ART. VII.-1. Climate and Time in their Geological Relations ; a Theory of the Secular Changes of the Earth's Climate. By James Croll, of Her Majesty's Geological Survey in Scotland. London, 1875.

2. The Great Ice Age and its relation to the Antiquity of Man. By James Geikie, F.R.S.E., F.G.S. London, 1874.

3. Flora Fossilis Arctica.'

Die Fossile Flora der Polarländer.

Von Dr. Oswald Heer. Zürich, 1869-1878.

4. The Primeval World of Switzerland. By Professor Oswald Heer, of the University of Zürich. 2 vols. London, 1876. 5. New Lands within the Arctic Circle. Narrative of the Discoveries of the Austrian Ship Tegethoff' in the years 18721874. By Julius Payer, one of the Commanders of the Expedition. 2 vols. London, 1876.

[ocr errors]

6. Narrative of a Voyage to the Polar Sea during 1875-6 in H. M. Ships Alert' and 'Discovery.' By Capt. Sir G. S. Nares, R.Ñ., K.C.B., F.R.S., Commander of the Expedition. With Notes on the Natural History, edited by H. W. Fielden, F.G.S., C.M.Z.S., Naturalist to the Expedition. 2 vols. London, 1878.

WHEN the late Professor Agassiz, in the year 1840, discovered clear traces of glacial action in the valleys of Scotland, his announcement was received with incredulity, even by geologists. The whole body of evidence afforded by fossil remains was supposed to prove that our climate had formerly been warmer that it is now, and that the farther we went back in geological time the more tropical were the forms of life which inhabited Europe. This was seen to be entirely consistent with the theory of a cooling earth, to which the high temperatures of the earlier periods were then almost universally attributed; and, as the idea of a much colder climate in former times did not harmonize with this theory, the glacialists were for some time looked upon as a set of wild enthusiasts, whose facts were not worth examining, and whose theories might be ridiculed or despised. Soon, however, the tide began to turn. Eminent geologists, after visiting the alpine valleys where glaciers are still at work, were struck with the identity of the phenomena. evidently produced by them with those to be still seen in all our mountain regions; the great importance of this identity was acknowledged; and thenceforth a body of enthusiastic and industrious workers arose, who have made this phase of the ancient history of our earth their special study. With admirable patience they have tracked the ice-marks far and wide over the northern hemisphere, and have so skilfully interpreted the


mysterious record, that we are now able to read with confidence the great outlines of this most marvellous chapter in the past history of our earth-the Glacial Period. More recently, the subject has been greatly extended by the growing belief that similar cold epochs have occurred during tertiary, secondary, and even paleozoic times-a belief founded on numerous facts which had previously been overlooked or misinterpreted; while the proofs of a corresponding succession of warm periods within the Arctic regions have accumulated so rapidly, that no doubt now remains as to this still more startling phase of climatic change.

To account for this wonderful series of phenomena the old theory of a cooling earth is evidently inadequate and has long been given up by geologists, while it has been shown by physicists to be altogether untenable. Another theory has however taken its place, which, though beset with many difficulties, does really account for almost the whole of the series of facts above referred to, and is therefore adopted with more or less reserve by most students of the subject. But the facts are so varied and widespread, their interpretation is sometimes so obscure, and the theory which explains them is often so difficult in its application, that the whole question appears to most persons to be a hopeless puzzle, while many even doubt the fundamental fact of there ever having been glacial epochs of the intensity and wide extent usually claimed for them. Within the last few years, however, much light has been thrown on the subject by the various Arctic expeditions and by discussions on oceanic circulation, and the time has probably now come when a condensed and intelligible account, both of the facts and of the best mode of accounting for the facts, will be acceptable to a wide circle of intelligent readers. Such an account will here be attempted.

The Glacial Epoch.-The phenomena that prove the comparatively recent occurrence of a glacial epoch in the temperate regions of the northern hemisphere are exceedingly varied and extend over very wide areas. It will be well therefore to state, first, what these facts are as exhibited in our own country, referring afterwards to corresponding phenomena in other parts of the world.

Perhaps the most striking and conclusive among these facts are the grooved, scratched, or striated rocks. These occur abundantly in Scotland, the Lake country, and North Wales, and no rational explanation of them has yet been given, except that they were formed by glaciers. In many valleys, as for instance that of Llanberis in North Wales, hundreds of examples may be seen of deep grooves several inches wide, smaller


furrows, and striæ of extreme fineness, whenever the rock is of sufficiently close texture and sufficiently hard to receive and retain such marks. These grooves or scratches often extend for many yards in length; they are found in the bottom of the valley as well as high up on its sides, and they are all (almost without exception) in one direction-that of the valley itself— even though the particular surface they are upon slopes in another direction. Where the native covering of turf is freshly removed, the grooves and striæ are often seen in great perfection, and there is reason to believe that such markings cover, or have once covered, a large part of the rocky surfaces of the countries where they are now found. Accompanying these striæ we find another phenomenon, hardly less curious, the rounding off or planing down of the hardest rocks to a smooth undulating surface. Hard crystalline schists, with the strata nearly vertical, and which one would expect to find exposing jagged edges, are often ground off to a smooth, but never to a perfectly flat surface. These smoothly-rounded surfaces are found not only on single rocks, but over whole valleys and mountain-sides, and form what are termed roches moutonnées, from their having the appearance at a distance of sheep lying down.

Now these two phenomena are actually produced elsewhere by existing glaciers, while there is no other known or even conceivable agency that could have produced them. Whenever the Swiss glaciers retreat a little, as they sometimes do, the rocks they have moved over are found to be rounded, grooved, and striated just like those we now see in Wales and Scotland. The two sets of phenomena are so exactly identical, that no one who has ever compared them can doubt that they are due to the same causes. But we have further and even more convincing evidence. Glaciers produce many other effects besides these two; and whatever effects they now produce in Switzerland, in Norway, or in Greenland, we find examples of similar effects having been produced in our own country. The most striking of these are moraines and travelled blocks, which must be briefly described.

Almost every existing glacier carries down with it great masses of rock, stones, and earth, which fall on its surface from the precipices and mountain slopes which hem it in. As the glacier slowly moves downward, this debris forms long lines on each side, or on the centre whenever two glacier-streams unite, and is finally deposited at its termination in a huge mound called the terminal moraine. The decrease of a glacier may often be traced by successive old moraines across the valley up which it has retreated. When once seen and examined in conVol. 148.-No. 295. nection


nection with an existing glacier, these moraines can always be distinguished almost at a glance. Their position is most remarkable, having no apparent relation to the valley or the surrounding slopes. They look like huge earthworks formed for purposes of defence, rather than works of nature; and their composition is equally peculiar, consisting of a mixture of earth and rocks of all sizes, without any arrangement whatever, the rocks often being huge angular masses just as they had split off the surrounding precipices. Some of these rocky masses often rest on the very top of the moraine, in positions where no other natural power but that of ice could have placed them. Exactly similar mounds are found in the valleys of Wales and Scotland, and always in districts where other evidences of ice-action can be abundantly traced, and where the course of the glacier which produced them can be readily understood.

The phenomenon of travelled or perched blocks is also a common one in all glacier-countries, marking out very clearly the former extension of the ice. The glacier which fills a lateral valley will often cross over the main valley and abut against the opposite slope, and will deposit there some portion of its terminal moraine. But in these cases the end of the glacier will spread out laterally and the moraine matter will be distributed over a large surface, so that the only well-marked feature left by it will be some of the larger masses of rock that may have been brought down. The same thing will occur when a glacier surrounds an isolated knoll or overrides a projecting spur, on both of which large blocks brought from a distance may be left stranded. Such blocks are found abundantly in many districts of our own country where other marks of glaciation exist, and they often consist of a rock different from that on which they rest, or from any in the immediate vicinity. They can, however, almost always be traced to their source in one of the higher valleys from which the glacier descended; and this remoteness of origin, combined with their great size, their angular forms, and their singular positions, often perched on the crest of a ridge, on a steep slope, or on the summit of a knoll, altogether preclude any other known mode of transport but by glaciers or floating ice.

Some of the most remarkable examples of these travelled blocks are to be seen on the southern slopes of the Jura mountains. They consist of enormous angular masses of granite, gneiss, and other crystalline rocks, quite foreign to the Jura range which consists entirely of Jurassic limestones and tertiary deposits, but exactly agreeing with those of the main Alpine range, more than fifty miles away across the great central valley


of Switzerland. These blocks have been proved by Swiss geologists to have been brought by the ancient glacier of the Rhone, which was fed by the snows of the whole Alpine range, from Mont Blanc to Monte Rosa and the head of the Rhone valley, a distance of nearly 120 miles, together with the southern slopes of the Bernese Alps-a district which comprises all the most extensive snowfields and glaciers in Switzerland. The whole area between these two ranges, which may be roughly described as about 100 miles long by 30 wide and bounded by mountains from 7000 to 10,000 feet high, must have been literally filled with ice; and this enormous glacier discharged itself at the mouth of the upper Rhone-valley, between the Dent de Morcles and the Dent de Midi, in an ice-stream of such enormous thickness as to fill up the bed of the Lake of Geneva, and, though spreading widely over the valley, retaining a thickness of more than 2000 feet when it reached the Jura.

The blocks brought by it are found scattered over the slopes of the Jura for a distance of about 100 miles, and it is a very interesting fact, that they reach the greatest height in a direct line with the course of the glacier, while on both sides of this point they are found lower and lower. One of the largest of the blocks is forty feet in diameter, and many others are of enormous size. They vary considerably in the material of which they are composed; but they have each been traced to their source, which has always been found in a part of the Alpine range corresponding to their actual position on the theory that they were brought, as described, by the great Rhone glacier. Thus, all the blocks found to the east of a central point near Neufchatel can be traced to the eastern side of the Rhone valley, while those found towards Geneva have all come from the west side. It is evident that, had these blocks been carried by floating ice during a period of submergence (as was at first supposed), their distribution must have been different. There could have been no accurate separation of those derived from the opposite sides of the same valley, while all would have been stranded at the same elevation, or in parallel lines indicating the different heights at which the water stood at different epochs. These considerations are so weighty, that they compelled Sir Charles Lyell to withdraw the explanation he first gave-of the carriage of the blocks by floating ice during a period of submergenceas quite untenable, and to accept, as the only explanation which covered the facts, the enormous glacier above described.* Similar phenomena, though nowhere on so grand a scale, are

*Antiquity of Man.' 4th ed., pp. 340-348.

Q 2


« VorigeDoorgaan »