The conclusions of some of our best scholars on this subject are so opposed to all that we would think possible, according to the present climate and surroundings, that they seem at first incredible, and yet they have been worked out with such care that there is no doubt of the substantial truth of the results.

The terminal moraine of the great glacier has been carefully traced through several States. We now know that one vast sea of ice covered the eastern part of North America, down to about the thirty-ninth parallel of lat.i.tude. We have every reason to think that the great glacier, extending many miles out in the Atlantic, terminated in a great sea of ice, rising several hundred feet perpendicularly above the surface of the water. Long Island marks the southern extension of this glacier.

From there its temporal moraine has been traced west, across New Jersey and Pennsylvania, diagonally across Ohio, crossing the river near Cincinnati, and thence west across Indiana and Illinois. West of the Mississippi it bears off to the north-west, and finally pa.s.ses into British America.<4>

All of North America, to the north and northeast of this line, must have been covered by one vast sea of ice.<5> Doubtless, as in Greenland to-day, there was no hill or patch of earth to be seen, simply one great field of ice. The ice was thick enough to cover from sight Mt.

Washington, in New Hampshire, and must have been at least a mile thick over a large portion of this area,<6> and even at its southern border it must in places have been from two hundred to two thousand feet thick.<7> This, as we have seen, is a picture very similar to what must have been presented by Europe at this time.<8>

Ill.u.s.tration of Antarctic Ice Sheet.-----------------

The Northern Atlantic Ocean must have presented a dreary aspect. Its sh.o.r.es were walls of ice, from which ever and anon great ma.s.ses sailed away as icebergs. These are startling conclusions. Yet, in the Southern Hemisphere to-day is to be seen nearly the same state of things. It is well-known that all the lands around the South Pole are covered by a layer of ice of enormous thickness. Sir J. A. Ross, in attempting to reach high southern lat.i.tudes, while yet one thousand four hundred miles from the pole, found his further progress impeded by a perpendicular wall of ice one hundred and eighty feet thick. He sailed along that barrier four hundred and fifty miles, and then gave up the attempt. Only at one point in all that distance did the ice wall sink low enough to allow of its upper surface being seen from the mast-head. He describes the upper surface as an immense plain shining like frosted silver, and stretching away as far as eye could reach into the illimitable distance.<9>

The foregoing makes plain to us one phase of the Glacial Age. Though it may not be quite clear what this has to do with the antiquity of man, yet we will see, in the sequel, that it has considerable. As to the periods of mild climate that are thought by some to have broken up the reign of cold, we do not feel that we can say any thing in addition to what has been said in a former chapter.<10>

We might, however, say, that the sequences of mild and cold climate are not as well made out in America as they seem to be in Europe; or at least our geologists are more cautious as to accepting the evidence as sufficient. And yet such evidences are not wanting: as in Europe, at various places, are found layers of land surfaces with remains of animals and plants, but both above and below such surface soil are found beds of bowlder clay. These offer undeniable evidence that animals and plants occupied the land during temperate inter-glacial epochs, preceded and followed by an Arctic climate, and ice-sheets like those now covering the interior of Greenland, and the Antarctic Continent.<11>

We have thus, though somewhat at length, gone over the evidence as to the reality and severity of the Glacial Age. It was during the continuance of such climate that Paleolithic man arrived in Europe, though it was not perhaps until its close. We must not lose sight of the fact that our princ.i.p.al object at present is to determine, if we can, a date for either the beginning or ending of this extraordinary season of cold, and thereby achieve an important step in determining the antiquity of man.

A moment's consideration will show us that a period of cold sufficient to produce over a large portion of the Northern Hemisphere the results we have just set forth must have a cause that is strange and far-reaching. It can not be some local cause, affecting but one continent, since the effect produced is observed as well in Europe as in America.

Every year we pa.s.s through considerable changes in climate. The four seasons of the year seem to be but an annual repet.i.tion, on a very small scale of course, of the great changes in the climate of the earth that culminated in the Glacial Age; though we do not mean to say, that periods of glacial cold come and go with the regularity of our Winter.

The changes in the seasons of the year are caused by the earth's position in its...o...b..t, and its annual revolution around the sun. It may be that the cause of the Glacial Age itself is of a similar nature; in which case it is an astronomical problem, and we ought, by calculation, to determine, with considerable accuracy, dates for the beginning and ending of this epoch.

Nothing is clearer than that great fluctuations of climate have occurred in the past. Many theories have been put forth in explanation. It has been suggested that it was caused by loss of heat from the earth itself.

That the earth was once a ball of incandescent matter, like the sun, and has since cooled down, is of course admitted. More than that, this process still continues; and the time must come when the earth, having yielded up its internal heat, will cease to be an inhabitable globe. But the climate of the surface of the earth is not dependent upon the heat of the interior. This now depends "according to the proportion of heat received either directly or indirectly from the sun; and so it must have been during all the ages of which any records have come down to us."<12> Some have supposed that the sun, traveling as it does through s.p.a.ce, carrying the earth and the other planets with him, might, in the course of ages, pa.s.s through portions of s.p.a.ce either warmer or colder than that in which it now moves. When we come to a warm region of s.p.a.ce, a genial climate would prevail over the earth; but, when we struck a cold belt, eternal Winter would mantle a large part of the globe with snow and ice. This, of course, is simply guess-work. No less than seven distinct causes have been urged; most of them either purely conjectural, like the last, or manifestly incompetent to produce the great results which we have seen must be accounted for. But, amongst these, two causes have been advanced--the one astronomical, the other geographical; and, to the one or the other, the majority of scholars have given their consent.

It will be no harm to see what can be said in favor of both theories.

So, we will ask the reader's attention, as it is our earnest desire to make as plain as possible a question that has so much to do with our present inquiry. In the course of our investigations, we can not fail to catch glimpses of wonderful changes in far away times; and can not help seeing what labor is involved in the solution of all questions relating to the same.<13>

The earth revolves around the sun in an orbit called an ellipse. This is not a fixed form, but slowly varies from year to year. It is now gradually becoming circular. It will, however, not become an exact circle. Astronomers a.s.sure us that, after a long lapse of time, it will commence to elongate as an ellipse again. Thus, it will continually change from an ellipse to an approximate circle, and back again. In scientific language, the eccentricity of, the earth's...o...b..t is said to increase and decrease.

Ill.u.s.tration of Earth's...o...b..t.--------------------

In common language we would state that the shape of the path of the earth around the sun was sometimes much more elongated and elliptical than at others. The line drawn through the longest part of an ellipse is called the major axis. Now the sun does not occupy the center of this line, but is placed to one side of it; or, in other words, occupies one focus of the ellipse. It will thus be seen that the earth, at one time during its yearly journey, is considerably nearer to the sun than at others. The point where it approaches nearest the sun is called _Perihelion,_ and the point where it reaches the greatest distance from the sun is called its _Aphelion._ It will be readily seen that the more elliptical its...o...b..t becomes the greater will be the difference between the perihelion and aphelion distance of the sun. At present the earth is about three millions of miles nearer the sun in perihelion than in aphelion. But we must remember the orbit of the earth is now nearly circular. There have been times in the past when the difference was about thirteen millions of miles. We must not forget to add, that the change in the shape of the earth's...o...b..t is not a regular increase and decrease between well-known extremes. It is caused by the attraction of the other planets. It has been calculated at intervals of ten thousand years for the last million years. In this way it has been found that "the intervals between connective turning points are very unequal in length, and the actual maximum and minimum values of the eccentricity are themselves variable. In this way it comes about that some periods of high eccentricity have lasted much longer than others, and that the orbit has been more elliptical at some epochs of high eccentricity than at others."<14> We have just seen that the earth is nearer the sun at one time of the year than at another. At present the earth pa.s.ses its perihelion point in the Winter of the Northern Hemisphere, and its aphelion point in the Summer. We will for the present suppose that it always reaches the points at the same season of the year. Let us see if the diminished distance from the sun in Winter has any thing to do with the climate.

If so, this effect will be greatly magnified during a period of high eccentricity, such as the earth has certainly pa.s.sed through in the past. We will state first, that the more elliptical the orbit becomes, the longer Summer we have, and the shorter Winter. Astronomically, Spring begins the 20th of March, and Fall the 22d of September. By counting the days between the epochs it will be found that the Spring and Summer part of the year is seven days longer than the Fall and Winter part. But if the earth's...o...b..t becomes as highly eccentrical as in the past, this difference would be thirty-six days.<15>

This would give us a long Spring and Summer, but a short Fall and Winter. This in itself would make a great difference. We must beer in mind, however, that at such a time as we are here considering, the earth would be ten millions of miles nearer the sun in Winter than at present.

It would certainly then receive more heat in a given time during Winter than at present.<16> Mr. Croll estimates that whereas the difference in heat received during a given time is now one-fifteenth,<17> at the time we are considering it would be one-fifth. Hence we see that at such a time the Winter would not only be much shorter than now, but at the same time would be much milder.

These are not all the results that would follow an increase of eccentricity. The climate of Europe and North America is largely modified by those great ocean currents--the Gulf Stream and the j.a.pan current. Owing to causes we will not here consider, these currents would be greatly increased at such a time. As a result of these combined causes, Mr. Croll estimates that during a period of high eccentricity the difference between Winter and Summer in the Northern Hemisphere would be practically obliterate. The Winter would not only be short, but very mild, and but little snow would form, while the sun of the long Summers, though not shining as intense as at present, would not have to melt off a great layer of snow and ice, but the ground became quickly heated, and so warmed the air. Hence, if Mr. Croll be correct, a period of high eccentricity would certainly produce a climate in the Northern Hemisphere such as characterized many of the mild interglacial epochs as long as the earth pa.s.sed its perihelion point in Winter.

We have so far only considered the Northern Hemisphere. As every one knows, while we have Winter, the Southern Hemisphere has Summer. So at the very time we would enjoy the mild short Winters, the Southern Hemisphere would be doomed to experience Winters of greatly increased length and severity. As a consequence, immense fields of snow would be formed, which, by pressure, would be changed to ice, and creep away as a desolating glacier. It is quite true that the short Summer sun would shine with increased warmth, but owing to many causes it would not avail to free the land from snow and ice.

As Mr. Geikie points out, "An increased amount of evaporation would certainly take place, but the moisture-laden air would be chilled by coming into contact with the vast sheets of snow, and hence the vapor would condense into thick fogs and cloud the sky. In this way the sun's rays would be, to a large extent, cut off, and unable to reach the earth, and consequently the Winter's snow would not be all melted away."

Hence it follows that at the very time the Northern Hemisphere would enjoy a mild interglacial climate, universal Spring, so to speak, the Southern Hemisphere would be encased in the ice and snow of an eternal Winter.

But the earth has not always reached its perihelion point during the Winter season of the Northern Hemisphere. Owing to causes that we need not here consider, the earth reaches its perihelion point about twenty minutes earlier each year, so if it now pa.s.ses its perihelion in Winter of the Northern Hemisphere, in about ten thousand years from now it will reach it in Summer, and in twenty-one thousand, years it will again be at perihelion in Winter. But see what important consequences follow from this. If during a period of high eccentricity we are in the enjoyment of short mild Winters and long pleasant Summers, in ten thousand years this would certainly be changed. Our Summer season would become short and heated; our Winters long and intensely cold. Year by year it would be later in the season before the sun could free the land from snow, and at length in deep ravines and on hill-tops the snow would linger through the brief Summer, and the mild interglacial age will have pa.s.sed away, and again the Northern Hemisphere will be visited by snow and ice of a truly. Glacial Age. If, therefore, a period of high eccentricity lasts through the many thousand years, we must expect more than one return of glacial cold interspersed by mild interglacial climates.

We have tried in these last few pages to give a clear statement of what is known as Croll's theory of the Glacial Age. There is no question but what the earth does thus vary in its position with regard to the sun, and beyond a doubt this must produce some effect on the climate, and we can truthfully state that the more the complicated question of the climate of the earth is studied, the more grounds do scholars find for affirming that indirectly this effect must have been very great. And yet we can not say that this theory is accepted as a satisfactory one even by the majority of scholars. Many of those who do not reject it think it not proven. Therefore, before interrogating the astronomer as to the data of the Glacial Age, according to the terms of this theory, let us see what other causes are, adduced; then we can more readily accept or reject the conclusions as to the antiquity of man which this theory would necessitate us to adopt.

The only other cause to which we can a.s.sign the glacial cold, that is considered with any favor by geologists, is geographical; that is to say, depending on the distribution of land and water. Glaciers depend on the amount of snow-fall. In any country where the amount of snow-fall is so great that it is not all evaporated or melted by the Summer's sun, and consequently increases from year to year, glaciers must soon appear, and these icy rivers would ere-long, flow away to lower levels. If we suppose, with Sir Charles Lyell, that the lands of the globe were all to be gathered around the equator, and the waters were gathered around the poles, it is manifest that there would be no such a thing as extremes of temperature, and it is, perhaps, doubtful whether ice would form, even in polar areas.<18> At any rate, no glaciers could be formed, as there would be no land on which snow could gather in great quant.i.ties.

If, however, we reverse this picture, and conceive of the land gathered in a compact ma.s.s around the poles, shutting out the water, but consider the equatorial region of the earth to be occupied by the waters of the ocean, we would manifestly have a very different scene. From the ocean moisture-laden winds would flow over the polar lands. The snowfall would necessarily be great. In short, we can not doubt but what all the land of the earth would be covered with glaciers.<19>

Although these last conceptions are purely hypothetical, they will serve the good purpose of showing the great influence that the geographical distribution of land and water have on the climate of a country. Of one thing, however, geologists have become more and more impressed of late years. That is, that continents and oceans have always had the same relative position as now; that is to say, the continents have followed a definite plan in their development. The very first part of North America to appear above the waters of the primal sea clearly outlined the shape of the future continent. Mr. Dana a.s.sures us that our continent developed with almost the regularity of a flower. Prof. Hitchc.o.c.k also points out that the surface area of the very first period outlined the shape of the continent. "The work of later geological periods seems to have been the filling up of the bays and sounds between the great islands, elevating the consolidated ma.s.s into a continental area."<20> So it is not at all probable that the lands of the globe were ever grouped, as we have here supposed them.

This last statement is liable, however, to leave us under a wrong impression; for although, as a whole, continental areas have been permanent, yet in detail they have been subject to wonderful and repeated changes. "Every square mile of their surface has been again and again under water, sometimes a few hundred feet deep--sometimes, perhaps, several thousand. Lakes and inland seas have been formed and been filled up with sediment, and been subsequently raised into hills, or even mountains. Arms of the sea have existed, crossing the continent in various directions, and thus completely isolating the divided portions for varying intervals. Seas have become changed into deserts and deserts into seas."<21>

It has been shown beyond all question that North-western Europe owes its present mild climate to the influence of the Gulf Stream.<22> Ocean currents, then, are a most important element in determining the climate of a country. If we would take the case of our hypothetical polar continent again, and, instead of presenting a continuous coast line, imagine it penetrated by long straits and fiords, possessing numerous bays, large inland seas, and in general allowing a free communication with the ocean, we are very sure the effect would be widely different.

Under these circ.u.mstances, says Mr. Geikie, the "much wider extent of sea being exposed to the blaze of the tropical sun, the temperature of the ocean in equatorial regions would rise above what it is at present.

This warm water, sweeping in broad currents, would enter the polar fiords and seas, and everywhere, beating the air, would cause warm, moist winds to blow athwart the land to a much greater extent than they do at present; and these winds thus distributing warmth and moisture, might render even the high lat.i.tude of North Greenland habitable by civilized man." So we see that it is necessary to look for such geographical changes as will interfere with the movements of marine currents.

Now, it is easy to see that comparatively small geographical changes would not only greatly interfere with these currents, but might even cause them to entirely change their course. An elevation of the northern part of North America, no greater in amount than is supposed to have taken place at the commencement of the Glacial Age, would bring the wide area of the banks of Newfoundland far above the water, causing the American coast to stretch out in an immense curve to a point more than six hundred miles east of Halifax, and this would divert much of the Gulf Stream straight across to the coast of Spain.<23>

Such an elevation certainly took place, and if continued westward, Behring's Strait would also have been closed. It is to such northern elevations, shutting out the warm ocean currents, that a great many geologists look for a sufficient explanation of the glacial cold.

Prof. Dana says: "Increase in the extent and height of high lat.i.tude lands may well stand as one cause of the Glacial Age." Then he points out how the rising of the land of Northern Canada and adjacent territory, which almost certainly took place, "all a sequel to the majestic uplift of the Tertiary, would have made a glacial period for North America, whatever the position of the ecliptic, or whatever the eccentricity of the earth's...o...b..t, though more readily, of course, if other circ.u.mstances favored it."<24>

It may occur to some that if high northern lands be all that is necessary for a period of cold, we ought to have had it in the Miocene Age, when there was a continuous land connection between the lands of high polar areas and both Europe and America, since we know that an abundant vegetation spread from there, as a center, to both these countries. But at that epoch circ.u.mstances were different. The great North Temperate lands were in a "comparatively fragmentary and insular condition."<25> There were great inland seas in both Europe and Asia, through which powerful currents would have flowed from the Indian Ocean to Arctic regions.

Somewhat similar conditions prevailed in North America. The western part was in an insular condition. A great sea extended over this part of the country, joining the Arctic probably on the north, through which heated water would pour into the polar sea. And so, instead of a Glacial Age, we find evidence of a mild and genial climate, with an abundant vegetation.

We thus see that there are two theories as to the cause of the Glacial Age presented for our consideration. Both of them have received the sanction of scholars eminent for their scientific attainments. On inspection we see they are not antagonistic theories. They may both be true for that matter, and all would admit that whatever effect they would produce singly would be greatly enhanced if acting together.

Indeed, there are very good reasons for supposing both must have acted in unison.

There seem to be very good reasons for not believing that the eccentricity of the earth's...o...b..t, acting alone, produced the glacial cold. If that were the case, then whenever the eccentricity was great we should have a Glacial Age. Now, at some period of time during the long-extended Tertiary Age we are certain the eccentricity of the earth's...o...b..t became very great, much more so, in fact, than that which is supposed to have produced the cold of the Quaternary Age. But we are equally certain there was no glacial epoch during this age.<26> What other explanation can we give for its non-appearance except that geographical conditions were not favorable?

But, on the other hand, there are certain features connected with the phenomena of the Glacial Age that seem very difficult of explanation, if we suppose that geographical changes alone produced them. We must remember that evidences of the former presence of glaciers are found widely scattered over the earth. We shall, therefore, have to a.s.sume an elevation not only for America and Europe, but extend it over into Asia, and take in the Lebanon Mountains, for they also show distinct traces of glaciers. And this movement of elevation must also have affected the Southern Hemisphere, the evidence being equally plain that at the same comparatively late date glaciers crushed over Southern Africa and South America.<27> This is seen to prove too much. Again, how can we explain the fact that some time during the Glacial Age we had a submergence, the land standing several hundred feet lower than now, but still remained covered with ice, and over the submerged part there sailed icebergs and ice-rafts, freighted with their usual _debris_? That such was the state of things in Europe we are a.s.sured by some very good authorities.<28>

Neither do geographical causes afford an adequate explanation of those changes of temperature that surely took place during the Glacial Age.

These last considerations show us how difficult it is to believe that geographical causes could have produced the Glacial Age.

We are a.s.sured that all through the geological ages the continents had been increasing in size and compactness, and that just at the close of the Tertiary Age they received a considerable addition of land to the north. The astronomer also informs us that at a comparatively recent epoch the eccentricity of the earth's...o...b..t became very great. The conditions being favorable, it is not strange that a Glacial Age supervened.

We have been to considerable length in thus explaining the position of the scientific world in regard to the cause of the Glacial Age. Our reason for so doing is that this age is, we think, so connected with the Paleolithic Age of man, that it seems advisable to have a clear understanding in regard to it. What we have to say is neither new nor original. It is simply an earnest endeavor to represent clearly the conclusions of some of our best scholars on this subject, and we have tried to give to each theory its due weight. Our conclusions may be wrong, but, if so, we have the consolation of erring in very good company.

We have now gone over the ground and are ready to see what dates can be given. Though the numbers we use seem to be very large indeed, they are so only in comparison with our brief span of life. They are insignificant as compared with the extent of time that has surely rolled by since life appeared on the globe. Let us, therefore, not be dismayed at the figures the astronomer sets before us.<29>

About two hundred and fifty thousand years ago the earth's path around the sun was much the same as that of the present. No great changes in climate were liable to take place at that time. During the next fifty thousand years the eccentricity steadily increased. Towards the end of that time all that was necessary to produce a glacial epoch in the Northern Hemisphere was favorable geographical causes, and that our earth should reach its point nearest the sun in Summer. This it must have done when about half that time had elapsed.

We can in imagination see what a slow deterioration of climate took place. Thousands of years would come and go before the change would be decisive. But a time must have at length arrived when the vegetation covering the ground was such as was suited only for high northern lat.i.tudes. The animals suited for warm and temperate regions must have wandered farther south; others from the north had arrived to take their place. We can see how well this agrees with the changes of climate at the close of the Pliocene Age. The snows of the commencing Glacial Age would soon begin to fall, finally the sun would not melt them off of the high lands, and mountain peaks, and so a Glacial Age would be ushered in.

We have referred to the fact that the earth reaches its perihelion point a little earlier each year, and, as a consequence, we would have periods of mild climate alternating the cold. This extended period of time, equal to twenty-one thousand of our ordinary years, has been named the Great Year of our globe. Mr. Wallace has pointed out some very good reasons for thinking Mr. Croll's theory must be modified on this point. He thinks that when once a Glacial Age was fairly fastened on a hemisphere, it would retain its grasp as long as the eccentricity remained high, but whenever the Summer of the Great Year came to that hemisphere, it would melt back the glacial ice for some distance, but this area would be recovered by the ice when the Winter of the Great Year supervened. These effects would be different when the eccentricity itself became low. Then we would expect the glacial conditions to vanish entirely when the Summer of a Great Year comes on.<30>