causes of glacial periods—actual conditions of existing glacial regions—high land in high latitudes—cold alone insufficient—large evaporation required—formation of glaciers—they flow like rivers—icebergs—greenland and antarctic circle—geographical and cosmic causes—cooling of earth and sun, cold spaces in space, and change in earth's axis, reviewed and rejected—precession alone insufficient—unless with high eccentricity—geographical causes, elevation of land—aerial and oceanic currents—gulf stream and trade winds—evidence for greater elevation of land in america, europe, and asia—depression—warmer tertiary climates—alps and himalayas—wallace's island life—lyell—croll's theory—sir r. ball—former glacial periods—correspondence with croll's theory—length of the different phases—summary—croll's theory a secondary cause—conclusions as to man's antiquity.
i turn from the effects to the causes of that great glacial period which has been described in the last chapter. this line of investigation is peculiarly interesting in the search after human origins, for it affords the only chance of reducing the vague periods of immense duration shown by geology, to something like a definite chronology of years and centuries. if astronomical causes, the dates of which admit of mathematical calculation, can be shown to have been, if not the sole or principal, yet one of the causes which must have influenced the phenomena of the glacial epoch, we may assume these dates for the occurrence of the human remains which accompany these phenomena. 294 otherwise we must fall back on immense antiquity, which may mean anything from 50,000 or 100,000, to 500,000 or 1,000,000 years, since the first authentic evidence for pal?olithic man.
the first step towards an investigation of the cause of glacial periods, is to consider what are the conditions of the actual ones which are now prevailing. we have one such period in greenland, another in the antarctic region, a third in high mountain chains like those of alaska, and of the swiss and new zealand alps. in all these cases we find certain common conditions. high land in high latitudes, rising in great masses above the snow-line or temperature which condenses water in the solid form; and winds which are charged with great quantities of watery vapour raised by evaporation, to be so condensed.
cold alone is insufficient to produce glaciers and ice-caps, as may be seen by the example of the coldest regions in the world, siberia and the tundras of northern asia and of north america, where the earth is permanently frozen to a depth of many feet; but there are no glaciers, the reason obviously is, that there is no sufficient supply of moist air from warm oceans to furnish more snow in winter than is melted in summer. heat is in a certain sense as necessary as cold to account for glacial periods, for snow and ice can no more than other things be made out of nothing, and every snowflake implies an equal amount of aqueous vapour raised somewhere else by evaporation. but if an abundant supply of liquid or gaseous water is combined with cold sufficient to condense it into the solid form, it becomes fixed, and if the summer heat is insufficient to melt the excess of snow, it necessarily 295 accumulates. the growth of glaciers, follows as an inevitable consequence. the snow is converted into ice by pressure and by alternate freezing and melting, and this grows year by year, until an equilibrium is established by the ice pushing down glaciers into lower levels, where the melting is more rapid, or into the sea, where the front is floated off in icebergs, and drifts into lower latitudes. the process is the same as that by which the rainfall on high levels is drained off by rivers into the sea, so that an equilibrium is established between waste and supply. and it is to be remarked that the glacier, though composed of solid ice, behaves exactly like a river, or rather like a river of some viscous fluid like pitch or treacle. its size depends on the magnitude of the reservoir or area drained by it; it conforms to the configuration of the valley by which it descends and the obstacles which it encounters; it flows rapidly, and with a broken current, through narrow gorges and down steep inclines; slowly and tranquilly over wide and level areas; its velocity is greatest at the surface and in the middle where friction is least, slowest at the bottom and sides where it is greatest. in short a glacier is simply a solid and slowly-flowing river, discharging an excess of solid ice to the lower level from which it came, just as a liquid river does with the rainfall of warmer regions. the cause of this tendency of solid and brittle ice to flow like a viscous fluid is not quite understood, though recent researches, especially those of tyndall, have thrown a good deal of light upon it; but all glacialists are agreed on the fact that it does so, and we can argue from it with great confidence as to the conditions under which glaciation has acted in the past and is now acting.
296 thus even if namsen had never crossed greenland, or ross had never discovered mounts erebus and terror, we might have inferred with certainty the existence of enormous ice-caps, implying continental masses of elevated land, in both the arctic and antarctic circles, from the number and size of the icebergs floated off into the northern atlantic and southern pacific oceans. icebergs are frequently met with in the latter down to 50° south latitude, or even lower, of a mile in length and 500 feet high above the sea; and in some instances icebergs three miles long and 1000 feet high have been recorded. as upwards of eight feet of ice must be under water for every foot that floats above it, some of these icebergs must be considerably over a mile in thickness, which implies that there must be land ice towards the south pole so thick that it is, in places, over 5000 feet in thickness at its outer margin. it has been estimated from the great size and abundance of these icebergs, that in the interior of the great antarctic continent the ice may be twenty miles or more thick, and in greenland the great interior ice-cap rises in a dome to at least 9000 or 10,000 feet above the sea-level, a great part of which is solid ice, while during the great glacial period it was certainly very much thicker.
as a first step therefore towards a solution of the problem of the glacial period we may start with the axiom that it requires abundant evaporation, combined with a temperature low enough to precipitate an excess of that evaporation in the solid form. this does not necessarily imply any great and permanent refrigeration of the whole earth, for although this would give the cold it would not give the evaporation, and would tend 297 rather to extend the conditions of siberia than those of greenland. longer and colder winters with shorter and hotter summers would seem more adapted to the growth of glaciers.
but for a more exact investigation our next step must be to inquire what are the causes which may have produced these postulates of a glacial period, lower temperature with larger evaporation. they may be classed under two heads.
1st. geographical causes, arising from latitude, a?rial and oceanic currents, and a different distribution of sea and land.
2nd. cosmic causes, such as variations of solar and terrestrial heat, passage through colder regions of space, the position of the poles, precession, and the eccentricity of the earth's orbit.
all these have had supporters in their time, but the result of the latest science has been to leave only two seriously in the field—lyell's theory of a different distribution and elevation of sea and land, carrying with it changes in a?rial and oceanic currents; and croll's theory of the effects of precession combined with high eccentricity of the earth's orbit.
thus, of the geographical causes, latitude is no doubt an important factor in determining temperature, but it cannot of itself be the cause of the glacial periods, for it has remained unchanged through all the vicissitudes of heat and cold in geological times. the latitude of greenland and spitzbergen is presumably the same now as it was in the miocene period, when they were the seat of a luxuriant temperate vegetation; and at the present day we have only to follow the isothermal lines to see to what a great extent climate in the same 298 latitudes is modified by other influences, such as the gulf stream.
of cosmic causes, the progressive cooling of the earth naturally presents itself, at the first blush, as sufficient to account for the glacial period. but although this has doubtless been an all-important factor in pregeological times, in fashioning our planet from glowing vapour into a habitable earth, it is no longer operative as an immediate cause of vicissitudes of temperature. it is enough to say that if it were, the cooling ought to be progressive, and having once got into a glacial period we never ought to have got out of it. but we clearly have recovered from the paroxysms of cold, both of the first and second great glaciations of the recent period; and according to most geologists, from the immensely earlier ones of the permian and carboniferous, and perhaps of the cambrian ages. as far as it acts at all on surface temperature, the secular cooling of the earth only acts indirectly by causing elevations and depressions of the outer crust, and crumpling it into wrinkles, which originate mountain chains, as the nucleus contracts, and thus affecting geographical conditions.
the same objection applies with equal force to the theory that the glacial period was caused by the sun giving out less heat owing to its cooling by radiation. here also it is obvious that if a glacial period were once established from such a cause it ought never to recover, but progress from bad to worse. we ought also, in this case, to have had a uniform progressive refrigeration from the beginning of geological time down to the present day, which has certainly not been the case. on the contrary, geologists are generally agreed that there are unmistakable traces of at least two glacial periods in the 299 carboniferous and permian ages, and the earliest eocene was certainly cooler than its later stages, as shown by their flora.
the conjecture that the sun is a variable star is also negatived by the consideration that in this case there ought to have been periodical variations in the earth's temperature, and hot and cold climates recurring at regular intervals throughout geological time, which has certainly not been the case.
again, the passage of the solar system through cold regions of space has been suggested, but it is a mere conjecture, unsupported by a particle of evidence, and opposed to all we know of the laws of heat, and of the constitution of the universe. it is hard to conceive how hot regions can exist surrounded by cold ones, or vice versa, without walls of a non-conducting medium to separate them, or that the faint heat from the fixed stars can ever have perceptibly affected the temperature of space. and such a theory, if it were possible, would fail to account for the frequent vicissitudes of hot and cold at short intervals within the glacial period, and for the great differences of temperature prevailing in the same latitudes.
an alteration in the position, of the poles has also been suggested, but this also is clearly inadmissible. there is no evidence that the present position has ever materially varied, and there is no known law that could cause such a variation. on the contrary, all the elaborate mathematical calculations by which the motions of the sun, moon, and planets are deduced from newton's law of gravity, tend to negative such a supposition.
and what is perhaps even more convincing to a nonmathematical 300 mind, the position of the poles implies the position of the equator, and cannot change without a corresponding change in the earth's shape. now the earth is not a sphere, but an oblate spheroid, of almost the exact shape which a fluid mass would take revolving about the present axis. the centrifugal force arising from the greater velocity of rotation in going from the poles to the equator would pile up a protuberant belt where the velocity was greatest, and in point of fact the earth's equatorial diameter is longer than the polar diameter by about twenty-eight miles. any displacement therefore of the poles, which carried them away from their present position, must displace the present equator to a corresponding extent. this mass of twenty-eight miles in thickness of earth and ocean must be thrown out of the old position, and driven to establish a new equilibrium in a position many degrees north or south of it in order to affect climates materially, submerging all existing lands, and leaving, until removed by denudation, miles upon miles of solid earth in unsymmetrical belts, like the moraines of retreating glaciers, as the equator shifted into new positions. and all this must have occurred, not once, but twice at least, and that with many minor vicissitudes, within the narrow limit of the quaternary period. it is unnecessary to say that nothing of the sort could by any possibility have occurred. some evidence has recently been adduced that some very slight changes in latitude are going on at the observatories of dorpat and greenwich, but if confirmed these can only be of very minute amount, arising from slight changes in the position of the earth's centre of gravity owing to partial elevations and depressions, and could never 301 have been sufficient to account for great variations of climate.[11]
neither could the precession of the equinoxes have been of itself a principal cause, for here also the limit of time negatives the supposition. this precessional circle carries the perihelion and aphelion, and with it the seasons, completely round, and brings them back to the old position, in about 21,000 years, and therefore if glacial periods were occasioned by them, there ought to be alternations from maximum of cold to maximum of warmth in each hemisphere every 10,500 years. but this has certainly not been the case even in recent times, and still less if we go back to the quaternary, tertiary, and earlier geological periods.
in fact it is only when combined with periods of high eccentricity of the earth's orbit, according to croll's theory, that precession can pretend to have any claim to be an important factor in the production of glacial periods. and even then the question is not of its being the sole or principal cause, but only whether it has had such a perceptible auxiliary effect on other more powerful causes, as may enable us to use it as a chronometer in assigning approximate dates for some of the more important phenomena of the long and varied period between the close of the tertiary and the establishment of the recent period.
as man certainly existed throughout the whole of this period, the possibility of finding such a chronometer becomes intensely interesting, and i proceed to discuss the latest state of scientific opinion respecting it. but 302 as croll's theory if a real is clearly only an auxiliary cause, i will, in the first instance, point out what are the certain and admitted causes which account for variations of temperature irrespective of latitude.
they may be summed up, in lyell's words, as different combinations of sea and land, for on these depend the secondary conditions which affect temperature. thus elevation of land is as certain a cause of cold as high latitude, and even kilimanjaro, under the equator, retains patches of unmelted snow throughout the year. it is estimated that a rise of 1000 feet in height is about equivalent to a fall of 3° f. in mean annual temperature, and that the line of perpetual snow is, on the average, a little higher than the line where this mean annual temperature is at 32° f., or freezing-point. if there is any mass of land so high as to be below this temperature, snow accumulates and forms glaciers, which descend some 4000 feet below the snow-line before the excess of ice pushing down is melted off by the summer heat unless it has been previously floated off in icebergs at a higher level. now the mean temperature of the north of scotland at sea-level is about 46° f., so that an elevation of 8000 or 10,000 feet would bring a great part of it well above the snow-line, and vast glaciers would inevitably accumulate, which would push down through the principal valleys almost to the sea-level; a state of things which actually exists in new zealand, where glaciers from the southern alps at about this elevation descend, in some instances to within 700 feet of the sea-level, in the latitude of devonshire. but a still more important factor of temperature is found in a?rial and oceanic currents, which again, to a great extent, are a product of the configuration of sea 303 and land. the most familiar instance is that of the gulf stream, which raises the temperature of western europe some 10°, and in norway as much as 15° f., above that due to latitude, and which prevails on the other side of the atlantic. the northern extremity of the british islands in shetland is on the same parallel of latitude as the southern extremity of greenland, cape farewell. one is buried under perpetual ice, in the other there is so little frost in winter that skating is an unknown art.
what is the reason of this? we must go to the tropics to find it. a vast mass of vapour is raised by the sun's heat from the oceans near the equator, which being lighter rises and overflows, the trade winds rushing in from the north to supply its place, and being deflected to the west by the earth's rotation. this prevalence of easterly surface winds sweeps the waters of the atlantic to the west, where they are intercepted by south america, turned northwards into the gulf of mexico, where they circle round under a tropical sun and become greatly heated, and finally run out through the straits of florida with a rapid current, and spread a surface return current eastwards over the northern atlantic. the shores of north-west europe are thus in the position of a house warmed by hot-water pipes, while their neighbours over the way in north-eastern america have no such apparatus.
this oceanic circuit of warm water has a counterpart in the a?rial circuit of heated air. the vapour which rose in the tropics overflows, and as it cools and gets beyond the region of the trade winds, descends mainly over the northern atlantic, carrying with it its greater velocity of rotation, and so causing westerly winds, 304 which reach our shores after blowing over a wide expanse of ocean heated by the gulf stream, thus bringing us warmth and wet, while the corresponding counter-currents which blow over continental europe and asia from the north-east bring cold and drought. the extreme effects of this may be seen by comparing the black sea at odessa, where ice often stops navigation, with the north sea at the lafoden islands, where the cod-fishing is carried on in open boats in the middle of winter. we in england are in the happy position where on the whole the mild and genial west winds prevail, though not exclusively, so as to give us the drenching rains of western ireland and scotland, or to prevent spells of a continental climate which give us bracing frosts in winter, and alternations of cold and heat in summer.
if we turn from these temperate regions to those in which exactly opposite conditions prevail, we find them still in the icy chains of a glacial period. greenland, for instance, which is a typical case, shows us what happens when a continental mass of land stands at a high elevation in high latitudes with no gulf stream, but instead of it cold currents from a polar ocean, and seas around it frozen or covered with icebergs for nine months out of the year. we have a dome of solid ice piled up to the height of 9000 feet or upwards, and sending millions upon millions of tons of glaciers down to the sea to be floated off as icebergs. the only trace we can see here of the old great glacial period is that these conditions were formerly more intense. thus the glaciation of some of the mountain sides and islands off the coast of greenland seem to show that the ice formerly stood 2000 or 305 3000 feet higher than at present, a result which would be attained if the whole continental mass, which is now slowly subsiding, had then been elevated to that extent.
the southern hemisphere affords a still more striking example of this on a larger scale, for we have there, in all probability, higher land in higher latitudes, surrounded by frozen seas, and washed by cold currents. i pass from this however, as beyond these general facts the special conditions of the antarctic circle are not known to us like those of greenland.
from the above facts we are very safe in drawing the conclusion that during the great quaternary glacial period the conditions which now cause glaciation must have existed in an aggravated degree, and those which now give us temperate climates in regions once glaciated must have disappeared or been reversed. on the other hand, the warm climates which prevailed during the tertiary and other geological epochs, and permitted a temperate flora to flourish as far north as grinnell land and spitzbergen, could only have occurred under conditions exactly the reverse of those which produced the cold. if high land in high latitudes is the principal cause of the present glaciation of greenland, still higher land must have been so in causing the still greater glaciation of the former period. scandinavia, laurentia, the british islands, the alps, apennines, rocky mountains, sierra nevada, and all great mountain ranges in the northern hemisphere must have stood at greater elevations. there must have been such an accumulation of ice and snow as to chill the air, cause fogs, and prevent the summer heat of the sun from melting off the water which fell in the solid form 306 during winter; and on the other hand, there must have been hot summers and great expanses of ocean to the south to supply the abundant evaporation which became condensed by contact with the chilly mountains and uplands of the north.
one supposition is that the isthmus of panama was then submerged, so that the gulf stream ran into the pacific. but this wants geological confirmation, as the isthmus shows no sign of such recent marine formations as must have been deposited if it had been submerged to a sufficient depth to let the gulf stream escape, and the extension of the ice-cap in north america to much lower latitudes than in europe, points rather to the conclusion that the gulf stream must have run very much in its present course.
the only geological evidence bearing on this question is the recent discovery of deep oceanic deposits such as the globigerena ooze, above tertiary deposits in barbadoes and jamaica, leading to the inference that the whole west indian area was a deep sea in comparatively modern times. this no doubt might affect both the temperature and the velocity of the gulf stream to a considerable extent.
but the geological evidence is much more conclusive for the greater elevation of the land during the periods of greater glaciation as well as for its depression during the inter-glacial period. american geologists estimate that a large part of eastern canada with adjacent regions must have been at least 2000, and may have been as much as 3000 feet above its present level during the first great glaciation; while the champlain marine beds show that it was some hundreds of feet below the present, sea-level during part of the inter-glacial period. scandinavia 307 stood at least 2000 feet higher than at present during the climax of the glacial period as proved by the depths of the fiords, and afterwards 500 or 600 lower as proved by the raised beaches. in great britain and ireland we have conclusive evidence both of higher elevation, and of depression of at least 1300 feet, and probably more than 2000 feet below the present sea-level, as proved by the marine shells on the top of moel-tryfen.
but these elevations and depressions are small in amount compared with the mountain building which is known to have occurred in asia in comparatively recent geological times. here the himalayas, stretching for 1500 miles from east to west, and rising to heights of from 20,000 to 29,000 feet above the sea, have been formed in great part during this period. within the same period the great table-lands of thibet and central asia have been uplifted, and the asian mediterranean sea, of which the black sea, the caspian, the salt deserts and lake balkash are the remnants, has been converted into dry land. movements of this magnitude, of which there are many other examples, may well account for great changes in isothermal lines and climates.
the complete removal of the conditions which produced the glacial period might go far to account for the preceding tertiary period. we have only to suppose a different configuration of sea and land; nothing but low lands and islands in high latitudes; free access for warm oceanic currents like the gulf stream into the limited area of the polar basin; no great continents or lofty mountain ranges to drain the return trade winds of their moisture; in short, all the 308 conditions of a mild and moist insular climate, as opposed to those of a continental one, to understand how forests of temperate trees might flourish as far north as greenland and spitzbergen. and the geological evidence which, as we have seen, shows that great elevation of land in the northern hemisphere did in fact inaugurate the glacial period, favours the conclusion that the reverse conditions actually prevailed during the tertiary and preceding epochs.
the presence of the nummulitic and other marine eocene and miocene formations over such extensive areas, and at such great elevations, is a conclusive proof that a great part of our existing continents were then at the bottom of deep oceans. the alps were certainly 10,000 feet lower than their present level, and the himalayas more so; and when this was the case a great part of europe and asia must have been sea, in which only a few of the highest peaks and elevated plateaux stood up as islands. the pacific and indian oceans as well as the atlantic might then have poured their gulf streams into the polar basin, and prevalent southerly and westerly winds, blowing over wide expanses of water, have deposited their vapour in genial showers instead of in solid snow. the effect of such geographical conditions in producing both heat and cold is admirably worked out by wallace in his island life, and few who read it can doubt that lyell was right in saying that they have been the principal causes of the vicissitudes of climate. and here i may say a word to express my admiration of the innate sagacity with which lyell, many years ago, and with comparatively few facts to work upon, sketched out the leading lines of geology, which have been 309 confirmed by subsequent research. details may have been corrected or added, but his main theories have stood the fullest test of the survival of the fittest. his law of the uniformity of natural causes, continued for long intervals of time, holds the field unchallenged. these causes may have operated a little more quickly or slowly in former ages than at present, but they have been of the same order. the waste of continents, instead, of averaging one foot in 4000 years, may have averaged ten or twenty feet during certain periods, and certain portions of the earth's crust may have been elevated or depressed at a quicker rate than is now going on in scandinavia; but no one any longer believes in paroxysms throwing up mountain chains or sinking continents below the ocean at a single blow.
in like manner later geologists have corrected details in the distribution of land and sea suggested by lyell to account for the glacial period, but his main law has only received confirmation—viz. that this distribution, and especially high land in high latitudes, has been the principal cause of such periods.
at the same time there is a pretty general consensus of the best and latest geologists, that, as lyell himself suggested, elevation and depression and other geographical changes, though the principal, are not the sole causes of the glacial period. the main argument is, that the phases of this period, though not exactly simultaneous over the whole world, are too nearly so to be due to mere local movements, and require the intervention of some general cosmic cause. we have already seen that of such causes there is none which appears feasible except croll's theory of the effects of precession combined with high eccentricity.
310 let us consider what this theory really asserts. if the earth were a perfect sphere, its orbit round the sun a perfect circle, and the equator coincided with the ecliptic, there would be no seasons. the four quarters of the year would each receive the same quantity of solar heat and light, and the days and nights would be always equal. but the inclination of the equator to the ecliptic, that is, of the earth's plane of daily rotation to that of its annual revolution, necessitates seasons. each pole must be alternately turned to and away from the sun every year. each hemisphere, therefore, must have alternately its spring, summer, autumn, and winter. but if the earth's orbit were exactly circular, these seasons would be of equal duration, and the distance from the sun no greater in one than in another. but the earth's orbit is not circular, but elliptic, and the eccentricity, or deviation of the oval from the circular form, varies considerably over very long periods, though always coming back to the amount from which it started. these variations are due to perturbations from the other bodies of the solar system acting according to the law of newton's gravitation, and therefore calculable.
again, the earth is not a perfect sphere, but a spheroid, and there is a factor called precession, due to the attraction on the protuberant mass at and towards the equator. the effect of this is, that instead of the earth's axis pointing uniformly towards the same celestial pole, it describes a small circle round it. this circle is completed in about 21,000 years, so that if the earth is nearest to the sun when the north pole is turned away from it, and it is winter in the northern hemisphere, as is now the case, in 10,500 years the conditions will be reversed, and the southern hemisphere will be in perihelion, 311 or nearest the sun, when its pole points away from it. and as the perihelion portion of the earth's orbit is, owing to its eccentricity, shorter and more quickly traversed than the aphelion portion, this means practically that winters will be shorter than summers in the hemisphere which precession favours, and longer in that to which it is adverse.
as precession now favours the northern hemisphere, which is warmer than the southern in corresponding latitudes, it might be thought at first sight that this was the cause of the glacial period. but it is evident that this is not the case, for the precessional revolutions come round far too rapidly, and it is impossible to suppose that there have been glacial and genial periods alternating every 10,500 years, with all the inevitable changes of seas and lands, and of fauna and flora, accompanying each alternation throughout the whole of geological time. in fact, it is abundantly evident, on historical evidence alone, that there has been no approach to any such changes during the last 10,500 years, which carries us back to a period when our northern summers were short and our winters long.
but croll's theory brings in the secular variation of the eccentricity, and contends that although precession may have little or no effect while the earth's orbit is nearly circular, as it is now, it must have a considerable effect when the orbit flattens out, so that the distances from the sun and the durations of summer and winter become exaggerated. croll calculated the periods when such maxima and minima of eccentricity occurred for several revolutions back from the formula of the great astronomer leverrier, and found that going back for the last 260,000 years there had been two maxima 312 of high eccentricity, one 100,000 years, and the other, and more intense, 210,000 years ago, with corresponding minima of low eccentricity between, which corresponded remarkably well with the refrigeration commencing in the pliocene, culminating towards its close or in the early quaternary, subsiding into a long inter-glacial period, rising again in the later quaternary to a second glacial maximum a little less intense than the first, and finally gradually subsiding into the low eccentricity and temperate climates of more recent times; especially as the geological evidence shows many minor oscillations of heat and cold and advances and retreats of glaciers during each phase of these periods, such as must have occurred from the shorter recurrent effects of precession according to croll's theory.
croll's calculations show that, at the period of maximum eccentricity 210,000 years ago, the earth would have been in mid-winter 8,736,420 miles further from the sun than it is now, and the winter half of the year nearly twenty-eight days longer than the summer half, instead of being six days shorter as at present. it appears, moreover, from a volume just published, on the astronomical causes of an ice age, by sir r. ball, one of the highest authorities on mathematics and astronomy, that croll had understated his case. ball says that "croll, misled by a statement of herschell's, had assumed the number of units of heat received from the sun, in a hemisphere of the earth, as equal in summer and winter. but in reality, of 100 such units, 63 are received in summer and only 37 in winter. as the maximum of eccentricity which is possible would produce an inequality between summer and winter of 33 days, they had the following possible conditions in a 313 hemisphere—summer 199 days and winter 166 days, or summer 166 days and winter 199 days. in each case it must be borne in mind that 63 heat units arrived in summer and 37 in winter. if the summer were a long one and the winter short, then the allotment of heat between the two seasons would be fairly adjusted. the 63 units were distributed over 199 days and the 37 units over 166 days, and a general inter-glacial state was the result on the hemisphere. if, however, a torrent of heat represented by 63 units was poured in during a brief summer of 166 days, whilst, the balance of 37 units is made to stretch itself over 199 days, a brief, intensely hot summer would be followed by a very long and cold winter, and as this condition lasted for many centuries, it seemed sufficient to produce a glacial epoch."
it would be going, too far, however, to assume that these conditions necessarily produced glacial periods whenever they occurred, and ball himself points out that even on astronomical grounds, several conditions must concur before high eccentricity alone would affect climate. but even with this reservation the same objection applies to assigning, this as the sole or principal cause of ice ages, as to precession alone, viz. that periods of high eccentricity occur too frequently to allow us to suppose that every such period in the past has had its corresponding glacial period. there was a maximum phase of eccentricity 700,000 years ago, even higher than that of 210,000 years, and there must have been at least two or three such maxima within each of the twenty-eight geological ages. but there are only two or three traces of glacial periods in past epochs on which geologists can rely with confidence, as proving 314 extensive ice-action—one in the permian, the other in the carboniferous age.
there are a few other instances which look like glacial action, as the conglomerate of the superga at turin, the flysch of switzerland, the great conglomerate at the base of the devonian; and professor geikie thinks that the oldest cambrian rocks in the west highlands have been rounded and smoothed by ice before the silurian strata were deposited on them. but even if these were authenticated and proved to be due to general and not merely local causes, they would not supply anything like the number of glacial periods required by croll's theory. croll attempts to meet this by the extensive denudation which has repeatedly carried away such large portions of land surface; but this scarcely explains the absence of the boulders of hard rocks, which accompany every moraine and iceberg; and still less the continuance of the same fauna and flora throughout whole geological periods with little or no change. we have no such abrupt changes as during the last glacial period, when at one time the canary laurel flourished in central france, while at another the reindeer moss and arctic willow extended to the pyrenees, both occurring within what may be called a short time, geologically speaking. on the contrary, there seems to have been no material changes in the flora throughout very long geological periods such as that of the coal measures.
the only real answer to this objection is that the question is, not whether croll's theory is the sole or even the principal cause of glacial periods, or able to influence them materially if the geographical conditions favour genial climates; but whether it has not a 315 co-operating effect, when these conditions are such as to produce glaciation. it seems difficult to suppose that such contrasts of conditions as are pointed out by sir r. ball can have had no perceptible effect on climates; or that such close coincidences as are shown between the astronomical theory and geological facts, during the last glacial period, can be due to mere accident.
geology shows six phases of this period:—(1) a refrigeration coming on in the pliocene; (2) its culmination in a first and most intense maximum; (3) a gradual return to a milder inter-glacial period; (4) a second refrigeration; (5) its culmination in a second maximum; (6) a second return to genial conditions, such as still prevail. croll's theory shows six astronomical phases, corresponding to these six geological phases. geology shows that each of its six phases involves several minor alternations of hot and cold; croll shows that this must have been the case owing to the effects of the shorter cycles of precession, occurring during the long cycles of variations in eccentricity. geology tells us that cold alone would not account for a glacial period; we must have heat to supply the evaporation which is condensed by the cold; croll shows that with high eccentricity cold and long winters must have been accompanied by short and hot summers. and sir r. ball's recent calculations show that the argument is really very much stronger than croll puts it.
the duration of each of the phases of croll's theory corresponds also, on the whole, remarkably well with that required for each phase of the geological record. they would average about 40,000 years each for croll's phases, and a less time can hardly be allowed for the immense amount of geological work in the way of 316 denudation and deposition, elevation and depression, and changes of fauna and flora which have occurred since the commencement of the great refrigeration in the late pliocene. in fact the only reasonable doubt seems to be whether croll's times are sufficient, and whether, as lyell was inclined to think, the first and greatest glaciation must not be carried back to the extreme period of high eccentricity which occurred about 700,000 years ago.
unless we are prepared to ignore all these considerations and deny that croll's theory, as amended by sir r. ball, has had any appreciable effect on the conditions of the glacial period, it follows with mathematical certainty, that this period, taking it from the commencement of the great refrigeration in the pliocene to its final disappearance in the recent, must have lasted for about 200,000 years. and as man clearly existed in the pre-glacial period, and was already widely spread and in considerable numbers in the early glacial, 250,000 years may be taken as an approximation to the minimum duration of the existence of the human race on the earth. to this must be added an indefinitely long period beyond, unless we are prepared to disprove the apparently excessively strong evidence for its existence in the pliocene and even in the miocene periods; evidence which has been rapidly accumulating of late years; and to which, as far as i know, there has been no serious and unbiassed attempt at scientific refutation; and to which confirmation is given by the undoubted fact that the dryopithecus, the hylobates, and other quadrumana, closely resembling man in physical structure, already existed in the miocene, and, if professor ameghino's discoveries referred to at p. 264 are confirmed, in the vastly more remote period of the early eocene.
