, as I knew we had no evidence of the existence of any such conditions. Besides, my aim was to account for that epoch from known and established facts and principles, without the introduction of hypo- thetical causes. I fear that the fact of my making little or no allusion to geographical conditions in my explanations may have unfortunately led Mr. Wallace ~ and others to conclude that I altogether ignore, or, at least, undervalue their importance, which is certainly not the case. Although Mr. Wallace so frequently alludes to the importance of geographical conditions, I am not sure if he believes that during the glacial epoch those condi- tions differed materially from what they are at present, or that glaciation could have been greatly influenced by any difference which did exist. 7. Mr. Wallace alludes to one or two ceoorapiiadl conditions which, 17 they had existed during the glacial epoch, would have greatly aided glaciation; as, for example, if a land-barrier had extended from the British Isles, across the Farde Islands and Iceland to Greenland, cutting off from Northern Europe the warm waters of the Atlantic, including the Gulf Stream. “The result,” he says,“would almost certainly | be that snow would accumulate on the high mountains of Scandinavia till they became glaciated to as great an extent as Greenland.” It would be easy to multiply cases of this kind whee a distribution of land and water different from the a ve > 7 a ¥ n pee Maas ig ee Les e a oe . ae. = 7 Fah a WO Bada ts ee ee ae ee Ter . om 7 ; a ae | MODIFICA TION OF THEORY EXAMINED. 99 ~ - psa might have been more favourable to glaciation _ than the present; but the question is, Did any such difference favouring glaciation actually exist during _ the glacial epoch? ai have never been able to find any ieeence that it did. Many a change in geographical ~ conditions has taken place during Tertiary times, some of which were doubtless favourable to glaciation; but _ have we any evidence that during the glacial epoch _ the geographical conditions were more favourable than they are at present? Unless this can be shown to be _ the case, there is no necessity for referring to a differ- ence in geographical conditions during that epoch as a cause of glaciation. This being so, it does not follow, because in my explanation of the cause of the glacial a epoch I may not, like Sir Charles Lyell and others, _ have speculated on the effects which might have resulted had the distribution of land and water been different from what it is now, that I ought on this account to be charged with undervaluing the import- ance of geographical conditions. Trusting that these preliminary considerations may tend to remove the partial confusion in which this somewhat complex subject has been involved, I shall now proceed to examine Mr. Wallace’s main argument. ___I shall consider it, first, in relation to physical prin- he ciples, and, secondly, in relation to geological and - paleontological facts. CHAPTER VII. EXAMINATION OF MR. ALFRED R. WALLACE’S MODIFI- CATION OF THE PHYSICAL THEORY OF SECULAR CHANGES OF CLIMATE.—Continued. Physics in relation to Mr. Wallace’s Modification of the Theory.— Another Impossible Condition Assumed. — A Geographical Change not Necessary in Order to Remove the Antarctic Ice.—Other Causes than Antarctic Ice affecting the North- ward-flowing Currents. — Climatic Conditions of the Two Hemispheres the reverse Ten Thousand Years ago; argu- ment from.—Mutual Reactions of the Physical Agents in Relation to the Melting of the Ice.—Another Reason why the Ice does not Melt. I. Physics in Relation to Mr. Wallace's Modification of the Theory. The grand modification, that during the height of the glacial epoch the snow and ice would not dis- appear when precession brought the winter solstice round to perihelion, I have already given in Mr. Wallace’s own words. As the reasons which he 1 assions for this modification are very briefly stated by him, I may here give them also in his words. After describing the state of North-Eastern America and the North Atlantic, to which I have already alluded, he says:— “But when such was the state of the North Atlantic (and, however caused, such must have been its state during the height of the glacial epoch), can we suppose that the mere | change from the distant sun in winter and near sun in a MODIFICATION OF THEORY EXAMINED. 101 summer, to the reverse, could bring about any important alteration—the physical and geographical causes of glaciation remaining unchanged? For, certainly, the less powerful sun of summer, even though lasting somewhat longer, could not do more than the much more powerful sun did during the phase of summer in perthelion, while during the less severe winters the sun would have far less power than when it was equally near and at a very much greater altitude in summer. It seems to me, therefore, quite certain that when- ever extreme glaciation has been brought about by high eccentricity combined with favourable geographical and physical causes (and without this combination it is doubtful whether extreme glaciation would ever occur), then the ice sheet will not be removed during the alternate phases of precession, so long as these geographical and physical causes remain unaltered. It is true that the warm and cold oceanic currents, which are the most important agents in increasing or diminishing glaciation, depend for their strength and efficiency upon the comparative extents of the northern and southern ice-sheets, but these ice-sheets cannot, I believe, increase or diminish to any important extent unless some geographical or physical change first occurs.” * Again,—‘“‘It is quite evident that during the height of the glacial epoch there was a combination of causes at work which led to a large portion of North-Western Europe and Eastern America being buried in ice to a greater extent even than Greenland. Among these causes we must reckon a diminution of the force of the Gulf Stream, or its being diverted from the north-western coasts of Europe ; and what we have to consider is, whether the alteration from a long cold winter and short hot summer, to a short mild winter and long cool summer would greatly affect the amount of ice af the ocean-currents remained the same. The force of these currents is, it is true, by our hypothesis modified by the increase or diminution of the ice in the two hemispheres alternately, and they then react upon climate; but they * “Tsland Life,” p. 150. — 102 DISCUSSIONS IN CLIMATOLOGY. cannot be thus changed till after the ice accumulation has: been considerably affected by other causes.” * There are some further reasons assigned, which will be considered as we proceed. From what has already been shown, it will be seen that the causes which led to the glacial epoch may be classed under three distinct groups:—(1) the astro- nomical, (2) the physical, and (3) the geographical. This threefold division is distinctly recognised by Mr. Wallace in the above quotations, as well as in all his reasoning on the subject of geological climate. In the astronomical group the main elements are the two following :—Ist, A high state of eccentricity producing, on the hemisphere whose winter solstice happens to be in aphelion, a long and cold winter with a short and hot summer, and on the other hemisphere, whose winter solstice, of course, at the time is in peri- helion, a short and mild winter with a long and cool summer ; 2nd, Precession, transferring these conditions from the one hemisphere to the other alternately every 10,000 or 12,000 years. The physical elements are, of course, the influence of snow and ice, ocean-currents, aqueous vapour, clouds, fogs, and a host of other things which have already been discussed at length in Chapters Il. and III; while the geographical consist of the particular distribution of land and water, elevations or depressions in the sea-bottom, contour of the sea coast, and other geographical con- ditions influencing the flow of ocean-currents. It is to the influence of physical agencies, however, — that the glacial epoch is more directly due. The main function of the astronomical agents is to set and keep the physical agencies in operation, and also to deter- * - such conservative power in the ice, a glacial epoch — . - resulting from the causes which I have been advocating would not have been possible. This conservative 4 tendency certainly renders it more difficult for the iS _ physical agencies to get rid of the ice during inter- — periods; but we evidently have no grounds for _ assuming that it will defy their melting-powers. CHAPTER VIII. EXAMINATION OF MR. ALFRED R. WALLACE’S MODIFICA- TION OF THE PHYSICAL THEORY OF SECULAR CHANGES OF CLIMATE—Continued. Professor J. Geikie on Condition of Europe during Interglacial Periods.—Scotland during Interglacial Periods.—Difficulty in Detecting the Climatic Character of the Earlier Interglacial Periods.—Objection as to the Number of Interglacial Periods. —Objection as to the Number of Submergences.—Interglacial Periods less strongly marked in Temperate Regions than Glacial. II. Geological and Paleontological Facts vn relation to Mr. Wallace’s Modification of the Theory. Mr. Wallace’s chief, and indeed only real, modification of my theory, is to the effect, as I have pointed out, that the alternate phases of precession, causing the winter of each hemisphere to be in aphelion and perihelion each 10,500 years, would produce a complete change of climate only when a country was partially snow- clad. According to his view, when the greater part of North-western Europe was almost wholly buried under snow and ice, those glacial conditions must have continued, and perhaps have even become intensified, when the winter solstice moved round to perihelion, instead of being replaced, as I have main- tained, by an almost perpetual spring. In short, Mr. | Wallace’s conclusion is that, during the Glacial Epoch proper, a warm and equable Interglacial Period could not have occurred. MODIFICATION OF THEORY EXAMINED. 127 In the preceding chapter, I have endeavoured to show that physical principles do not warrant such a conclusion. I shall now proceed to consider what the direct testimony of Geology and Palzontology is on the subject ; and I believe we shall find that the facts - of Geology and Paleontology are as much opposed to the conclusion as are the principles of Physics. On this point I may quote the evidence of a geologist who, more than any other, has devoted special attention to all points relating to Glacial and Interglacial periods. Prof. J. Geikie, after devoting upwards of five hundred pages of his “Prehistoric Europe” to the consideration and accumulation of facts from all parts of this country and the Continent relating to Glacial and Interglacial periods, gives the following as the result of his investigations :— “We note,” he says, “as we advance from Pliocene times, how the climatic conditions of the colder epochs of the Glacial Period increase in severity until they culminate with the appearance of that great northern mer cle glace which overwhelmed all Northern Europe, and reached as far south as the 50th parallel of lati- tude in Saxony. Thereafter the glacial epochs decline -in importance, until, in the Postglacial Period, they cease to return. The genial climate of Interglacial ages probably also attained a maxvmum towards the middle of the Pleistocene Period, and afterwards became less genial at successive stages, the temperate and equable conditions of early Postglacial times being probably the latest manifestation of the Interglacial phase.” * I shall now quote the same author’s description of an Interglacial Period as demonstrated by its flora and fauna. The reader must, however, observe that, * “Prehistoric Europe,” p. 561. 128 DISCUSSIONS IN CLIMATOLOGY. by Pleistocene Period, Professor Geikie means the so- | called Glacial Period, with its alternations of severe Arctic climate and mild and genial conditions. * “ An examination,” he says, “of Pleistocene organic remains leads us to conclude that strongly-contrasted climatic conditions alternated during the Period. At one time an extremely equable and genial climate prevailed, allowing animals, which are now relegated to widely-separated zones, to live throughout the year in one and the same latitude. Hippopotamuses, elephants, and rhinoceroses, Irish deer, horses, oxen, and bisons then ranged from the borders of the Mediterranean as far north at least as Middle Eng- land and Northern Germany. In like manner, plants which no longer occur together—some being banished to hilly regions, while others are restricted to low grounds, and yet others have retreated to the extreme south of the Continent or to warmer regions beyond the limits of Hurope—lived side by side. The fig-tree, the judas-tree, and the Canary laurel flourished in Northern France along with the sycamore, the hazel, and the willow. And we encounter in the Pleistocene deposits of various countries in Europe the same remarkable commingling of northern and southern forms—of forms that demand a humid climate and are capable of enduring considerable cold, together with species which, while seeking moist conditions, yet could not survive the cold of our present winters. The testimony of the mammals and plants is confirmed by that of the land and fresh-water mollusca—all the evidence thus conspiring to demonstrate that the climate of Pleistocene Europe was, for some time at all events, remarkably equable and somewhat humid. * “ Prehistoric Europe,” p. 544. wey ee a rn MODIFICATION OF THEORY EXAMINED. 129 The summers may not indeed have been warmer than they are now; the winters, however, were certainly much more genial.” * This, be it observed, is a description of a condition of things which existed during an Interglacial Period belonging, not to the close, but to the very climax of the Glacial Epoch. For, immediately preceding and succeeding this Period, almost the whole of Northern Europe was enveloped in one continuous sheet of ice. “But if,’ continues Professor J. Geikie, “the evidence of such a climate having formerly obtained be very weighty, not less convincing are the proofs, supplied by the Pleistocene deposits, of extreme conditions. Think what must have been the state of Middle and Northern Europe when Paleolithic man hunted the reindeer in Southern France, and when the Arctic willow and its congeners grew at low levels in Central Europe. Reflect upon the fact that in the very same latitude in France, where at one time the Canary laurel and the fig-tree flourished, the pine, the spruce, and northern and high-alpine mosses at another time found a congenial habitat. Bear in view, also, that the land and fresh-water molluscs testify in like manner to the same strongly-contrasted climate. Besides those that tell of more equable and genial conditions than the present, there are species now restricted to the higher Alps and northern latitudes that formerly abounded in Middle Europe, and their shells occur commingled in the same deposits with the remains of lemmings, marmots, reindeer, and other northern and mountain-loving animals.” + But more convincing still is another range of facts, some of which have been adduced by Mr. Wallace * << Prehistoric Europe,” p. 540. ¢ Ibid, p. 541. 130 DISCUSSIONS IN CLIMATOLOGY. himself. In a section on alternations of warm and cold periods during the Glacial Epoch,* he says :— “The evidence that such was the case” (alternate warm and cold periods) “is very remarkable. The 4 ‘Till” as we have seen, could only have been formed when the country was entirely buried under a large ice-sheet of enormous thickness, and when it must therefore have been, in all the parts so covered, almost entirely destitute of animal and vegetable life. But in several places in Scotland fine layers of sand and gravel, with beds of peaty matter, have been found resting on ‘Till, and again covered by ‘Till’ Some- times these intercalated beds are very thin, but in other cases they are twenty or thirty feet thick, and in them have been found remains of the extinct ox, the Irish elk, the horse, reindeer, and mammoth. Here we have evidence of two distinct periods of intense cold, and an intervening milder period sufficiently — prolonged for the country to become covered with vegetation and stocked with animal life.” Let us now see to what all this leads. It has been proved beyond the possibility of a doubt that, at the time the Till was being formed which overlies the Scottish interglacial beds, the whole of Scotland, Scandinavia, the bed of the North Sea, and a great part of the North of England were covered with one continuous sheet of ice upwards of 2000 feet in thickness. This sheet overwhelmed the Hebrides, the Orkney and Shetland Islands, extended into Russia, filled the basin of the Baltic, overflowed Denmark and Holstein, and advanced into North Germany as far at least as Berlin. It has also been demonstrated that, — at the time the Lower Till was being formed which underlies these interglacial beds, North - western * “Tsland Life,” p. 114. f ” ‘4 ae - = . ~ - ‘ 2 Ey es Rat ee ee ee s z * Pa * aa i m4 ¥ % MODIFICATION OF THEORY EXAMINED, 131 Europe was under a still more severe state of glacia- tion. The ice-sheet at this time advanced farther south into England, and extended into North Germany as far as Saxony. It is perfectly obvious that this sheet must have destroyed all plant and animal life in Scotland ; and before the country could have become covered with vegetation and stocked with those inter- glacial animals to which Mr. Wallace refers, the ice must have disappeared and the climate become mild. Equally conclusive are the facts adduced by Mr. _ Wallace in reference to the interglacial beds of England. “In the east of England, Mr. Skertchly,” he _ says, “enumerates four distinct boulder-clays with intervening deposits of gravels and sands. Mr. Searles VY. Wood, jun., classes the most. recent (Hessle) boulder- clay as ‘Postglacial, but he admits an intervening warmer period, characterised by southern forms of mollusca and insects, after which glacial conditions again prevailed with northern types of mollusca. Elsewhere Mr. Wood says :—‘ Looking at the presence of such fluviatile mollusca as Cyrena fluminalis and Unio littoralis, and of such mammalia as the hippo- potamus and other great pachyderms, and of such a - littoral Lusitanian fauna as that of the Selsea bed, where it is mixed up with the remains of some of those pachyderms, as well as of some other features, it has seemed to me that the climate of the earlier part of the Postglacial Period in England was possibly even warmer than our present climate; and that it was succeeded by a refrigeration sufficiently severe to cause ice to form all round our coasts, and glaciers to 2 99 accumulate in the valleys of the mountain districts. That these fauna indicate a warm and equable con- dition of climate is further evident from Mr. Wallace’s remarks :—“ The fact,’ he says, “of the hippopotamus 132 DISCUSSIONS IN CLIMATOLOGY. having lived at 54° N. lat. in England, quite close to ~ the time of the Glacial Epoch, is absolutely inconsistent with a mere gradual amelioration of climate from that time till the present day. The immense quantity of — vegetable food which this creature requires, implies a mild and uniform climate with hardly any severe winter; and no theory that has yet been suggested renders this possible, except that of alternate cold and warm periods during the Glacial Epoch itself. Thus the very existence of the hippopotamus in Yorkshire, as well as in the south of England, in close association with glacial conditions, must be held to be a strong corroborative argument in favour of the reality of an interglacial warm period.” I trust that Mr. Wallace has not been misled by ~ Mr. Wood’s unfortunate use of the term “ Postglacial” as applied to the Hessle boulder-clay. The Hessle boulder-clay as surely belongs to the Glacial Period proper as does the true Till of Scotland, which covers the Lowlands and overlies the interglacial beds of that country. It is the moraine profonde of the last mer dle glace, which covered the greater part of North- western Europe. The Upper Till of Scotland and the Hessle boulder-clay of England belong to. the same period. This has been clearly shown by Professor J. Geikie in his “Great Ice-Age,” chap. xxx. (2nd edit.), and in “ Prehistoric Kurope,” chap. xii., and elsewhere. The Hessle boulder-clay is, in short, a continuation of — the Upper Till of Scotland. The position of these Hessle beds to which Mr. Wallace refers, like that of the interglacial bed: f Scotland, is between two boulder-clays—the Hessle and the Purple boulder-clays, both of which indicate a period of extreme glaciation: only the Purple boulder-clay period was somewhat the more severe — hs MODIFICATION OF THEORY EXAMINED. 133 of the two. At both periods, the greater part of North-western Europe was buried under ice. We know that during the last great ice-period, which was undoubtedly the period of the Hessle boulder-clay, the ice-sheet reached in North Germany as far as Berlin; while during the period of the Purple boulder- clay it advanced to about Saxony. The accompanying chart, reproduced from ‘ Climate and Time,’ which was sketched out during the summer of 1870, shows pretty correctly the condition of North- western Europe both before and after the interglacial period referred to by Mr. Wood. The observations of Professor J. Geikie and Mr. A. Helland have since shown, however, that the Scandinavian land-ice did not pass over the Farde Islands, as represented in the chart; but the chart has, in almost every other parti- cular, been now proved: by geologists to be accurate. The chart exhibits in a striking manner the enormous amount of ice which must have been melted off the ground before the warmth of the interglacial period could even have commenced. The observations of Prof. Torrell, Dr. A. Penck, Prof. Credner, Prof. Berendt, Dr. Jentzsch, A. Helland, FF. Wahnschaffe, H. Habenicht, and other geologists, have shown that there are in North Germany three distinct boulder-clays—an Upper, Middle, and Lower, with two series of interglacial beds. In these inter- glacial beds have been found organic remains which evidently indicate a mild and genial condition of climate. The younger interglacial period (the one prior to the last great extension of the ice) in all probability corresponds to the last interglacial period of Scotland, England, and Ireland. Interglacial beds belonging to the same period have been found in Switzerland, Italy, Denmark, North America, and other HE PERIOD OF MAXIMUM GLAGIATION —— THE H JABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE DURING T The lines also represent the: actual direction of the 4 1 ¥ 4 1 ; ‘i it *. a = r << x > d 7 t = =) ” bs - . » > . . = o —_ ¢ ~ ~ 2 : 134 DISCUSSIONS IN CLIMATOLOGY. places, all indicating a mild and equable condition of climate. There is another class of facts, almost entirely over- looked, which will doubtless yet prove as conclusively — the warm character of interglacial periods. These facts will be referred to when we come to consider ~— the question of warm polar climates. It would be impossible at present to give even the briefest outline of the recent discoveries in regard to interglacial periods. But though this were possible it would be wholly unnecessary, as the facts which have already been adduced by Mr. Wallace himself are perfectly sufficient for our present purpose. If now it be true, as it undoubtedly is, that the Hessle boulder-clay of England belongs to the same age as the Upper Till of Scotland, and that the last warm interglacial period—when the Cyrena fluminalis and Umo littoralts, the hippopotamus, the Hlephas — antiquus, and other animals of a southern type lived in England—occurred between two glacial periods so severe as to envelop the greater part of North-western Kurope in a continuous sheet of ice, then this particular — interglacial period must have supervened during a high state of eccentricity, and not, as Mr. Wallace assumes, at a period subsequent to the Glacial Epoch proper, when the eccentricity had greatly diminished. This is obvious; for if the last great ice-sheet could have been produced without a high state of eccentri- city, then there seems no reason why the one preceding it should not also have been produced without high eccentricity. If so, then all the previous ice-sheets may in like manner have been so produced. For the difference in magnitude between the last and penulti- mate ice-sheets was not so great as to warrant the supposition of any considerable difference in the Paes MODIFICATION OF THEORY EXAMINED. 135 amount of eccentricity at the two periods when these ice-sheets were respectively developed. Im short, if the last great ice-sheet can be explained without the supposition of a high state of eccentricity, then there does not appear to be any real necessity for any theory of eccentricity in accounting for the Glacial Epoch. If we adopt the Physical theory of the cause of the Glacial Epoch, we are compelled to maintain that the last two great Ice-periods were the indirect results of a high state of eccentricity, and in this case we can hardly avoid the conclusion that the mild intervening period was due to the same cause. The occurrence of a mild interglacial period between the two ice-periods is directly in opposition to Mr. Wallace’s view—that during a high state of eccentricity the ice would not disappear but be continued. It is in perfect harmony, however, with that which I advocate; for during high eccentricity a mild and equable condition of climate, when the winters occur in perihelion, is as much a necessary result as a cold and glacial condition when they occur in aphelion. The facts of Geology thus to me appear, so far, to be as much opposed to Mr. Wallace’s modifications as are _ the principles of Physics. Difficulty vn detecting the Climatic Character of the Earlier Interglacial Periods.—lt follows according to theory that, other things being equal, the greater ‘the amount of eccentricity the more equable and mild will the interglacial periods be. It is probable, therefore, that some of the earlier interglacial periods were milder and more equable than the last. It may be difficult, in the present state of our knowledge, to prove this conclusion by direct geological and palzon- tological evidence; but, on the Shee eee it is certainly impossible to disprove it by that means, The absence 136 DISCUSSIONS IN CLIMATOLOGY. of deposits containing organic remains, indicative of a superior mildness of climate having obtained during early interglacial periods, cannot certainly be regarded as satisfactory evidence against the conclusion just referred to. When we consider the enormous pressure and destructive power of an ice-sheet some 2000 or 3000 feet in thickness grinding down the face of a country, our surprise is that so much evidence remains of even the last interglacial period. That so few relics of the flora and fauna of preceding interglacial periods have been preserved, is a conclusion which we might a priory anticipate. This fact has been clearly pointed out by Mr. Wallace himself, who says :—“If there have been, not two only, but a series of such alternations of — climate, we could not possibly expect to find more than the most slender indications of them, because each succeeding ice-sheet would necessarily grind down or otherwise destroy much of the superficial deposits left by its predecessors, while the torrents that must always have accompanied the melting of these huge masses of ice would wash away even such fragments as might have escaped the ice itself.” * When we pass beyond the limits reached by the ice-sheets of the Glacial Epoch, we may expect, of course, to find the remains of many of the plants and animals which lived during the earlier interglacial periods. But here, again, we encounter another diffi- culty ; for we have in this case seldom any means of determining the age to which these remains belong. Unless in relation to overlying and underlying boulder- clays, there seems in many cases no way of knowing to what interglacial period they ought to be assigned ; or, in fact, whether they are really interglacial. or not. If the remains in question indicated a condition of * «Tsland Life,” p. 118. MODIFICATION OF THEORY EXAMINED. 137 climate much milder than the present, the probability is that they would be classified as preglacial. I fully agree with Prof. J. Geikie, that many of those plants and animals of a southern type which have been regarded as preglacial are in reality of interglacial age. Objection as to the Number of Interglacial Periods. —It has been urged as an objection to the Physical theory of the Glacial Epoch, that, according to it, there ought to have been more interglacial periods than we have direct evidence of having actually occurred. I am doubtful as to the force of this objection. I do not think that there could have been more than about five well-marked interglacial periods during the entire Glacial Epoch ; three probably during the former half of the epoch, and certainly not more than two during the latter half. There would be a large interval between the two maxima of eccentricity of 100,000 and 200,000 years ago, when the alternations of climate would be comparatively moderate in extent. Besides, it is not correct to assume, as is generally done, that the interval between two consecutive inter- glacial periods is only 21,000 years; for the mean rate of motion of the perihelion during the Glacial Epoch ~ was considerably less than has been assumed. It will be seen from the Table of the Longitude of the Perihelion, given in ‘Climate and Time, p. 320, that it has taken the perihelion 231,000 years to make one complete revolution. If, therefore, we assume, what of course is not certain, that the mean rate of precession during the Glacial Epoch was the same as the present, then the rate of precession to that of the perihelion’s motion would, in this case, be as nine to one. The equinoxial point will take 25,811 years to make one revolution; but as the perihelion moves in the opposite direction, it will reduce the time taken by the point in 138 DISCUSSIONS IN CLIMATOLOGY. passing from perihelion round to perihelion to 23,230 years, which will represent the mean interval between two consecutive interglacial periods. But as the motion of the perihelion was very irregular, the length of the interval between the periods would, of course, differ considerably. When we consider how difficult it must be to detect in the drift covering glaciated countries even a relic of early interglacial deposits, and when, moreover, we remember that it is only within the past few years that geologists have begun to bestow any attention on the subject, it is certainly not surprising that direct geological evidence of so few interglacial periods has as yet been discovered. In England, geologists have, however, already detected evidence of three interglacial periods, with four or five ice-periods. In Germany, quite recently, two interglacial periods and three or more ice-periods have been recognised by competent observers. In Denmark there are four boulder-clays separated by intercalated beds of sand and clay. In severely -glaciated Scotland, where traces of former interglacial periods can hardly be expected, there have nevertheless been found in old preglacial buried | channels, and other sheltered hollows, three, four, and in some places five, boulder-clays, separated from one another by immense beds of sand, gravel, and clay. Some of these beds are found to be continuous for long distances. It is true that these intercalated beds have yielded few or no organic remains, but it may well be that further research will yet result in the discovery of more abundant fossils; for frequently the beds in question are too thick and too extensive to allow us to infer their subglacial origin. They do not in such respects resemble the deposits which have been accumulated by aqueous action under ice, but have MODIFICATION OF THEORY EXAMINED, 139 all the characteristics of deposits which have been laid down in lakes and lacustrine hollows. As some have already yielded organic remains, a more extended scrutiny will probably lead to the discovery of similar fossils in those beds which are at present believed to be unfossiliferous. Objection as to the number of Submergences.—It has also been urged as an objection to the Physical theory, that, according to it, there ought to have been a greater number of submergences of the land during the glacial epoch than is known to have taken place ; for according to the theory there ought to have been a submergence to a greater or lesser extent correspond- ing to each ice period. Submergence ought regularly to accompany glaciation, and emergence the disappear- ance of the ice; whereas geologists have detected only about three such periods. In reply to this objection, it will not do to estimate the number of submergences by the number of observed raised beaches or well-marked terraces. These in most cases must have resulted from a subsidence of the land, and not from a rise of the sea due to a displace- ment of the earth’s centre of gravity. Oscillations of sea level resulting from an alternate increase and decrease of ice would not likely produce well-marked terraces. In order to cut a terrace the sea must continue for a long period at the same level; but this could hardly be expected in the case of a rise resulting from the accumulation of ice; for, according to theory, the mass of the ice gradually increases till a maximum is reached, when it then begins as gradually to decrease. It is true that when the ice is near its maximum it will change but slowly, and when at the turning point it may remain stationary for centuries before any sensible decrease takes place. In this case it might in 140 DISCUSSIONS IN CLIMATOLOGY. some places leave its mark in the form of a terrace or a raised beach; but no doubt, such cases would be exceptional. The condition of things becomes further complicated by another cause referred to in ‘Climate and Time’ (p. 888), which will occasionally come into operation ; viz., a lowering of the general level of the ocean resulting from the abstraction of the ice, or a rise resulting from a general decrease of the ice. The submergences and emergences arising from dis- placement of the earth’s centre of gravity would of course leave evidence of their existence in the form of stratified deposits; but, as we have already seen, no one could possibly determine from such deposits the number of elevations or depressions of sea level which actually took place. I think it is probable, however, that some of the more recent well-marked changes of sea level, such as | those indicated by the Carse-clays and the submarine Forest-beds, were due to displacements of the earth’s centre of gravity. I am inclined also to believe that the rise of the land, or rather the lowering of the sea level during the interglacial or continental periods, in many cases, resulted from the same cause. If we admit, © with some geologists, that the sinking of the land was due to the weight of the ice, we shall have an explana- tion of glacial submergences; but such a theory will in no wise explain the elevation of the land during the continental periods. It is true, the removal of the ice might allow the land to regain its former level; but its removal could have no tendency to raise the land above that level. The whole matter of glacial submergence is too. obscure and complicated an affair to allow us to determine, with anything like certainty, how often the land might have been under water during the olacial epoch. MODIFICATION OF THEORY EXAMINED. 141 Interglacial Periods Less Strongly Marked in Tem- perate Regions than Glacial._—I quite agree with Mr. Wallace that the interglacial deposits never exhibit any indication of a climate whose warmth corresponded to the severity of the preceding cold. This, however, cannot be urged as an objection, for it is a result which follows as a necessary consequence from theory. It theoretically follows that the cold of the glacial periods will not only exceed in severity the heat of the inter- glacial, but will also be of longer duration. During the glacial periods extreme cold is the characteristic of the winters, which, owing to the presence of snow and ice, only becomes moderated, although, of course, considerably, during the summers. But, on the other hand, during interglacial periods mildness and equa- bility of temperature rather than heat are the characteristics both of summer and winter. That the cold of the glacial periods must have con- tinued longer than the warmth of the interglacial will, I think, be apparent from the following considerations. As long as a country remains permanently covered with snow and ice, the climate, as has been repeatedly shown, must continue cold, no matter what the direct heat of the sun may be. Astronomically considered, the interglacial periods are, of course, of the same length as the glacial—the mean length of which, during the Glacial Epoch, was about 11,600 years; but the cold of a glacial period would not, as we shall presently see, actually terminate at the end of the period, but would be continued on probably for cen- turies into the succeeding interglacial period. Suppose that during a glacial period the country is covered with a sheet of ice, which, during the continuance of the period, had accumulated to the thickness of 2000 or 3000 feet. All this enormous quantity of ice would 142 DISCUSSIONS IN CLIMATOLOGY. have to be melted off the ground before the warmth of the interglacial period would eommence. So long asa single inch of ice covered the surface of the country, the cold would continue. Ice, as we have seen, by chilling the air, induces fresh snow to fall; and, of course, it is only when the amount of ice annually melted exceeds that being formed from the falling snow, that a diminution in the thickness of the sheet would begin to take place. A real melting of the ice, and consequent decrease in the thickness of the sheet, would probably not commence till the astronomical and physical agencies in operation during the glacial period began to act in an opposite direction. In short, it would be the favourable conditions of the inter- glacial period that would effectually remove the ice; and it would be then, and only then, that the warmth would begin; while, again, at the close of the period, when the first inch of ice made its appearance on the surface of the country, the interglacial condition of climate would come to an end. The time required to remove the ice does not prevent an interglacial con- dition of climate; it only somewhat shortens its duration. There is another circumstance worthy of notice here. It is this: as the mild and equable character of the climate during interglacial periods resulted to a large extent from the enormous transference of equatorial heat, and its distribution over temperate and polar regions, the difference of climatic conditions between the subtropical and the temperate and polar regions would be less marked than at present; in other words, — the temperature would not differ so much with latitude as it does at present. This, as we have seen, is a con- ~ clusion which is fully borne out by geological and paleontological facts. CHAPTER IX. THE PHYSICAL CAUSE OF MILD POLAR CLIMATES. The Probably True Explanation.—Sir William Thomson on Mild Arctic Climates.—Mr. Alfred R. Wallace on Mild Arctic Climates. — Influence of Eccentricity during the Tertiary Period. THERE are few facts within the domain of geology better established than that at frequent periods in the past the polar regions enjoyed a comparatively mild and equable climate, and that places now buried under permanent snow and ice were then covered with a rich and luxuriant vegetation. Various theories have been advanced to account for this remarkable state of things, such as a different distribution of sea and land, a change in the obliquity of the ecliptic, a displace- ment in the position of the earth’s axis of rotation, - and so forth. The true explanation will, I feel per- suaded, be found to be the one I gave many years ago. The steps by which my conclusions were reached were as follows :— The annual quantity of heat received from the sun at the equator is to that at the poles as 12 to 4°98, or, say,as 12 to 5. This is on the supposition that the same percentage of rays is cut off by the atmosphere at the equator as at the poles, which, of course, is not the case. More is cut off at the poles than at the equator, and consequently the difference in the amount of heat received at the two places is actually 144 DISCUSSIONS IN CLIMATOLOGY. ereater than that indicated by the ratio 12 to 5. But, assuming 12 to 5 to be the ratio, the question arose what ought to be the difference of temperature between the two places in question on the supposition that the temperature was due solely to the direct heat received from the sun? This wasa question difficult to answer, for its answer mainly depended upon two con- siderations, regarding both of which a very consider- able amount of uncertainty prevailed. First, it was necessary to know how much of the total amount of heat received by the earth was derived — from the sun, and how much from the stars and other sources, or, in other words, from space. Absolute zero is considered to be —461° Fahr. The temperature of the equator is about 80°. This gives 541° Fahr. as the absolute temperature of the equator. Now, were all the heat received by the earth derived simply from the sun, and were the temperature of each place pro- portionate to the amount directly received, then the absolute temperature of the poles would be 53, of that of the equator, or 225°. This would give a difference of 316° between the temperature of the equator and that of the poles. According to Pouillet and Herschel, space has a temperature of — 239°, or 222° of absolute temperature. If this be the temperature of space, then only 319° of the absolute temperature of the equator are derived from the sun; consequently, as the poles receive from the sun only 7% of this amount of temperature, or 133°, this will give merely 186° as the difference which ought to exist between the equator and the poles. There is, however, good reason for believing that the temperature of space is far less than that assigned by Pouillet and Herschel - —that, in fact, it is probably not far above absolute zero. Therefore, by adopting so high a temperature CAUSE OF MILD POLAR CLIMATES. 145 as — 239°, we make the difference between the tempera- ture of the equator and that of the poles too small. Second, it was necessary to know at what rate the temperature increased or decreased with a given increase or decrease in the amount of heat received. It was well known that Newton’s law—that the change of temperature was directly proportionate to the change in the quantity of heat received—was far from being correct. The formula of Dulong and Petit was found to give results pretty accurate within ordinary limits of temperature. But it would not have done, in making my estimate, to take that formula, if I adopted Herschel’s estimate of the tem- perature of space; for it would have made the difference of temperature between the equator and the poles by far too small. Newton’s law, if we adopt Herschel’s estimate of the temperature of space, would give results much nearer the truth; for the error of the one would, to a large extent at least, neutralise that of the other. From such uncertain data it was, of course, impos- sible to arrive at results which could in any way be regarded as accurate. But it so happens that perfect accuracy of results in the present case was not essential ; all that really was required was a rough estimate of what the difference of temperature between the equator and the poles ought to be. The method adopted showed pretty clearly, how- ever, that the difference of temperature could not be less (although probably more) than 200°; but the present actual difference does not probably exceed 80°. We have no means of ascertaining with certainty what the mean annual temperature of the poles is; but as the temperature of lat. 80° N. is 4°°5, that of the poles is probably not under 0°. If the present difference be L 146 DISCUSSIONS IN CLIMATOLOGY. 80°, it is then 120° less than it would be did the tem- perature of each place depend alone on the heat received directly from the sun. This great reduction from about 200° to 80° can, of course, be due to no other cause than to a transference of heat from the equator to the poles. The question then arose, by what means was this transference effected? There were only two agencies available—the transference must be effected either by aerial or by ocean-currents. It was shown at considerable length (‘Climate and Time,’ pp. 27-30, and other places) that the amount of heat that can be conveyed from the equator to the poles by means of aerial currents is trifling, and that, consequently, the transference must be referred to the currents of the ocean. It became obvious then that the influence of ocean-currents in the distribution of heat over the globe had been enormously under- estimated. In order to ascertain with greater certainty that such had been the case, I resolved on determining, if possible, in absolute measure, the amount of heat actually being conveyed from the equator to temperate and polar regions by means ot ocean-currents. The only great current whose volume and tempera- ture had been ascertained with any degree of certainty is the Gulf Stream. On computing the absolute amount of heat conveyed by that stream, it was found — to be-more than equal to all the heat received from the sun within 32 miles on each side of the equator. The amount of equatorial heat carried into temperate and polar regions by this stream alone is therefore equal to one-fourth of all the heat received from the sun by 'the North Atlantic from the Tropic of Cancer up to the Arctic cirele.* Although the heating- * ‘Climate and Time,’ pp. 34, 35; ‘‘ Phil. Mag.,” February, 1870. d ‘a ate, a CAUSE OF MILD POLAR CLIMATES. 147 power of the Gulf Stream had long been known, yet no one had imagined that the warmth of our climate was due, to such an enormous extent, to the heat conveyed by that stream. The amount of heat received by an equatorial zone 64 miles in breadth represents, be it observed, merely the amount conveyed by one current alone. There are several other great currents, _ some of which convey as much heat polewards as the { Gulf Stream. On taking into account the influence of the whole system of oceanic circulation, it is not surprising that the difference of temperature between the equator and the poles should be reduced from 200° to 80°. From these considerations, the real cause of former comparatively mild climates in Arctic regions becomes now apparent. All that was necessary to confer on, say, Greenland a condition of climate which would admit of the growth of a luxuriant vegetation is simply an increase in the amount of heat transferred from equatorial to Arctic regions by means of ocean-- currents. And to effect this change of climate no very great amount of increase is really required; for it was shown that the severity of the climate of that region is about as much due to the cooling effect of the permanent snow and ice as to an actual want of heat. An increase in the amount of warm water entering the Arctic Ocean, just sufficient to prevent the formation of permanent ice, is all that is really necessary ; for were it not for the presence of ice the summers of Greenland would be as warm as those of England. Were the whole of the warm water of the Gulf Stream at present to flow into the Arctic Ocean, it would probably remove the ice of Greenland. Any physical changes, such as those that have been discussed 148 DISCUSSIONS IN CLIMATOLOGY. — on former occasions, which would greatly increase the volume and temperature of the stream and deflect more of its waters into the Arctic Ocean would, there is little doubt, confer on the polar regions a climate suitable for plant and animal life. At present the Gulf Stream bifurcates in mid-Atlantic, one branch passing north-eastwards into the Arctic regions, whilst the larger branch turns south-eastwards by the Azores, and after passing the Canaries re-enters the equatorial current. As the Gulf Stream, like other great currents of the ocean, follows almost exactly the path of the prevailing winds *, it bifurcates in mid-Atlantic simply because the winds blowing over it bifurcate also. Any physical change which would prevent this bifurcation of the winds and cause them to blow north-eastwards would probably impel the whole of the Gulf Stream waters into the Arctic seas. All this doubtless might quite well be effected without any geographical changes, although changes in the physical geography of the North lene might be helpful. These considerations regarding the influence of the Gulf Stream point to another result of an opposite character. It is this: if a large vncrease in the volume and temperature of the stream would confer on Green- land and the Arctic regions a condition of climate somewhat like that of North-western Europe, it is obvious, as has been shown at length on former — occasions, that a large decrease in its temperature and _ volume would, on the other hand, lead to a state of things in North-western Europe approaching to that which now prevails in Greenland. A decrease leads to a glacial, an increase to an interglacial condition of things. Sir William Thomson on Mild Arctic Climates * See ‘ Climate and Time,’ p. 213, CAUSE OF MILD POLAR CLIMATES. 149 In a paper read before the Geological Society of Glasgow in February, 1877, Sir William maintains also that an increase in the amount of heat conveyed by ocean-currents to the Arctic regions, combined with the effect of Clouds, Wind, and Aqueous Vapour, is perfectly sufficient to account for the warm and temperate condition of climate which is known to have prevailed in those regions during former epochs. The following quotations will show Sir William’s views :— “A thousand feet of depression would submerge the continents of Europe, Asia, and America, for thousands of miles from their present northern coast-lines; and would give instead of the present land-locked, and therefore ice- bound Arctic sea, an open iceless ocean, with only a number of small steep islands to obstruct the free interchange of water between the North Pole and temperate or tropical regions. That the Arctic sea would, in such circumstances, _ be free from ice quite up to the north pole may be, I think, securely inferred from what, in the present condition of the globe, we know of ice-bound and open seas in the northern hemisphere and of the southern ocean abounding in icebergs, but probably nowhere ice-bound up to the very coast of the -circumpolar Antarctic continent, except in more or less land- locked bays. “Suppose now the sea, eee cted by land from either pole to temperate or tropical regions, to be iceless at any time, would it continue iceless during the whole of the sun- less polar winter? Yes; we may safely answer. Supposing the depth of the sea to be not less than 50 or 100 fathoms, and judging from what we know for certain of ocean-currents, we may safely say that differences of specific gravity of the water produced by difference of temperature, not reaching anywhere down to the freezing point, would cause enough of circulation of water between the polar and temperate or tropical regions to supply all the heat radiated from the 150 DISCUSSIONS IN CLIMATOLOGY. water within the Arctic circle during the sunless winter, if air contributed none of it. Just think of a current of three- quarters of a nautical mile per hour, or 70 miles per four days flowing towards the pole across the Arctic circle. The area of the Arctic circle is 700 square miles for each mile of its circumference. Hence 40 fathoms deep of such a current would carry in, per twenty-four hours, a little more than water enough to cover the whole area to a depth of | fathom; and this, if 7°-1 Cent. above the freezing point, would bring in just enough of heat to prevent freezing, if in twenty-four hours as much heat were radiated away as taken from a tenth of a fathom of ice-cold water would leave it ice at the freez- ing-point. This is no doubt much more than the actual amount of radiation, and the supposed current is probably much less than it would be if the water were ice-cold at the pole and 7°:0 Cent. at the Arctic circle. Hence, without any assistance from air, we find in the convection of heat by water alone a sufficiently powerful influence to prevent any freezing-up in polar regions at any time of year.” * That an amount of warm water flowing into the Arctic Ocean, equal to that assumed by Sir William Thomson, along with the effects of clouds, wind, dew, and other agencies to which he refers, would wholly prevent the existence of permanent ice in those regions, is a conclusion which, I think, can hardly be doubted. It is with the greatest deference that I venture to differ from so eminent a physicist; but I am unable to believe that such a transference of water from inter- tropical and temperate regions could be effected by the agency to which he attributes it. Certainly the amount of heat conveyed by means of a circulation resulting from difference of specific gravity, produced by difference of temperature, must be trifling when. compared with that of ocean-currents produced by the * Trans. of the Geol. Soc. of Glasgow, 22nd February, 1877. CAUSE OF MILD POLAR CLIMATES. 151 impelling force of the winds. Take, for example, the case of the Gulf Stream. If. the amount of heat conveyed from intertropical regions into the North Atlantic, by means of difference of density resulting from difference of temperature, were equal to that conveyed by the Gulf Stream, it would follow, as has been proved,* that the Atlantic would be far warmer in temperate and Arctic than in intertropical regions. Taking the annual quantity of heat received from the sun per unit surface at the equator as 1000, the quantities received by the three zones would be respectively as follows :-— EOE tc eel) eRe oe SOOO Horrid zone... 2.6.16 O75 Temperate zone . . . 757 Meicrid Zone... ..)....., 404 Assume, then, that as much heat is conveyed from intertropical regions into the Atlantic and Arctic seas by this circulation from difference of specific gravity as by the Gulf Stream, and assume also that one half of the total heat conveyed by the two systems of circulation goes to warm the Arctic Ocean, and the other half remains in temperate regions, the following would then be the relative quantities of heat possessed by the three zones :— Atlantic intorrid zone. . . 671 4 in temperate zone . 940 He ) wtrioid:zone... <5). 2 766 There is a still more formidable objection to the theory. It has been demonstrated, from the temperature - soundings made by the ‘Challenger’ Expedition, + that the general surface of the North * Climate and Time,’ Chap. XI. ; ‘‘ Phil. Mag.,” March, 1874. { ‘Climate and Time,’ pp. 220-225 ; ‘‘ Phil Mag.,” September and December, 1875; ‘‘ Nature,” November 25, 1875. 152 DISCUSSIONS IN CLIMATOLOGY. Atlantic must, in order to produce equilibrium, stand at a higher level than at the equator: in other words, the surface of the Atlantic is lowest at the equator, and rises with a gentle slope to well nigh the latitude of England. This curious condition of things is owing to the fact that, in consequence of the enormous quantity of warm water from intertropical regions which is being continually carried by the Gulf Stream into temperate regions, the mean temperature of the Atlantic water, considered from its surface to the bottom, is greater, and the specific gravity less, in temperate regions than at the equator. In consequence of this difference of specific gravity, the surface of the Atlantic at latitude 23°N. must stand 2 feet 3 inches above the level of the equator, and at latitude 38°N. 3 feet 3 inches above the equator. In this case it is absolutely impossible that there can be a flow in the Atlantic from the equatorial to the temperate regions resulting from difference of specific gravity. If there is any motion of the water from that cause, it must, in so far as the Atlantic is concerned, be in the opposite direction, viz., from the temperate to the equatorial regions. All, or almost all, the heat which the Arctic seas receive from intertropical regions in the form of warm water comes from the Atlantic, and not from the Pacific; for the amount of warm water entering by Behring Strait must be comparatively small. It there- fore follows from the foregoing considerations that none of that equatorial heat can be conveyed by a circulation resulting from difference of specific gravity produced by difference of temperature. It is assumed as a condition in this theory that a submergence of the Arctic land of several hundred feet. must have taken place in order to convert that land CAUSE OF MILD POLAR CLIMATES. 153 into a series of islands allowing of the free passage of water round them. But the evidence of Geology, as was shown on a former occasion,* is not altogether favourable to the idea that those warm climates were in any way the result of a submergence of the polar land. Take the Miocene epoch as an example: all the way from Ireland and the Western Isles, by the Faroes, Iceland, Franz-Joseph Land, to North Greenland, the Miocene vegetation and the denuded fragmentary state of the strata point to a much wider distribution of Polar land than that which now obtains in those regions. Mr. Alfred R. Wallace on Mild Arctic Cluemates.— The theory that the mild climates of Arctic regions were due to an inflow of warm water from intertropical and temperate regions has also been fully adopted by Mr. Alfred R. Wallace. But, unlike Sir William Thomson, he does not attribute this transference of warm water to a circulation resulting from difference of density produced by difference of temperature, but to currents caused by the impelling force of the wind. Mr. Wallace shares in the opinion, now entertained _ by a vast number of geologists, that during the whole of the Tertiary period the climate of the north tem- perate and polar regions was uniformly warm and mild, without a trace of any intervening epochs of cold. According to him, there were no glacial or interglacial periods during Tertiary times. Im this case he, of course, does not suppose that the inflow of warm water into Arctic regions, on which the mild condition of climate depended, was in any way due to those physical agencies which came into operation during an interglacial period. Mr. Wallace accounts for the mild Arctic climate during the Tertiary period by the * “Geol. Mag.,” September, 1878. 154 DISCUSSIONS IN CLIMATOLOGY. supposition that at that time there were probably several channels extending from equatorial to Arctic regions through the eastern and western continents, allowing of a continuous flow of intertropical water into the Arctic Ocean. Mr. Wallace expresses his views on the point thus :-— “The distribution of the Eocene and Miocene forma- tions shows that during a considerable portion of the Tertiary period an inland sea, more or less occupied by an archipelago of islands, extended across Central Kurope, between the Baltic and the Black and Caspian Seas, and thence by narrower channels south-eastward to the valley of the Euphrates and the Persian Gulf, thus opening a communication between the North Atlantic and the Indian Ocean. From the Caspian, also, a wide arm of the sea extended during some part of the Tertiary epoch northwards to the Arctic Ocean; and there is nothing to show that this sea may not have been in existence during the whole Tertiary period. Another channel probably existed over Egypt into the eastern basin of the Mediterranean and the Black Sea; while it is probable that there was a communication between the Baltic and the White Sea, leaving Scandinavia as an extensive island. Turning to India, we find that an arm of the sea of great width and depth extended from the Bay of Bengal to the mouths of the Indus; while the enormous depression indicated by the presence of marine fossils of Eocene age, at a height of 16,500 feet in Western Thibet, renders it not improbable that a more direct channel across Afghanistan may have opened a communication between the West-Asiatic and Polar seas.” * My acquaintance with the Tertiary formations of — the globe, and with the distribution of land and water * ah 9 Pe, ie ey 7 OF 7 MELD POLAR CLIMATES, of the “ Flysch” of the Eocene formation, and the tter the date of the period when the Miocene erratics ere deposited in the icy sea near Turin. _ Some geologists have maintained that the climatic a onditions of the Tertiary period are utterly hostile to Eee Physical Theory of Secular changes of Climate. The very reverse, however, is the case ; for, as we have seen, several of the facts of Tei climate can be r of this thickness 158 will be equal to 9 feet. A layer of ice about 9 feet in thickness, therefore, is the total amount that gravity * < Antarctic Regions,” p. 16, > "ae y= y a 2 © | |. PROBABLE ORIGIN OF NEBUL#. — 303 greatest concussion), and would continue there while _ the blocks retained their solid condition. It is difficult in imagination to realize what the temperature of the surfaces would be at this moment. For, supposing the heat were uniformly distributed through the entire mass, each pound, as we have already seen, would possess 100,000,000,000 foot-pounds of heat. But as the greater part of the heat would at this instant be concentrated on the outer layers of the _ blocks, these layers would be at once transformed into _ the gaseous condition, thus enveloping the blocks and filling the interspaces. The temperature of the incan- descent gas, owing to this enormous concentration of heat, would be excessive, and its expansive force inconceivably great. As a consequence the blocks would be separated from each other, and driven in all directions with a velocity far more than sufficient to carry them to an infinite distance against the force of gravity were no opposing obstacle in their way. The blocks, by their mutual impact, would be shivered into smaller fragments, each of which would consequently become enveloped in incandescent gas. These smaller fragments would in a similar manner break up into still smaller pieces, and so on until the whole came to assume the gaseous state. The general effect of the explosion, however, would be to disperse the blocks in all directions, radiating from the centre of the mass. Those towards the outer circumference of the mass, meeting with little or no obstruction to their onward progress, would pass outwards into space to indefinite distances, leaving in this manner a free path for the layers of blocks behind them to follow in their track. Thus eventually a space, perhaps twice or even thrice that included within the orbit of Neptune, might be filled with fragments by the time the whole had assumed the gaseous condition, 304 DISCUSSIONS IN COSMOLOGY. — It would be the suddenness and almost instantaneity — with which the mass would receive the entire store of energy, before it had time even to assume the molten, far less the gaseous condition, which would lead to such fearful explosions and dispersion of the materials. If the heat had been gradually applied, no explosions, and consequently no dispersion, of the materials would have taken place. There would first have been a eradual melting; and then the mass would pass by slow degrees into vapour, after which the vapour would rise in temperature as the heat continued, until it became possessed of the entire amount. But the space thus occupied by the gaseous mass would neces- sarily be very much smaller than in the case we have been considering, where the shattered materials were first dispersed into space before the gaseous condition was assumed. Reason why Nebule are of such various Shapes— The latter theory accounts also for the various and irregular shapes assumed by the nebule; for although the dispersion of the materials would be in all direc- tions, it would, according to the law of chances, very rarely take place uniformly in all directions. There would generally be a greater amount of dispersion in certain directions, and the materials would thus be carried along various lines and to diverse distances ; and although gravity would tend to bring the widely scattered materials ultimately together into one or more spherical masses, yet, owing to the exceedingly rarified condition of the gaseous mass, the nebula: would change form but ca Reason why Nebule emit such feeble Light. —The feeble light emitted by nebulz follows as a necessary result fo the theory. The light of nebulee is mainly derived from glowing hydrogen and nitrogen in a ae on Be, oF : od, ‘de PROBABLE ORIGIN OF NEBULZ. 305 gaseous condition; and it is well known that these gases are exceedingly bad radiators. The oxyhydrogen flame, though its temperature is only surpassed by that of the voltaic arc, gives nevertheless a light so feeble as scarcely to be visible in daylight. Now, even supposing the enormous space occupied by a nebula were due to excessive temperature, the light emitted would yet not be intense were it derived from nitrogen or hydrogen gas. The small luminosity of nebulze, however, is due to a different cause. The enormous space occupied by those nebulz is not so much owing to the heat which they possess, as to the fact that their materials were dispersed into space before they had time to pass into the gaseous condition; so that, by the time this latter state was assumed, the space occupied was far greater than was demanded either by the temperature or the amount of heat received. If we adopt the nebular hypothesis of the origin of _ our solar system, we must assume that our sun’s mass, when in the condition of nebula, extended beyond the orbit of the planet Neptune, and consequently filled the entire space included within that orbit. Supposing Neptune’s orbit to have been its outer limit, which it evidently was not, it would nevertheless have then occupied 274,000,000,000 times the space that it does at present. We shall assume, as before, that 50 million years heat was generated by the concussion. Of course there might have been twice, or even ten times that quantity; but it is of no importance what number of years is in the meantime adopted. Enormous as 50 million of years’ heat is, it yet gives, as we shall pre- sently see, only 32 foot-pounds for each cubic foot. The amount of heat due to concussion being equal, as before stated, to 100,000,000,000 foot-pounds for each Y 308 DISCUSSIONS IN COSMOLOGY. & pound of the mass, and a cubic foot of the sun at his 4 present density of 1:43 weighing 89 lbs. each cubic foot must have possessed 8,900,000,000,000 foot- pounds. But when the mass was expanded to occupy 274,000,000,000 times more space, which it would do when it extended to the orbit of Neptune, the heat 4 possessed by each cubic foot would then amount to only 32 foot-pounds. In point of fact, however, it would not even amount to that ; for a quantity equal to upwards of 20 million years’ heat would necessarily be consumed in work against gravity in the expansion of the mass; all of © q which would, of course, be given back in the form of = heat as the mass contracted. During the nebulous condition it would not exist as heat, so that only 19 foot-pounds out of the 32 foot-pounds generated by concussion would then exist as heat. The density of the nebula would be only rsz#sr1s0 that of hydrogen at ordinary temperature and pressure. The 19 foot- pounds of heat in each cubic foot would nevertheless — ,, 7 be sufficient to maintain an excessive ae for there would be in each cubic foot only zzo000 of a grain. a of matter. But although the temperature would be excessive, the guantity both of light and heat in each cubic foot would of necessity be small. The heat being — a. only 7: of a thermal unit, the light emitted would certainly be exceedingly feeble, resembling very much the electric light in a vacuum-tube. : Heat and Light of Nebule cannot result from Con- 4 densation.—The fact that nebule are not only self- luminous but indicate the existence of hydrogen and nitrogen in an incandescent condition proves that they — a must possess a considerable temperature. And it is scarcely conceivable that the temperature could have = been derived from the condensation of their masses. PROBABLE ORIGIN OF NEBULZ. 307 When our sun was in the nebulous condition it no doubt was self-luminous like other nebule, and doubt- less would have appeared, if seen from one of the fixed stars, pretty much like other nebule as viewed from our earth. The spectrum would no doubt have revealed in it the presence of incandescent gas. At all events we have no reason to conclude that our nebula was in this respect an exception to the general rule, and essentially different from others of the same class. The heat which our nebula could have derived from condensation up to the time that Neptune was formed, no matter how far the outer circumference of the mass may originally have extended beyond the orbit of that planet, could not have amounted to over zovs000 of a thermal unit for each cubic foot; and the quantity of light given out could not possibly have rendered the mass visible. Consequently the heat and light pos- sessed by the mass must have been derived from some other source than that of gravity. We have further evidence that the heat and light of nebulze cannot have been derived from condensation. If there be any truth, as there doubtless is, in Mr. Lockyer’s view of the evolution of the planets, then the nebulz out of which these bodies were evolved must have originally possessed a very high temperature—a temperature so high, indeed, as to produce perfect chemical dissociation of the elements. In short, “the _ temperature of the nebule,’ as Mr. Lockyer remarks,* _ “was then as great as the temperature of the sun is now. Mr. Lockyer’s theory is that the metals and the metalloids, owing to excessive temperature, existed in the nebulous mass uncombined—the metals, owing to their greater density, assuming the central position, and the metalloids keeping to the outside. The denser * «Why the Earth’s Chemistry is as it is,” p. 55, 1877. 308 DISCUSSIONS IN COSMOLOGY. the metal the nearer would its position be to the centre of the mass, and the lighter the metalloid the nearer to the outside. Asa general rule the dissociated elements would arrange themselves according to their densities; and it is for this reason, he considers, that the outer planets Neptune, Uranus, Saturn, and Jupiter, are less dense than the inner planets, since they must have been formed chiefly of metalloids, while the inner and more dense planets would consist chiefly of metallic elements. “The hypothesis,” says Mr. Lockyer, “is almost worthless unless we assume very high temperatures, because unless you have heat enough to give perfect dissociation, you will not have that sorting-out which always seems to follow the same law.” But the heat which produced this dissociation previous to the forma- tion of the planets could not have been derived from the condensation of the nebula; for the quantity so derived prior to the existence of the outermost planet must have been infinitesimal indeed. The heat exist-— ing in the nebula previous to condensation must have — come from some other source; and we can conceive of no other save that which we have been considering. The Gaseous State the first Condition of a Nebula.— If the foregoing be the true explanation of the origin of nebule, it will follow that the gaseous state will in most cases be the first or original condition, and that a nebula giving a continuous spectrum will only be found after it has condensed to a considerable extent. The irresolvable nebulze which exhibit bright lines, in all probability consist, as Mr. Huggins maintains, of glowing gas without anything solid in them. In short, they are nebule in their first stage of development, and have not as yet condensed sufficiently to become possessed of nuclei. If we adopt the generally accepted ; ¥ \ = { : - aes ) met « ‘ \ § “om pha as PoE Pe - 6 — 262 Gastaldi, M., on glacial deposits in . Miocene beds, 172 Pe: _ Geikie, Prof. A., on large faults, J., on climate of 269 Geikie, Prof. « Pleistocene times, ~ 127 , on post - glacial : elevation, 97 — zs on distribution of Arctic flora, 197 = on land - connec- tion between Europe and Farées, 197 s on tufa of Europe, 187 8 on Carse-clays, 114, 115 INDEX. 319 Geike, Prof. J., on slopes down which icemoves 244 ns cited, 116, 132 Geographical conditions, misap- prehensions regard- ing, 104, 106 3 conditions necessary for glaciation, 165 a conditions part of Physical Theory, 93, 94 Geological facts in relation to modification of theory, 126 Georgia, South, why snow is perpetual, 51 Germany, mammoth remains found in, 186 SS glaciation of by Scandi- navian ice-sheet, 169 Giescke, M., on Greenland, 69 Glacial epoch, condition of Europe at the time, 129, 132 x three main causes of, 102 ie might be produced without very high eccentricity, 52 a ice of, 246 ~ how affected by geographical con- ditions, 165 Be dates of, derived from eccentricity, 174 Glacial periods of Tertiary age, 164 Glaciation not the result of eccentricity alone, 165 Glaciation, M, Woekiof on cause of, 57 Glacier-motion, heat transformed into, 256 i regelation as a cause of, 248 Glen Brora, moraine of, 116 Glen Torsa, moraines on 30 feet beach, 116 Graham Land under glaciation, 113 Gravitation, theory of sun’s heat irreconcilable with geological facts, 291 320 Gravitation alone cannot account for sun’s heat, 299 Be alone cannot originate suns or nebule, 300 Gravity, a cause of descent of glaciers, 254 Greenland, heat received from the sun, 48 5 coldness of air during summer, 84 a how its ice may be removed, 147 a3 misapprehensions re- regarding, 81 ". why the summers are cold, 54 3 free from ice during any of the inter- glacial periods, 195 5 Nordenskjold on ab- sence of boulders in strata of, 170 . rainfall and snowfall in, 237 = ice of, 64 a whale in Carse-de- posits, 116 a Rink, Heyes, Brown, &c., on, 64-66 ae glaciers, high velocity of, 239 Gulf Stream, influence in distribu- tion of heat, 198 a absolute amount of heat conveyed by, 146 ABENICHT, H., on German inter-glacial beds, 133 Haeckel, Prof., on age of the earth, 289, cited, 291 Hamilton, Sir William, definition of Cause, 103 Hampshire deposits, 163 Haughton, Prof., on change of axis of rotation, 4 cited, 294, 295 Heat of Aérial currents, 23 ,, cut of by the atmosphere, 37 », trapped,” 45 », rays ‘‘sifting” of, 44 INDEX. Heat evolved by freezing, 45 » asa cause of glacier motion, », absoluteamountconveyed by Gulf-stream, 146 Hedenstrom, M., on the mammoth, 179, 181 Helix schrencki, 183 Helland, Mr. es on German inter-_ glacial beds, 133 ee on Scandinavian ice-sheet, 133 Ps on density of ice, 209 ES on rate of motion of Greenland ice, 239 34 on thickness of ice-sheet of Nor- way, 247 Helmholtz, theory of origin of sun’s heat, 265 Bi formula of condensa- tion of sun’s mass, 265- —" 53 cited, 282, 290 Herschel, Sir W., cited, 309 Hessle boulder-clay of Mr. Wood, 131 », not post-glacial, 132 Heyes, Dr., on Greenland ice, 64 High Land in relation to glaciation, 85 Hill, Rev. E., evaporation in re- lation to glaciation, 23 ss on change of axis of rotation, 5 ss answer to question, ce objection sidered, 46 £2 cited, 48 Hippopotamus in England, 134 Hooker, Sir Joseph D., theory of Antarctic i ice, 75 5 on tropical plants, 10 es on Antarctic ice, Lips cited, 76 Hopkins, Mr., on internal heat, 2 Howorth, Mr. , on the mammoth in Siberia, 178 con- ca 4 - Howorth, Mr., on mammoth in . Europe, 186 “4 on evidence from . shells, 183 e on plants of Can- stadt, 189 ne cited, 180, 182, 187 Huggins, Mr., on nebula, 308 Hull, Prof., on great Irwell fault, 268 Hutton, Dr., on expansive force of ice in freezing,. 215 Hypothesis, reason for avoiding, 26 CE of Greenland and Antarctic continent, 64 », at S. Pole, why it is thick, », another reason why it does not melt, 123 »» permanent without perpetual snow, 89 », melting point lowered by pressure, 215, 227 »» melting, resultingfrom mutual reaction of the physical causes, 122 », paleochrystic, 77 = ,, cliffs of America, 192 ____ ,, how moved up hill slopes, 255 Icebergs, Antarctic, peculiar ice formation of, 70 Ds testimony of, 207 Ae. » found in SouthernOcean, ; 207 a of Southern Ocean, enor- mous size of, 208 ee mean density of, 209 & estimates of their thick- ness, 209 “4 height of, determined by their mean density, 210 - Ice-sheet, Antarctic, conditions of, 202 ofnorth-western Europe, extent of, 246 interior of, warmer than surface, 217 FR ” & : ¥ ” across valleys, 255 INDEX. 321 Ice strata, variations is not due to pressure or melting, 231 4s thinning out of, due to dispersion, 232 Incandescence of Nebulous masses, how produced, 284 Indiga, port of, 182 Interglacial beds of Scotland, 130 A of England, 131 in of Germany, 133 at of Switzerland, 133 Interglacial climate, characteris- tics of, 185 Ms periods in Siberia, 184 Interglacial periods in Greenland, 195 A of Pleistocene times, 127 fed interval _be- tween, 137 om difficulty in : detecting cli- matic charac- ter, 135 - objection as to the number of, 137 is probably five, 137 a number de- tected in Germany, Denmark, &c., 138 ” shorter than Glacial, 141 at less marked in temperate regions than Glacial, 141 Intertropical water, effect on polar climate, 165 Internal heat of the earth, 2 AMIESON, Mr. on ice mark- ings on Schiehallion, 212 Jenson, on Greenland ice, 64 . explanation of passage | Jentzsch, Dr., on German Inter- glacial beds, 133 322 AMSKATKA, mammoth found there, 179 Kotzebue Sound, ice-cliffs of, 192 Krestowkoje, trees at, 183 A CELLE, tufa of, 189 Land-connection with the Farodes, objection to, 199 », between Europe, Fardes, and Greenland, 197 Land-ice theory, objections to con- sidered, 246 Land-surface, absence of in Terti- ary strata, 168 Langley, Prof., on temperature of space, 20 ss observations at Mount-Whit- ney, 20 Be remarkable con- clusion by, 32 Le Coq., Prof. H., on change of climate, 3 Lena, river, there, 179 Leverrier’s formuleof eccentricity, 38 Liachof Archipelago, found there, 179 Life on earth, testimony of geology as to age of, 266 ay age of, 299 LInimaz agrestis, 184 LTimnea auricularia, 183 Lockyer, Mr., on evolution of the planets, 307 _ cited, 309 Lyell, Sir Charles, on change of climate, 3 5 on geographical ditions, 97 A on Mississippi, 275 mammoth found mammoth con- ‘CLURE, Capt., Arctic trees, 193 M‘Farland, Prof., on eccentricity of earth’s orbit, 38, 39 Mackenzie River, 192 Maak, M., shells found by, 183 INDEX. Main characteristics of Interglacial climate, 185 Mammoth in Siberia, 178. i », Hurope, 186 e Interglacial, 184 a Glacial as well as Inter- glacial, 190 he Dr. Rae, on, 191 Mars, why not under glaciation, 92 Mecham, Lieut., on Arctic trees, 193 Meech, cited, 48 Melville Island, wood found at, 193 . Merklin, Prof., on limit of wood, 183 Meyer, Dr., meteoric theory, 265 Mild polar climates, 143 a Mr, ~ ASE Wallace on, 153-157 * 5 “A due to inter- tropical water, 166 Miocene glacial deposits of Italy 172 99 period, date of, 174 -Misconception, fundamental, 46 Mississippi, amount of sediment carried by, 275 Moisture in relation to glaciation, 85 Mollusca of the tufa, from, 190 Montpellier, tufa of, 188 Moret, in the valley of the Seine, deposit near, 188, 190 Mosley, Canon, on insufficiency of gravity to shear ice, 248 Motion in space, 291 primal cause of sun’s light and heat, 287 evidence i) 2? Motion of stars, difficulty of estimating, 313 ) > our knowledge ~ of, dependent on their direc- tions, 313 Mount-Whitney expedition, 20 Mountains, carved out by denuda- tion, 278 Mousson on the lowering of the melting point of ice, 215, 227 Sail so = ~ . wt 3 i Mutual reaction of physical agents, 52, 54, 120 - Mutual reaction of physical agents, misapprehension regarding, 52 Mytilus edulis at Spitzbergen, 194 — probable origin of, 297 as reason why they occupy so much space, 301 lye why of such various shapes, 304 be why they emit such feeble light, 304 x heat and light can- not result from condensation, 306 = gaseous, first condi- tion of, 308 + irresolvability due to gaseous condi- tion, 308 a Prof. Tait’s sugges- tion as to nebula, 310 Newcomb’s, Prof., reason for con- sidering his objec- tions, 18 . objections to my terms, 19 - on aérial currents, 23, 24 i on Prevost’s theory of exchanges, 31 Neumayer, Dr., on drift currents, 1 Newton’s law of radiation, 22 » in relation to tempera- ture, 145 ‘*Noashina,” 181, 193 Nordenskjéld, Prof., on absence of boulders in Green- land strata, 170 > on Spitzbergen in- terglacial bed, 194 $s on interglacial de- posits, 194 Northward-flowingcurrents, causes affecting, 112 Norther Siberia, mammoth in, 178 former limit of wood in, 180 INDEX. 323 North-Western Europe, path of ice in, 133 Norway, ice-sheet of, 247 BJECTIONS to theory of stellar heat and light considered, 310 Obliquity of ecliptic, change of, 4,115 Ocean-currents, influence of, in Arctic regions, 8 Orkney Islands, glaciation of, 169 Osborn, Capt., on Arctic trees, 193 ALZONTOLOGY in relation to modification of theory, 126 Paleochrystic sea, 77 Parallax of stars, few known, 314 Payer,Commander Julius, on Franz ; Josef Land, 76 »» ON permanent ice at sea level, 76 Peach, Mr. B. N., on great fault between Silurian and O.R. sand- stone, 269 Penck, Dr. A., on German inter- glacial beds, 133 Perihelion summer not hot, 53 Permanent ice without perpetual snow, 89 Physics in relation to Mr. Wal- lace’s modification, 100 Physical conditions of Antarctic ice, 202 Physical agents in relation to melting of ice, 119 e mutual reaction of, 52, 119 Physical theory, no hypothesis, 16 - three factors of, 84 ve modification exa- mined, 89 ag general statement of, 91 sr does not account for all condi- tions, 91 misapprehen- sion regarding, 92 93 a 324 Physical theory, why so named, 92 “5 recognises geo- graphical con- ditions, 93 a points of agree- ment, 95-97 Pisidium fontinale, 184. Planets, elements of, 308 Planorbis albis, 183 Polar regions, mild climates ex- ceptional in, 171 Poisson, M., on cause of geological climate, 2 Prevost’s theory of exchanges, 31 Prince Patrick’s Island, wood found at, 193 Provence, tufa of, 188 Pouillet, M., on temperature of space, 259 Pouillet and Herschel on tempera- ture of space, 144 Poles, heat received to that at the Equator, 143 Polar regions, glaciation, normal condition, 171 ; ADIATION of a particle, 22 of heat through plates, Professor Balfour Stewart on, 261 “6 effect of, on sur- face of ice-sheets, 217 Rae, Dr., on mammoth in Siberia, Raised beach with moraine matter on, 116 Ramsay, Prof, A. C., on great faults in North Wales, 268 Reaction of the physical agents, 54, 120 . Reade, Mr. Mellard, cited, 279 Regelation, three theories of, 249 a theory of, from ‘Cli- mate and Time,’ 251 “‘Rejoinder,” Prof. Newcomb’s, 31, 32, 34 Rhinoceros tichorhinus, 180 Rink, Dr., on Greenland ice, 64 4 on rainfall and snow- fall in Greenland, 237 INDEX Rink, Dr., cited, 48 Rock- basins, how excavated by land ice, 254 Rogers, Prof. H. D., on fault in the Appalachians, 270 Ross, Sir John, on snowfall of Antarctic regions, 79 ie on absence of water on Artarctic ice- sheet, 218 Ruprecht, Herr von, on Siberia, 182 Russia, mammoth remains found in, 186 ANDWICH Land under glacia- tion, 113 Sannikow on former climate of © Siberia, 182 Saporta, M., on tufa, 188, 189 Schmidt on depositson the Tundra, 182 Scoresby, Capt., on Greenland, 69 A on coldness of” Greenland air, 84 Scotland, inter-glacial beds of, 130 Sea, temperature higher than that of land, 26 », mean temparature, why it is high? 26-31 Sea-level, oscillations of, 139, 140 Secular cooling of the earth, Sir W. Thomson on, 295 Shaler, Prof., theory of Antarctic ice, 76 nf on Antarctic 111 Shells in Tufa, 190 Siberia, why snow is not per- petual, 88 a mammoth in, 178 a inter-glacial period of, 190 ice, aa North warmer during mammoth epoch, 180 sy wood found in, 181 Siemens, Dr., theory of the sun, 298 ‘‘Sifting” of rays, 44 Skeletons of mammoth in Europe, 186 Smithers, Capt., measurement of icebergs by, 210 Snow, misapprehensions regarding, 40 », influence of, 40 », amount melted, 41 ;, influence on climate, 41, 45 », melting not proportional to heat received, 46-55 », melting misconception re- garding, 47 », conservation of, 46 5; permanent in northern hemi- sphere at sea level, 76 »» permanent source of cold, 84 perpetual high lands, 87 Snow-fall, why great at south pole, 79 without os in relation to glaciation, 85 South Georgia, heat received from sun, 44, 48 > _ under glacial con- ditions, 113 South Pole, why the ice must be thick there, 78 —81 e probable thickness of ice at, 227, 241, 245 a ice at below freezing point, 227 Ss greatest thickness of ice there indepen- dent of amount of snowfall, 236 a mode of dispersion of ice from, 233 South-polar ice-cap, Mr. Wallace on, 78 South Shetland under glaciation, 113 Southern Europe, deposits of, 188 Southern hemisphere warmer than northern, 33 Space, temperature of,an important factor, 19 », Prof. Langley on tempera- ture of, 20 », Pouillet and Herschel on temperature of, 20, 21 », temperature of, 258 INDEX. 325 Spectrum analysis of stellar motions, on, 313 Spencer, Mr. Herbert cited, 288 Spitzbergen, interglacial beds of, 194 Star-clusters, formation of, 309 Stars of great velocities, why not observed, 312 Stellar space, on bodies moving in, 286 Stellar bodies, motions of, con- verted into heat by collision, 301 Stewart, Prof. Balfour, on radia- tion, 22 a on radiation through plates, 261 Stockwell, Mr., on obliquity of ecliptic, 4 “e formulz of eccen- tricity, 39 Stoney, Mr. Johnstone, cited, 287, - 288 Stratified rocks, age of, as deter- mined by denu- dation, 277 a age of, 278 a evidence as to age of earth, 267 Submergences, oscillations of sea- level in relation to, 139 ne objection as to the number of, 139 es number unknown, 139 Succinea putris, 184 Sun, minimum age of, determined by that of the earth, 280 s; loss of heat, estimate of, 290 »» probable origin and age of, 282 ,, how original temperature ob- tained, 283 ,, nebulous mass of, how super- heated, 283 Sun’s heat, origin and age of, 264 os as derived from gravi- tation, 265 Surface temperature of Antarctic ice, 225 Sweden, mammoth remains found at, 186 Switzerland, interglacial beds, 133 326 INDEX. Switzerland, ‘‘ Flysch” of, 173 Thomson, Sir William, on age of i illustration from ice earth, 292 of, 245 5 cited, 14 me glaciers, temperature pos on underground determined by pres- heat, 213 sure, 222 Thomson, Sir Wyville, on Ant- i nee Prof., on age of the sun, 266. on nebula, 310 Tay Valley, Carse-clays of, 115 Temperature of space, bearing on terrestrial phy- Sics, 258 Be Prof. Langley on, 20 53 animportant factor, 1 - mean of ocean, reason why it is high, 26-31 be of ocean greater than that of land, 26 Temporary stars, why so rare, 315 causes of, 315 Terrestrial physics, most important problems in, 1 Tertiary period, influence of eccen- tricity during, 157 s as affected by ec- centricity, 160 5 evidence of glacia- tion in, 172 Tertiary fossil floras, Mr. J. Starkie Gardner on, 161 », glacial epochs, 164 Thermodynamics, misapprehen- sions regarding, 33 Thomson, Prof. James, on cause of regelation, 250 te on lowering of melt- ing point of ice, 215, 227 es cited, 203 formula, 227 Thomson, Sir William, on internal heat, 2 i theory of climates, 6 a on mild Arctic cli- mates, 148 several arctic regions, 69- 73, 203 Me on Antarctic ice- bergs, 70-74 oe on thickness. of Antarctic ice- sheet, 225 be on thickness of ice- bergs, 209 ae on stratification of Antarctic ice, 229 HN on Antarctic ice- cap, 232 Tidal retardation, argument from, 292 Torrell, Prof., on German inter- glacial beds, 133 Tournonér, M., on shells, 190 Towson, Mr., on icebergs of Southern Ocean, 207 cited, 211 Tropical and Arctic floras, Mr. J. S. Wood on commingling of, 162 Tufa, Prof. J. Geikie on, 187 ;, Mr. Howorth on, 188, 189 ,, of Provence, 188; of Moret, 188; of Canstadt, 189; of Le Celle, 189 ,, of Europe, 187 », mollusca of, 190 », of Tuscany, 188 Twisden, Rev. J. F., on changes of axis of rotation, 5 cited, 294 Prof., experiments on passage of heat through ice, 252 5, experimentson heat-absorb- ing power of aqueous vapour, 260 Tyndall, NDERGROUND heat, 214 9, in re- lation to Antarctic ice-sheet, 225 Unio littoralis, 131, 134 ee ad i & iE oes" te