of Greenland to-day--The Terminal Moraine--Appearance of the North Atlantic--Interglacial Age--Causes of the Glacial Age--Croll's Theory--Geographical causes--The two theories not Antagonistic--The date of the Glacial Age--Probable length of the Paleolithic Age--Time since the close of the Glacial Age--Summary of results. As we have already remarked, geological periods give us no insight as to the actual passage of years. To say that man lived in the Glacial Age, and that we have some faint traces of his presence in still earlier periods, after all conveys to our minds only vague ideas of a far-away time. The more a geologist studies the structure of the earth, the more impressed is he with the magnitude of the time that must have passed since "The Beginning." At present, however, there are no means known of accurately measuring the time that has passed. It is just as well that it is so, since, were it known, the human mind would be utterly incapable of comprehending it. But as to the antiquity of man, it is but natural that we should seek more particularly to solve the problem and express our answer in some term of years. Now, we have seen that the question of the antiquity of man is intimately connected with that of the Glacial Age. That is to say, the relics of man as far as we know them in Europe, are found under such circumstances that we feel confident they are not far removed from the period of cold. For it will be found that those conservative scholars who do not think that man preceded the Glacial Age, or inhabited Europe during the long course of years included in that period, do think he 141 came into Europe as soon as it passed away. So, in any case, if we can determine the date of the Glacial Age, we shall have made a most important step in advance in solving the problem of the antiquity of man himself. So it seems to us best to go over the subject of the Glacial Age again, and see what conclusions some of our best thinkers have come to as to its cause, when it occurred, and other matters in relation to it. It is best to state frankly at the outset that this topic is one of the great battle-grounds of science to-day, and that there are as yet but few points well settled in regard to it. One needs but attempt to read the literature on this subject to become quickly impressed with the necessity of making haste slowly in forming any conclusions. He must invoke the aid of the astronomer, geologist, physical-geographer, and physicist. Yet we must not suppose that questions relating to the Glacial Age are so abstruse that they are of interest only to the scholar. On the contrary, all ought to be interested in them. They open up one of the most wonderful chapters in the history of the world. They recall from the past a picture of ice-bound coasts and countries groaning under icy loads, where now are harbors enlivened by the commerce of the world, or ripening fields attesting the vivifying influence of a genial sun. Let us, therefore, follow after the leaders in thought. When we come to where they can not agree we can at least see what both sides have to say. Somewhat at the risk of repetition, we will try and impress on our readers a sense of the reality and severity of the Glacial Age. There is danger in regarding this as simply a convenient theory that geologists have originated to explain some puzzling facts, that it is not very 142 well founded, and is liable to give way any day to some more ingenious explanation. On the contrary, this whole matter has been worked out by very careful scholars. There is, perhaps, no great conclusion in any science which rests upon a surer foundation than this, and if we are to be guided by our reason at all in deducting the unknown from the known, the past from the present, we can not refuse our assent to the reality of the Glacial Age of the Northern Hemisphere in all its more important features. At the present day glaciers do exist in several places on the earth. They are found in the Alps and the mountains of Norway, and the Caucasus, in Europe. The Himalaya mountains support immense glaciers in Asia; and in America a few still linger in the more inaccessible heights of the Sierra Nevada. It is from a study of these glaciers, mainly however, those of the Alps, that geologists have been enabled to explain the true meaning of certain formations they find in both Europe and America, that go by the name of drift. When in an Alpine valley we come upon a glacier, filling it from side to side, there will be noticed upon both sides a long train of rock, drift, and other
debris that have fallen down upon its surface from the mountain sides. If two of these ice-rivers unite to form one glacier, two of these trains will then be borne along in the middle of the resulting glacier. As this glacier continues down the valley, it at length reaches a point where a further advance is rendered impossible by the increased temperature melting the ice as fast as it advances. At this point the train of rocks and dirt are dumped, and of course form great mounds, called moraines. The glacier at times shrinks back on its rocky bed and allows explorers to examine it. In such cases they find the rocks smoothed and polished, but here and 143 there marked with long grooves and striae. These points are learned from an examination of existing glaciers. Further down the valley, where now the glaciers never extend, are seen very distinctly the same signs. There are the same moraines, striated rocks, and bowlders that have evidently traveled from their home up the valley. The only explanation possible in this case is that once the glaciers extended to that point in the valley. It required a person who was perfectly familiar with the behavior of Alpine glaciers, and knew exactly what marks they left behind in their passage, to point out the proofs of their former presence in Northern Europe and America, where it seems almost impossible to believe they existed. Such a man was Louis Agassiz, the eminent naturalist. Born and educated in Switzerland, he spent nine years in researches among the glaciers of the mountains of his native country. He proved the former wide extension of the glaciers of Switzerland. With these results before them, geologists were not long in showing that there had once been glacial ice over a large part of Europe and North America. The proofs in this case are almost exactly the same as those used to show that the ancient glaciers of Switzerland were once larger than now. But as the great glaciers of the glacial age were many times larger than any thing we know of at the present day, there were of course different results produced. For instance, the water circulating under Alpine glaciers is enabled to wash out and carry away the mass of pulverized rock and dirt ground along underneath the ice. But when the glaciers covered such an enormous extent of country as they did in the Glacial Age, the water could not 144 sweep away this detritus, and so great beds of gravel, sand, and clay would be formed over a large extent of country. But to go over the entire ground would require volumes; it is sufficient to give the results. The interior of Greenland to-day is covered by one vast sea of ice. Explorers have traversed its surface for many miles; not a plant, or stone, or patch of earth is to be seen. In the Winter it is a snow-swept waste. In the Summer streams of ice-cold water flow over its surface, penetrating here and there by crevasses to unknown depths. This great glacier is some twelve hundred miles long, by four hundred in width. Vast as it is, it is utterly insignificant as compared with the great continental glacier that geologists assure us once held in its grasp the larger portion of North America. 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 latitude. 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 145 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 northwest, and finally passes into British America. All of North America, to the north and northeast of this line, must have been covered by one vast sea of ice. 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, and even at its southern border it must in places have been from two hundred to two thousand feet thick. This, as we have seen, is a picture very similar to what must have been presented by Europe at this time. Illustration of Antarctic Ice Sheet.----------------- The Northern Atlantic Ocean must have presented a dreary aspect. Its shores were walls of ice, from which ever and anon great masses 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
