Popular Science Monthly/Volume 16/April 1880/Croll's Climate and Time
By W. J. McGEE.
THE recent publication in "The Popular Science Monthly" of a paper on "The Age of Ice," and its apparently favorable reception and republication elsewhere, prompt the writer to submit the following incomplete notice of a work in which the field barely entered by the author of that paper is most thoroughly and exhaustively examined.
In addition to a convenient abstract of the line of argument pursued, and a statement of some fundamental principles of geology, it is pointed out in an introductory chapter that the earlier theories framed to account for climatal variations during the geological æons are utterly inadequate; that the earth could not pass through hotter or colder portions of space without seriously deranging the mechanism of the solar system; that a diminution of heat from this or any other cause could never inaugurate a glacial epoch; that considerable changes in the obliquity of the ecliptic have never occurred, and could not have caused glacial periods if they had; and, finally, here as well as in a more recently published paper, which may be considered as supplementary to this chapter, that material changes in the position of the terrestrial axis can never have taken place: in short, he shows, by bringing together the independent results arrived at by eminent geologists, physicists, and mathematicians, that the various cataclysmic theories of geological climate are alike untenable. Telluric causes being thus shown to be incompetent, no alternative remains but to seek for the cause of secular climatal variations in the earth's astronomical relations. In the same chapter, but more fully in the Appendix, Dr. Croll goes on to show that the variable length of the seasons consequent upon the ellipticity of the terrestrial orbit had begun to attract attention before the close of the last century; and that, as early as 1830, Sir Charles Lyell had expressed the idea that the long winters and short summers of the southern hemisphere might have some influence in lowering the temperature of that portion of the globe. Sir John Herschel and others, however, soon after demonstrated that the light and heat received by any portion of the earth's surface during any year is practically invariable, whatever the eccentricity of the terrestrial orbit; the greater proportionate length of winter in the hemisphere whose winters occur in aphelion being exactly counterbalanced by the greater proximity of the sun in summer. This is, indeed, a legitimate deduction from Kepler's second law, and was long ago demonstrated by D'Alembert. The hypothesis, therefore, fell into disrepute. Over fifteen years ago, however, the author of the work under consideration began to point out, in a series of papers (chiefly in the "London and Edinburgh Philosophical Magazine"), the substance of which is reproduced in "Climate and Time," that while the variable length of the seasons resulting from this cause could never produce a glacial epoch directly, yet the same cause might, especially when intensified by a high degree of eccentricity, bring into operation a chain of physical agencies which could not fail to very materially affect the climate of the globe.
As a further introduction to that portion of the work devoted to the elucidation and application of the above-named astronomical and physical principles, and as an illustration of the efficiency of one of the secondary agencies on the operation of which the theory is based, the heat-conveying power of ocean-currents is discussed at length in the second and third chapters. The importance of these currents is shown to be immense. Thus, according to Professor Dove's "Temperature Tables," the temperature of the British Isles, and of western Europe generally, is 12° Fahr. above the normal—or, more properly, the mean—for that latitude, while the temperature of corresponding portions of eastern North America is nearly as much below the normal. Dr. Croll attributes this difference to the effect of the Gulf Stream in warming western Europe, and of the cold counter-current in chilling our American coasts. The same subject is recurred to frequently throughout the volume, notably in Chapters XI. and XII., in the latter of which Mr. Findlay's objections are answered by calculating from his own data that the heat liberated from the Gulf Stream in the North Atlantic is equal to more than one half of that received directly from the sun in the same latitude. An analogous condition of things exists on the shores of the North Pacific, which are similarly affected by the Kuro Siwo; though there the effect is less distinctly marked, owing to the more unfavorable conformation of the coast. So effective are ocean-currents in distributing the heat of tropical and the cold of polar regions, that our author concludes that the globe would not be habitable for existing orders of beings were their influence to cease.
The heat-conveying power of aërial currents is also discussed; but it seems possible that Dr. Croll has under-estimated their capacity, many times increased as it is by the aqueous vapor with which they are laden. Space will not, however, permit the discussion of this interesting point.
The combined effect of aërial and aqueous currents is estimated to reduce the difference in temperature between equator and poles from 218° to 80°. This astonishing result may be substantially verified by a simpler and probably more accurate method than that employed by Dr. Croll. Meech shows that the relative solar intensity at the equator, at the poles, and over the whole earth, varies in the ratio of 81·50, 33·83, and 66·73, respectively; and Dove, many years ago, determined the mean annual temperature of the earth to be approximately 58°. Now, the solar energy elevates the temperature of the earth from that of stellar space, or 239° (as determined by Herschel and Pouillet), to 58°. 297° is, therefore, the mean elevation of the earth's temperature by solar action. Eliminating the distributing power of aerial and aqueous currents, and assuming the present mean relation between absorption and radiation to remain constant, it is quite probable that the temperature of the various parts of the earth's surface would vary directly as the solar intensity. A simple proportion, then, shows us that while the equator would be raised 363°, or to 124° of absolute temperature, the poles would be elevated only 150°, or to 88°; making a difference of 212° between equator and poles. If, as is extremely probable, the temperature of stellar space is really below 239°, this difference would be still greater. Moreover, it can be shown that any decrease in temperature tends to increase the radiating capacity of the earth by rendering the surrounding atmosphere diathermous. Hence the poles would sink lower in temperature, proportionally, than the equator.
Six chapters (VI.-XI.) are devoted to the "gravitation theory" of oceanic circulation, with the object of completely refuting it. Two reasons for the very full discussion of this branch of the subject are given: 1. Because the gravitation theory "lies at the root of a great deal of the confusion and misconception which have prevailed in regard to the whole subject of ocean-currents"; and, 2. Because, "if the theory is connect, it militates strongly against the physical theory of secular changes of climate advanced in this volume." The advocates of this theory are separated into two arbitrary classes: 1. Those who consider the difference in density between equatorial and polar waters to be due to difference in saltness; and, 2. Those who attribute the difference in density to difference in temperature. Lieutenant Maury's theory, stated in his "Physical Geography of the Sea," is rejected because that eminent meteorologist recognized both of the above-named causes of difference in density, while they are very nearly equal and quite antagonistic, and because the actual differences in specific gravity due to these causes are mathematically demonstrable to be incompetent to produce so powerful currents; being, in fact, as Dr. Croll pointedly insists, only one seventh of that necessary to produce the slightest motion. Dr. Carpenter's theory, based chiefly on differences of density due to differences in temperature, is discussed at still greater length than Lieutenant Maury's, and is rejected on the grounds—1. Of being counteracted by differences in saltness (in which, view Dr. Croll has the support of Sir Wyville Thomson); and, 2. Of resting on assumed causes utterly inadequate either (a) to produce existing currents, or (b) to convey northward so great an amount of heat as that shown to be given out by the Gulf Stream; the apparently paradoxical conclusion, that the Gulf Stream actually conveys an absolutely greater quantity of heat to high latitudes than the whole Atlantic could convey, being explained by the proposition that the Gulf Stream obtains the greater portion of its heat in the southern hemisphere, while according to Dr. Carpenter's hypothesis the circulation should be independent in each hemisphere.
In Chapter XIII. the "wind theory" of oceanic circulation is enunciated and elaborated, and in the succeeding chapter its relations to climatal variations are discussed. It is first pointed out that the various ocean-currents are not due to the trade-winds alone, as was until recently supposed by advocates of the wind theory, but to the general impulse of the prevailing winds of the globe, viewed as a single grand system, and acting, not upon several separate and independent oceans, but upon a single grand oceanic system, the various parts of which are most intimately related. The correspondence between the supposed cause and the observed effect seems to be all that could be desired. "All the principal currents of the globe are in fact moving in the exact direction in which they ought to move, assuming the winds to be the sole impelling cause. In short, so perfect is the agreement between the two systems, that, given the system of winds and the conformation of sea and land, and. . . the system of oceanic circulation might be determined a priori. Or, given the system of the ocean-currents, together with the conformation of sea and land; and the direction of the prevailing winds could also be determined a priori. Or, thirdly, given the system of winds and the system of currents, and the conformation of sea and land might be roughly determined." Return currents are formed through the tendency of the ocean to maintain its level, and naturally select the path of least resistance. Hence they are usually undercurrents.
The general agreement between the systems of winds and of oceanic circulation is shown on a chart. It may be mentioned that the direction of the winds, as laid down on this chart, does not precisely correspond with the direction determined by the late Professor Coffin, and indicated on the charts in his cyclopean work, "The Winds of the Globe," recently published by the Smithsonian Institution; and it is quite probable that more exhaustive observations will show that there are inaccuracies of as great magnitude in the representation of oceanic currents. It should be borne in mind, however, that any minor discrepancies between the two systems do not militate against the theory, unless it can be shown that they are not such as would be produced by the conformation of the coasts; for the wind system, as a cause of oceanic circulation, is modified by this important and frequently antagonistic factor.
In seeking for demonstrative evidence of the correctness of the wind theory. Dr. Croll seems to fall into an error which he has repeatedly had occasion to point out in others. Thus, in discussing one of Dr. Carpenter's sections, he shows, from Professor Muncke's coefficients of the density of sea-water, at various temperatures, that the surface of the Atlantic is, at north latitude 23° 10′, two feet six inches, and at north latitude 37° 54′, fully three feet six inches higher than at the equator, on account of the greater thickness at these points of the upper layer of warm water. It is urged that "gravitation never could have caused the ocean to assume this form," and hence that "gravitation can no more cause the surface-water of the Atlantic to flow toward the Arctic regions than it can compel the waters of the Gulf of Mexico up the Mississippi into the Missouri." Now, by Dr. Croll's own showing, it is gravity alone that causes the surface of the Atlantic to assume that form—that is, if that form is actually assumed; a fact not determined by measurements. It is merely calculated from the data given to what height above the level at the equator the waters of the North Atlantic would have to be raised in order to maintain a condition of static equilibrium; it is not shown that the waters are so elevated, but only that they would have to be in order that the influence of gravitation, in producing oceanic currents, should be eliminated. How the warm water, on which this supposed configuration depends, was conveyed thither, is a dynamical and not a statical problem.
Moreover, two important elements in the problem of oceanic circulation are disregarded by Dr. Croll: 1. The water borne from equatorial to polar regions by aerial currents, in the form of aqueous vapor, must exercise a powerful influence not only on the earth's temperature but also upon marine currents; for, as shown by Lieutenant Maury, it is sufficient to permanently render the Arctic seas much less saline than those of tropical regions. Sir Wyville Thomson strongly insisted upon the importance of this agency in his presidential address before the geographical section of the British Association, at its Dublin meeting, in which he even went so far as to maintain that one part of the general oceanic circulation takes place through the atmosphere; though in thus considering the aërial circulation to be telluric, or universal over the whole surface of the earth, instead of hemispheric, or comparatively independent on opposite sides of the equator, this distinguished explorer seems to assume to be true that which is most emphatically contradicted, not only by the almost innumerable observations collated by Professor Coffin in the great work already referred to, but by those of all other observers. It is true, though, that, as the transportation of the vapor is accomplished by aërial currents, these currents indirectly cause those marine currents formed by the precipitation of vapor. 2. The earth's axial rotation, which is one of the two great causes of aërial circulation, must exercise considerable effect on the marine currents, though of course its operation is more seriously interfered with by the inequalities of the terrestrial surface in the case of the earth's discontinuous liquid envelope, than in the universal gaseous one. Still, if, as suggested by Guilleman, this force is adequate to influence the course of rivers flowing to the north or south, it will vitiate any theory which neglects it. Moreover, the actual effect of the earth's axial rotation would be very likely to elude observation, as the currents so produced would necessarily correspond approximately with the principal aërial currents of the globe; and hence the combined effect of the two causes would be likely to be attributed to the most obvious one. The influence of axial rotation on the oceanic circulation may be determined mathematically, however, and indeed such an investigation has already been entered upon by Mr. Ferrel; but his results are not generally known.
While both branches of the gravitation theory are summarily dealt with, analogy with the second prime cause of aërial circulation (i. e., difference in density, owing to differences in temperature and humidity) would indicate that any difference in density, whether due to difference in temperature or in saltness, would be sure to play a minor part in the phenomenon of oceanic circulation, at least in some cases. Therefore, of the five distinct agencies which probably coöperate in disturbing the equilibrium of the ocean. Dr. Croll recognizes but one; and, singularly, in view of his usual method, he nowhere endeavors to prove mathematically, or in any other conclusive manner, that the impulse of the winds is adequate to produce the effects attributed to it. The wind theory can not, therefore, be said to be demonstrated. Nevertheless, the agreement between the winds and the marine currents is so striking that geographers and navigators are generally disposed to adopt that theory. The late distinguished geographer of Gotha, Dr. Petermann, was one of the most prominent advocates of the theory.
With Dr. Croll's connection with this subject a new era may be said to have been inaugurated. Previously it had been deemed sufficient to point out certain agencies which seemed to be adequate to produce the observed effect, without making any effort to show mathematically that they were adequate; but this physicist contended from the first that cause and effect should be determined in absolute measure, just as in the other branches of physical science. His failure to observe this excellent rule in one case is to be attributed to the same paucity of trustworthy observations which is the occasion of the obscurity enveloping the whole subject.
In Chapter IV. the physical agencies leading to changes of climate are discussed, and an explanation of the present low temperature of the southern hemisphere is offered in Chapter V. As has already been intimated, that hemisphere, which has its winters in aphelion, has a longer winter and a shorter summer than the mean. Now, it is perfectly obvious that this variation in the length of the seasons increases with any increase in the eccentricity of the terrestrial orbit, and similarly diminishes with any diminution of eccentricity; for the eccentricity of the planetary orbits may vary within pretty "wide limits, which have been determined by La Grange, Leverrier, and, more recently as well as more satisfactorily, by Mr. J. N. Stockwell, of Cleveland. The present eccentricity (0·0168) is such that there is a difference of about eight days in the length of summer and winter in either hemisphere, when the solstices coincide with the apsides; and, the winter solstice of the southern hemisphere being now not far from aphelion, that hemisphere has the long winter and short summer. It is admitted, however (even too readily, it would seem), that the present degree of eccentricity is too insignificant to exercise much influence on the climate of the globe; but 210,000 years ago, when, according to Dr. Croll's elaborate computations, the eccentricity was 0·0575, the excess of winter over summer, due to this cause, amounted to 26·7 days; and 850,000 years ago, the eccentricity then being 0·0747, it amounted to 34·7 days; and it is argued that so great a difference in the relative length of the seasons would indirectly, through the intervention of a number of physical agencies, materially affect the earth's climate. These agencies are shown to be such as would be brought into operation by an increase in the length of winter, even if its severity was not increased; and they are mainly dependent on the increased proportion of moisture precipitated as snow instead of rain. Of course, this snow would remain until melted by approaching summer, just as it does in every region where much snow falls.
"There are three separate ways whereby accumulated masses of snow and ice tend to lower the summer temperature": 1. By means of direct radiation [and by direct contact]; 2. By direct reflection back into space of the solar rays; and, 3. By chilling the air and condensing the vapor into thick fogs which intercept the solar rays. This third influence had better not be insisted upon, however, as it is counteracted by—1. The immense amount of latent heat liberated in the condensation of the vapor; and, 2. The interception by these fogs of the heat radiated from the earth. A substitute may be offered for it, viz., by rendering the atmosphere diathermous, and therefore incapable of absorbing the solar rays or of intercepting the radiation and reflection from the earth. In consequence of the operation of these agencies, the air in snow-covered regions seldom rises above the freezing-point, and the solar heat is conveyed away into space and utterly lost to the earth; for, if a portion of it is absorbed by the snow and ice during the hours of sunshine, it is not rendered sensible, owing to the high latent and specific heat of these forms of water, and is radiated away, unchecked by any "protecting blanket of vapor" (which Professor Tyndall shows to be so efficacious in protecting the earth from radiation, but which can not exist above snow-covered regions), during the succeeding hours of darkness.
This waste of solar energy, in turn, still further curtails the already short summer, and permits the same causes to operate with increased efficiency during the succeeding season. Moreover, the reaction of each effect upon its cause is such as to strengthen the cause, and the interaction of all the agencies is such as to increase the efficiency of each. Each winter would thus add to the snow which had remained unmelted during the intervening summer, until the accumulation of snow was checked by the absence of vapor for condensation and precipitation; for, as pointed out by Tyndall, the presence of large quantities of vapor is the first essential for the formation of extensive glaciers.
It has been objected to the theory, that this picture has been overdrawn—that no such slight cause could so seriously disturb the equilibrium of the seasons; it has even been shown mathematically that the heat of a few days in summer would melt the total accumulation of the previous winter. The answer to these objections is, that in such calculation the operation of the agencies just described was disregarded, and hence that its results are unreliable; that, though the solar intensity is greater over polar regions in summer than in the tropics, as shown by Meech, it is not sufficient to melt the annual accumulation of ice, else this ice never could have accumulated to so vast an extent as to annually send forth thousands of colossal bergs to be melted in temperate seas; that not only in the Alps, but even in the almost tropical Himalayas, where the sun shines with undiminished intensity throughout the year, the direct effect of the solar energy is so far below the accumulation of congealed vapor that the ice is only prevented from piling up indefinitely by that property which enables it to flow down to lower levels where the conditions described do not exist; that even in the northern portions of our own country the slight annual film of snow retards the coming of summer by weeks if not months; that while the reception and dissipation of heat by any portion of the earth's surface are equal, the physical properties of ice are such that ice-covered regions lose their heat immediately without being sensibly increased in temperature. The effect on temperature of accumulations of ice and snow must, therefore, be enormous. "Were it not for the ice," strikingly remarks Dr. Croll, "the summers of Greenland. . . would be as warm as those of England; but, instead of this, the Greenland summers are colder than our winters. Cover India with an ice-sheet, and its summers would be colder than those of England." Even on our own coasts the grounding of a single berg appreciably lowers the temperature and greatly increases the danger of frosts.
On the hemisphere whose summers occurred in aphelion during the period of high eccentricity, an exactly opposite tendency would be manifested: the snow and ice would gradually melt and perhaps entirely disappear, and vegetation might flourish even under the pole. That hemisphere would then enjoy an interglacial period. These periods occupy an important place in the theory under examination.
Dr. Croll then proceeds to show how the accumulation of ice in polar regions would affect the general oceanic circulation: "Owing to the difference between the temperature of the equator and the poles there is a constant flow of air from the poles to the equator. It is to this that the trade-winds owe their existence. Now, as the strength of these winds, as a general rule, will depend on the difference of temperature that may exist between the equator and higher latitudes, it follows that the trades on the cold hemisphere will be stronger than those on the warm. . . . Suppose, now, the northern hemisphere to be the cold one. The northeast trade-winds of this hemisphere will far exceed in strength the southeast trade-winds of the southern hemisphere. The median-line between the trades will consequently lie to a very considerable distance to the south of the equator. . . . Let us now consider how this would affect the Gulf Stream. The South American Continent is shaped somewhat in the form of a triangle, with one of its angular comers, called Cape St. Roque, pointing eastward. The equatorial current of the Atlantic impinges against this corner, but, as the greater portion of the current lies a little to the north of the corner, it flows westward into the Gulf of Mexico and forms the Gulf Stream. . . . Now, it is perfectly obvious that the shifting of the equatorial current of the Atlantic only a few degrees to the south of its present position—a thing which would certainly take place under the conditions we have been detailing—would turn the entire current" to the south of Cape St. Roque, and thence along the Brazilian shores and into the Southern Ocean, and "the Gulf Stream would consequently be stopped."
Now, it is quite manifest that if the wind theory of oceanic circulation is incorrect—and, as already shown, its correctness has not been demonstrated—no such deflection of the Atlantic current would be likely to occur. This possibility is fairly confronted by Dr. Croll when he admits that the gravitation theory "militates strongly" against his theory of secular changes of climate. But there is another self-evident proposition which seems to have escaped his attention. Assuming that the results detailed above follow in the order laid down when the eccentricity is at a high value, it is perfectly manifest that the deflection of the Gulf-Stream feeder is an effect of glaciation, and, if a cause at all, only a secondary one. Hence, if the purely physical agencies alone are capable of causing glaciation, Dr. Croll's theory of secular changes in climate will stand, whether or not the wind theory of oceanic circulation is correct; but, if they are not capable of producing a glacial period alone, the theory will fall, even if the wind theory of oceanic circulation be correct. This can not be too strongly emphasized. Yet, not only in "Climate and Time," but in his other publications on the subject, Dr. Croll dwells upon the deflection-of ocean-currents as the principal telluric element in his theory!
The only positive evidence adduced to prove that the Gulf Stream was deflected during the glacial period is the fact, pointed out by Mr. Crosskey, that there is more difference between the glacial and recent shells of Scotland than between the glacial and recent shells of Canada. But this only proves that the present temperature of that part of Canada is lower than that of Scotland; for the temperature of the waters at the edge of the ice-sheet must have been approximately the same, whatever the latitude to which it extended. This evidence is, therefore, utterly valueless. Furthermore, we have pretty reliable positive evidence that the Gulf Stream was not stopped during the glacial epoch, in the more northerly limits of continental glaciation in those parts of Europe so greatly affected by the Gulf Stream to-day, any more than in the United States, where its influence is comparatively unfelt. Moreover, Professor Dana has shown in his "Journal" that the distribution of ice during the glacial period coincided in a general way with the present distribution of rainfall in the same latitudes. Now, the greater part of the moisture of the United States, especially of the great Mississippi Basin, is derived from the Gulf of Mexico; and the stoppage of the South Atlantic feeder of the Gulf Stream would cool the waters of the Gulf so considerably as to materially diminish our vapor-supply, and at the same time the distribution would be altered. Similarly, the stoppage of that current during the glacial period would have so altered the distribution that its relation to the present precipitation would not be recognizable, even if it did not so completely cut off the vapor-supply as to prevent glaciation. The mass of evidence is therefore against the hypothesis of the shifting of this important marine current during the glacial period; and, as this would, as Dr. Croll points out, be likely to be the first out of all of the ocean-currents to be deflected, it may reasonably be doubted whether any were seriously affected by the cause specified. It must he admitted, however, that the physical agencies which have already been described seem competent to inaugurate a glacial epoch during any period of high eccentricity of the terrestrial orbit, without any assistance from the meteorological influences. These periods, recurring at long and irregular intervals, must have alternately refrigerated and revivified the circumpolar regions again and again during the immeasurable ages whose lapse is so dimly recorded in the rocky strata. The theory is, therefore, in the highest accord with the modern uniformitarian doctrine which rejects all hypothetical explanations of phenomena which are not in harmony with the present course of nature.
The present low temperature of the southern hemisphere as compared with the northern is explained by the assertion that the warm waters of the southern hemisphere are borne into the northern, while the cold waters of the northern hemisphere are conveyed into the southern. The argument supporting this conclusion is that of a radical advocate of the wind theory, and is as strongly opposed to the first principles of the physical theory of climate as the most conservative critic could wish. Space will not permit the discussion of this chapter.
Chapter XVIII. contains a résumé of the evidence of former glacial eras which had been collected up to the time of the preparation of the volume. There had already been placed on record more or less decisive evidence of former glaciers, not only in the Quaternary but in the Miocene, the Eocene, the Cretaceous, the Oölitic, the Permian, the Carboniferous, the Old Red Sandstone, the Silurian, and even the Cambrian. In some of these cases, notably in the formations of the Permian age, the evidence is so voluminous, so distinct, and from such widely separated localities, that it seems impossible not to conclude that our Pleistocene ice age was but the homologue of long antecedent secular winters. The reasons for the paucity of evidence regarding these early glacial eras are summarized in the preceding chapter.
A statement of the method of computing the eccentricity of the terrestrial orbit, elaborate tables (laboriously computed by the author, with the exception of about a dozen periods) showing the eccentricity for 3,000,000 years in the past and 1,000,000 years in the future, and conclusions as to the probable date of the glacial epoch, constitute Chapter XIX. It has since been pointed out, by an undoubted authority in such matters (Professor Newcomb), that Leverrier's formulæ, which were employed in making the computations embraced in the tables, are defective, and hence that the figures given are not rigidly correct; but for the present these minor inaccuracies may be disregarded. The dates given may, therefore, be assumed to be correct, though they are undoubtedly only approximations.
It has already been stated that periods of high eccentricity occurred 210,000 and 850,000 years ago, respectively. Sir Charles Lyell, the founder of the uniformitarian school of geology, was inclined to believe that it was the first of these periods that produced the glacial epoch. Guided by the rate and amount of post-glacial erosion, however, Dr. Croll concludes that it was the more recent period which corresponded with the Quaternary glacial epoch; but he suggests that the earlier period may have coincided with the Miocene glacier. He thinks that the glacial period of the Quaternary lasted from shortly before the last great maximum until about 80,000 years ago (when the eccentricity was 0·0398, corresponding to a difference of twenty-two days in the length of the seasons), or for about 160,000 years—including, of course, the alternating interglacial periods.
Aside from the strong inherent evidence of the approximate correctness of this determination of the date of the glacial epoch (any uncertainty being due to the imperfection of Leverrier's formulæ), there is an abundance of independent testimony leading to substantially the same conclusion. Many eminent geologists have calculated the duration of post-glacial time from various data—generally the rate and amount of erosion or deposition in stated localities—with results usually ranging from 100,000 to 300,000 years. The mean of several of the most reliable is a trifle less than 200,000 years. Now, while each of these results, when viewed singly, may properly be regarded, in the words of Sir John Lubbock, "not as a proof, but as a measure of antiquity," they may, when viewed collectively, justly be considered reliable within wide limits, and to prove that no less a period than 40,000 or 50,000 years can have elapsed since the retreat of the ice sheet from temperate latitudes; and the time has now come when he who endeavors to fix a later date for that event, without showing why these estimates should be rejected, need not be astonished if his efforts only bring him into contempt. So great is the weight of this independent testimony, indeed, as to warrant the suggestion that the glacial epoch of the Quaternary did not extend down to the period of high eccentricity 80,000 years ago, as Dr. Croll intimates, but closed 160,000 or 170,000 years ago. This last maximum might, then, be represented by the Reindeer epoch of Europe, and possibly (if it may be permitted in this strongly reactionary age to suggest the bare possibility that the pioneers in the field of American archæology, At water, Morton, Squier, and their compeers, did not err most egregiously in their estimates of the antiquity of the earlier works of our prehistoric races) by the migration into Mexico and Central America of the mound-builders of the Mississippi Valley—a migration which might thus furnish a parallel with that southerly migration of the Pliocene mammalian fauna at the inception of the glacial period, to which the Indian geologists attribute the richness and variety of the Siwalik and related fossil fauna of the Orient. It might further be urged that, if the duration of the ice age were so great as Dr. Croll suggests, we would be likely to find more unequivocal evidence of the fact in structural variations in those species which survived the cataclysm.
Interglacial periods are treated of at considerable length in Chapters XV. and XVI.; and it is justly held that any theory which does not explain the occurrence of the plant-beds of the drift, as well as its bowlder clays, is unworthy of acceptance. The fossiliferous strata of the marine formations found in Arctic regions, as well as the whole series of fossiliferous deposits found in the drift, are referred to these brief periods of unusually mild climate; but this seems almost too radical. If the general series of marine deposits in the Arctic regions were as nearly unfossiliferous as are the sedimentary strata of tropical India, such an hypothesis would be a little more likely to find acceptance among geologists. The fact developed by Meech, that the polar regions ought to have a warmer summer than the equator, if the solar intensity is a fair criterion, would indicate that these regions should have only a temperate climate if the ice were removed and the summer's heat stored up in the earth; and so slight an additional quantity of heat would accomplish this in a few years that, in view of the known variability of the solar emission and of the terrestrial absorption, it seems quite unnecessary to attach so much importance to the interglacial periods in their relations to Arctic formations. That coal is an interglacial formation, as is suggested in Chapter XXVI., seems still less probable, chiefly because these periods are too short to admit of so great an accumulation of vegetable matter as is stored up in each coal-seam.
It is not improbable, indeed, especially if the marine currents were not seriously affected by the polar snows, that the greater part of each of these periods would be required to melt the ice which had accumulated during the preceding glacial period. It seems very doubtful, too, even if the melting of the ice took place with the greatest conceivable rapidity, whether terrestrial animals or plants would spread over the barren wastes of crude glacial débris so rapidly as to people so wide a zone in the brief period assigned. Some of the intercalated fossiliferous beds of the drift, too, are very rich in numbers as well as species of both animals and plants—the latter sometimes forming extensive deposits of lignite—which must have required an immense time for their development. It seems scarcely possible that these terrestrial deposits can be interglacial, in the sense in which Dr. Croll employs the term, though the aqueous deposits, containing fossil shells of marine and estuarine mollusks, may justly be so considered; for such animals would be likely to keep close to the margin of the icecap as it retreated. To explain the two principal divisions of the drift, which have been recognized over immense areas on both sides of the Atlantic, it seems equally reasonable to refer the uppermost to the last period of high eccentricity, and the lower to that which Sir Charles Lyell supposed to coincide with the glacial epoch; in fact, in support of this collocation, we have the striking coincidence that the ice extended some degrees farthest during the period of greatest eccentricity. Besides, such a collocation affords a sufficient period for the development of the rich and characteristic fauna and flora of the forest-bed—a widespread fossiliferous and carbonaceous deposit that must represent a lapse of time of geological importance. The belief that any glacier would necessarily remove all the débris formed by an antecedent one—a belief which has done much to prevent the acceptance of theories of successive glaciation—is best shown to be fallacious by the unequivocal evidence that the ice of the last glacial epoch did pass over older deposits of unconsolidated materials without removing them. Some such evidence is cited by Dr. Croll in the last-named chapters.
Among the interesting cognate subjects taken up in the remaining eleven chapters of the work are—methods of measuring the rate of subaërial denudation, and of determining the mean thickness of the rocks of the globe; the age and origin of the sun (which is an able effort to reconcile the existing disagreement between the geologist and the astronomer and physicist as to the age of the earth); the physical cause of continental submergence and emergence during the glacial period; the influence of the obliquity of the ecliptic on terrestrial climate; some glacial phenomena of Scotland and England; and the physical cause of glacier-motion. Appendices and an index are added.
It has been the aim in the foregoing pages to convey a general idea of the nature and scope of the work under review, and at the same time to indicate those points which do not seem to be sustained; and, as is natural in view of this double object, justice has not been done to the work as a whole. "Climate and Time" represents years of study and an almost incredible amount of conscientious labor by perhaps the most competent living man to deal with this obscure subject, which occupies a position intermediate between geology, physics, and astronomy, and requires a thorough knowledge of all of these branches of science for its adequate comprehension. As a geologist, Dr. Croll occupies an important and responsible position; and, as an astronomer and physicist, his reputation in scientific circles is even more enviable. Owing to the confusion in which he found the subject, to the absence of reliable data, and perhaps to a rather radical disposition, he seems to have fallen into a few errors; but, with some reservations, his ingenious theory has been received with much favor, and has been pretty widely adopted, especially on the other side of the Atlantic. Here, it is comparatively unknown, and, in too many cases, lack of acquaintance with the principles on which it is based has led to its being unfavorably regarded; but even those who reject the theory would do well to familiarize themselves with its details before they undertake to investigate the subject anew.