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 ele............