The World Before the Deluge

by Louis Figuier

GLACIAL PERIOD.

The two cataclysms, of which we have spoken, surprised Europe at the moment of the development of an important creation. The whole scope of animated Nature, the evolution of animals, was suddenly arrested in that part of our hemisphere over which these gigantic convulsions spread, followed by the brief but sudden submersion of entire continents. Organic life had scarcely recovered from the violent shock, when a second, and perhaps severer blow assailed it. The northern and central parts of Europe, the vast countries which extend from Scandinavia to the Mediterranean and the Danube, were visited by a period of sudden and severe cold: the temperature of the polar regions seized them. The plains of Europe, but now ornamented by the luxurious vegetation developed by the heat of a burning climate, the boundless pastures on which herds of great Elephants, the active Horse, the robust Hippopotamus, and great Carnivorous animals grazed and roamed, became covered with a mantle of ice and snow.

To what cause are we to attribute a phenomenon so unforeseen, and exercising itself with such intensity? In the present state of our knowledge no certain explanation of the event can be given. Did the central planet, the sun, which was long supposed to distribute light and heat to the earth, lose during this period its calorific powers? This explanation is insufficient, since at this period the solar heat is not supposed to have greatly influenced the earth’s temperature. Were the marine currents, such as the Gulf Stream, which carries the Atlantic Ocean towards the north and west of Europe, warming and raising its temperature, suddenly turned in the contrary direction? No such hypothesis is sufficient to explain either the cataclysms or the glacial phenomena; and we need not hesitate to confess our ignorance of this strange, this mysterious, episode in the history of the globe.

There have been attempts, and very ingenious ones too, to explain these phenomena, of which we shall give a brief summary, without committing ourselves to any further opinion, using for that purpose the information contained in M. Ch. Martins’ excellent work. “The most violent convulsions of the solid and liquid elements,” says this able writer, “appear to have been themselves only the effects due to a cause much more powerful than the mere expansion of the pyrosphere; and it is necessary to recur, in order to explain them, to some new and bolder hypothesis than has yet been hazarded. Some philosophers have belief in an astronomical revolution which may have overtaken our globe in the first age of its formation, and have modified its position in relation to the sun. They admit that the poles have not always been as they are now, and that some terrible shock displaced them, changing at the same time the inclination of the axis of the rotation of the earth.” This hypothesis, which is nearly the same as that propounded by the Danish geologist, Klee, has been ably developed by M. de Boucheporn. According to this writer, many multiplied shocks, caused by the violent contact of the earth with comets, produced the elevation of mountains, the displacement of seas, and perturbations of climate—phenomena which he ascribes to the sudden disturbance of the parallelism of the axis of rotation. The antediluvian equator, according to him, makes a right angle with the existing equator.

“Quite recently,” adds M. Martins, “a learned French mathematician, M. J. Adhémar, has taken up the same idea; but, dismissing the more problematical elements of the concussion with comets as untenable, he seeks to explain the deluges by the laws of gravitation and celestial mechanics, and his theory has been supported by very competent writers. It is this: We know that our planet is influenced by two essential movements—one of rotation on its axis, which it accomplishes in twenty-four hours; the other of translation, which it accomplishes in a little more than 365¹⁄₄ days. But besides these great and perceptible movements, the earth has a third, and even a fourth movement, with one of which we need not occupy ourselves; it is that designated nutation by astronomers. It changes periodically, but within very restricted limits, the inclination of the terrestrial axis to the plane of the ecliptic by a slight oscillation, the duration of which is only eighteen hours, and its influence upon the relative length of day and night almost inappreciable. The other movement is that on which M. Adhémar’s theory is founded.

“We know that the curve described by the earth in its annual revolution round the sun is not a circle, but an ellipse; that is, a slightly elongated circle, sometimes called a circle of two centres, one of which is occupied by the sun. This curve is called the ecliptic. We know, also, that, in its movement of translation, the earth preserves such a position that its axis of rotation is intercepted, at its centre, by the plane of the ecliptic. But in place of being perpendicular, or at right angles with this plane, it crosses it obliquely in such a manner as to form on one side an angle of one-fourth, and on the other an angle of three-fourths of a right angle. This inclination is only altered in an insignificant degree by the movement of nutation. I need scarcely add that the earth, in its annual revolution, occupies periodically four principal positions on the ecliptic, which mark the limits of the four seasons. When its centre is at the extremity most remote from the sun, or aphelion, it is the summer solstice for the northern hemisphere. When its centre is at the other extremity, or perihelion, the same hemisphere is at the winter solstice. The two intermediate points mark the equinoxes of spring and autumn. The great circle of separation of light and shade passes, then, precisely through the poles, the day and night are equal, and the line of intersection of the plane of the equator and that of the ecliptic make part of the vector ray from the centre of the sun to the centre of the earth—what we call the equinoctial line.

“Thus placed, it is evident that if the terrestrial axis remained always parallel to itself, the equinoctial line would always pass through the same point on the surface of the globe. But it is not absolutely thus. The parallelism of the axis of the earth is changed slowly, very slowly, by a movement which Arago ingeniously compares to the varying inclination of a top when about to cease spinning. This movement has the effect of making the equinoctial points on the surface of the earth retrograde towards the east from year to year, in such a manner that at the end of 25,800 years according to some astronomers, but 21,000 years according to Adhémar, the equinoctial point has literally made a circuit of the globe, and has returned to the same position which it occupied at the beginning of this immense period, which has been called the ‘great year.’ It is this retrograde evolution, in which the terrestrial axis describes round its own centre that revolution round a double conic surface, which is known as the precession of the equinoxes. It was observed 2,000 years ago by Hipparchus; its cause was discovered by Newton; and its complete evolution explained by D’Alembert and Laplace.

“Now, we know that the consequence of the inclination of the terrestrial axis with the plane of the ecliptic is—

“1. That the seasons are inverse to the two hemispheres—that is to say, the northern hemisphere enjoys its spring and summer, while the southern hemisphere passes through autumn and winter.

“2. When the earth approaches nearest to the sun, our hemisphere has its autumn and winter; and the regions near the pole, receiving none of the solar rays, are plunged into darkness, approaching that of night, during six months of the year.

“3. When the earth is most distant from the sun, when much the greater half of the ecliptic intervenes between it and the focus of light and heat, the pole, being then turned towards this focus, constantly receives its rays, and the rest of the northern hemisphere enjoys its long days of spring and summer.

“Bearing in mind that, in going from the equinox of spring to the autumnal equinox of our hemisphere, the earth traverses a much longer curve than it does on its return; bearing in mind, also, the accelerated movement it experiences in its approach to the sun from the attraction, which increases in inverse proportion to the square of its distance, we arrive at the conclusion that our summer should be longer and our winter shorter than the summer and winter of our antipodes; and this is actually the case by about eight days.

“I say actually, because, if we now look at the effects of the precession of the equinoxes, we shall see that in a time equal to half of the grand year, whether it be 12,900 or 10,500 years, the conditions will be reversed; the terrestrial axis, and consequently the poles, will have accomplished the half of their bi-conical revolution round the centre of the earth. It will then be the northern hemisphere which will have the summers shorter and the winters longer, and the southern hemisphere exactly the reverse. In the year 1248 before the Christian era, according to M. Adhémar, the north pole attained its maximum summer duration. Since then—that is to say for the last 3,112 years—it has begun to decrease, and this will continue to the year 7388 of our era before it attains its maximum winter duration.

“But the reader may ask, fatigued perhaps by these abstract considerations, What is there here in common with the deluges?

“The grand year is here divided, for each hemisphere, into two great seasons, which De Jouvencel calls the great summer and winter, which will each, according to M. Adhémar, be 10,500 years.

“During the whole of this period one of the poles has constantly had shorter winters and longer summers than the other. It follows that the pole which experiences the long winter undergoes a gradual and continuous cooling, in consequence of which the quantities of ice and snow, which melt during the summer, are more than compensated by those which are again produced in the winter. The ice and snow go on accumulating from year to year, and finish at the end of the period by forming, at the coldest pole, a sort of crust or cap, vast, thick, and heavy enough to modify the spheroidal form of the earth. This modification, as a necessary consequence, produces a notable displacement of the centre of gravity, or—for it amounts to the same thing—of the centre of attraction, round which all the watery masses tend to restore it. The south pole, as we have seen, finished its great winter in 1248 b.c. The accumulated ice then added itself to the snow, and the snow to the ice, at the south pole, towards which the watery masses all tended until they covered nearly the whole of the southern hemisphere. But since that date of 1248, our great winter has been in progress. Our pole, in its turn, goes on getting cooler continually; ice is being heaped upon snow, and snow upon ice, and in 7,388 years the centre of gravity of the earth will return to its normal position, which is the geometrical centre of the spheroid. Following the immutable laws of central attraction, the southern waters accruing from the melted ice and snow of the south pole will return to invade and overwhelm once more the continents of the northern hemisphere, giving rise to new continents, in all probability, in the southern hemisphere.”

Such is a brief statement of the hypothesis which Adhémar has very ingeniously worked out. How far it explains the mysterious phenomena which we have under consideration we shall not attempt to say, our concern being with the effects. Does the evidence of upward and downward movements of the surface in Tertiary times explain the great change? For if the cooling which preceded and succeeded the two European deluges still remains an unsolved problem, its effects are perfectly appreciable. The intense cold which visited the northern and central parts of Europe resulted in the annihilation of organic life in those countries. All the watercourses, the rivers and streams, the seas and lakes, were frozen. As Agassiz says in his first work on “Glaciers”: “A vast mantle of ice and snow covered the plains, the valleys, and the seas. All the springs were dried up; the rivers ceased to flow. To the movements of a numerous and animated creation succeeded the silence of death.” Great numbers of animals perished from cold. The Elephant and Rhinoceros perished by thousands in the midst of their grazing grounds, which became transformed into fields of ice and snow. It is then that these two species disappeared, and seem to have been effaced from creation. Other animals were overwhelmed, without their race having been always entirely annihilated. The sun, which lately lighted up the verdant plains, as it dawned upon these frozen steppes, was only saluted by the whistling of the north winds, and the horrible rending of the crevasses, which opened up on all sides under the heat of its rays, acting upon the immense glacier which formed the sepulchre of many animated beings.

How can we accept the idea that the plains, but yesterday smiling and fertile, were formerly covered, and that for a very long period, with an immense sheet of ice and snow? To satisfy the reader that the proof of this can be established on sufficient evidence, it is necessary to direct his attention to certain parts of Europe. It is essential to visit, at least in idea, a country where glacial phenomena still exist, and to prove that the phenomena, now confined to those countries, were spread, during geological times, over spaces infinitely vaster. We shall choose for our illustration, and as an example, the glaciers of the Alps. We shall show that the glaciers of Switzerland and Savoy have not always been restricted to their present limits; that they are, so to speak, only miniature resemblances of the gigantic glaciers of times past; and that they formerly extended over all the great plains which extend from the foot of the chain of the Alps.

To establish these proofs we must enter upon some consideration of existing glaciers, upon their mode of formation, and their peculiar phenomena.

The snow which, during the whole year, falls upon the mountains, does not melt, but maintains its solid state, when the elevation exceeds the height of 9,000 feet or thereabouts. Where the snow accumulates to a great thickness, in the valleys, or in the deep fissures in the ground, it hardens under the influence of the pressure resulting from the incumbent weight. But it always happens that a certain quantity of water, resulting from the momentary thawing of the superficial portions, traverses its substance, and this forms a crystalline mass of ice, with a granular structure, which the Swiss naturalists designate névé. From the successive melting and freezing caused by the heat by day and the cold by night, and the infiltration of air and water into its interstices, the névé is slowly transformed into a homogeneous azure mass of ice, full of an infinite number of little air-bubbles—this was what was formerly called glace bulleuse (bubble-ice). Finally, these masses, becoming completely frozen, water replaces the bubbles of air. Then the transformation is complete; the ice is homogeneous, and presents those beautiful azure tints so much admired by the tourist who traverses the magnificent glaciers of Switzerland and Savoy.

Such is the origin of, and such is the mode in which the glaciers of the Alps are formed. An important property of glaciers remains to be pointed out. They have a general movement of translation in the direction of their slope, under the influence of which they make a certain yearly progress downward, according to the angle of the slope. The glacier of the Aar, for example, advances at the rate of about 250 feet each year.

Under the joint influence of the slope, the weight of the frozen mass, and the melting of the parts which touch the earth, the glacier thus always tends downwards; but from the effects of a more genial temperature, the lower extremity melting rapidly, has a tendency to recede. It is the difference between these two actions which constitutes the real progressive movement of the glacier.

The friction exercised by the glacier upon the bottom and sides of the valley, ought necessarily to leave its traces on the rocks with which it may happen to be in contact. Over all the places where a glacier has passed, in fact, we remark that the rocks are polished, levelled, rounded, and, as it is termed, moutonnées. These rocks present, besides, striations or scratches, running in the direction of the motion of the glacier, which have been produced by hard and angular fragments of stones imbedded in the ice, and which leave their marks on the hardest rocks under the irresistible pressure of the heavy-descending mass of ice. In a work of great merit, which we have before quoted, M. Charles Martins explains the physical mechanism by which granite rocks borne onwards in the progressive movements of a glacier, have scratched, scored, and rounded the softer rocks which the glacier has encountered in its descent. “The friction,” says M. Martins, “which the glacier exercises upon the bottom and upon the walls, is too considerable not to leave its traces upon the rocks with which it may be in contact; but its action varies according to the mineralogical nature of the rocks, and the configuration of the ground they cover. If we penetrate between the soil and the bottom of the glacier, taking advantage of the ice-caverns which sometimes open at its edge or extremity, we creep over a bed of pebbles and fine sand saturated with water. If we remove this bed, we soon perceive that the underlying rock is levelled, polished, ground down by friction, and covered with rectilinear striæ, resembling sometimes small grooves, more frequently perfectly straight scratches, as though they had been produced by means of a graver, or even a very fine needle. The mechanism by which these striæ have been produced is that which industry employs to polish stones and metals. We rub the metallic surface with a fine powder called emery, until we give it a brilliancy which proceeds from the reflection of the light from an infinity of minute striæ. The bed of pebbles and mud, interposed between the glacier and the subjacent rock, here represents the emery. The rock is the metallic surface, and the mass of the glacier which presses on and displaces the mud in its descent towards the plain, represents the hand of the polisher. These striæ always follow the direction of the glacier; but as it is sometimes subject to small lateral deviations, the striæ sometimes cross, forming very small angles with one another. If we examine the rocks by the side of a glacier, we find similar striæ engraved on them where they have been in contact with the frozen mass. I have often broken the ice where it thus pressed upon the rock, and have found under it polished surfaces, covered with striations. The pebbles and grains of sand which had engraved them were still encased in the ice, fixed like the diamond of the glazier at the end of the instrument with which he marks his glass.

“The sharpness and depth of the striæ or scratches depend on many circumstances: if the rock acted upon is calcareous, and the emery is represented by pebbles and sand derived from harder rocks, such as gneiss, granite, or protogine, the scratches are very marked. This we can verify at the foot of the glaciers of Rosenlaui, and of the Grindenwald in the Canton of Berne. On the contrary, if the rock is gneissic, granitic, or serpentinous, that is to say, very hard, the scratches will be less deep and less marked, as may be seen in the glaciers of the Aar, of Zermatt, and Chamounix. The polish will be the same in both cases, and it is often as perfect as in marble polished for architectural purposes.

“The scratches engraved upon the rocks which confine these glaciers are generally horizontal or parallel to the surface. Sometimes, owing to the contractions of the valley, these striæ are nearly vertical. This, however, need not surprise us. Forced onwards by the superincumbent weight, the glacier squeezes itself through the narrow part, its bulk expanding upwards, in which case the flanks of the mountain which barred its passage are marked vertically. This is admirably seen near the Châlets of Stieregg, a narrow defile which the lower glacier of the Grindenwald has to clear before it discharges itself into the valley of the same name. Upon the right bank of the glacier the scratches are inclined at an angle of 45° to the horizon. Upon the left bank the glacier rises sometimes quite up to the neighbouring forest, carrying with it great clods of earth charged with rhododendrons and clumps of alder, birches, and firs. The more tender or foliated rocks were broken up and demolished by the prodigious force of the glacier; the harder rocks offered more resistance, but their surface is planed down, polished, and striated, testifying to the enormous pressure which they had to undergo. In the same manner the glacier of the Aar, at the foot of the promontory on which M. Agassiz’ tent was erected, is polished to a great height, and on the face, turned towards the upper part of the valley, I have observed scratches inclined 64°. The ice, erect against this escarpment, seemed to wish to scale it, but the granite rock held fast, and the glacier was compelled to pass round it slowly.

“In recapitulation, the considerable pressure of a glacier, joined to its movement of progression, acts at once upon the bottom and flanks of the valley which it traverses: it polishes all the rocks which may be too hard to be demolished by it, and frequently impresses upon them a peculiar and characteristic form. In destroying all the asperities and inequalities of these rocks, it levels their surfaces and rounds them on the sides pointing up the stream, whilst in the opposite direction, or down the stream, they sometimes preserve their abrupt, unequal, and rugged surface. We must comprehend, in short, that the force of the glacier acts principally on the side which is towards the circle whence it descends, in the same way that the piles of a bridge are more damaged up-stream, than down, by the icebergs which the river brings down during the winter. Seen from a distance, a group of rocks thus rounded and polished reminds us of the appearance of a flock of sheep: hence the name roches moutonnées given them by the Swiss naturalists.”

Another phenomenon which plays an important part in existing glaciers, and in those, also, which formerly covered Switzerland, is found in the fragments of rock, often of enormous size, which have been transported and deposited during their movement of progression.

The peaks of the Alps are exposed to continual degradations. Formed of granitic rocks—rocks eminently alterable under the action of air and water, they become disintegrated and often fall in fragments more or less voluminous. “The masses of snow,” continues Martins, “which hang upon the Alps during winter, the rain which infiltrates between their beds during summer, the sudden action of torrents of water, and more slowly, but yet more powerfully, the chemical affinities, degrade, disintegrate, and decompose the hardest rocks. The débris thus produced falls from the summits into the circles occupied by the glaciers with a great crash, accompanied by frightful noises and great clouds of dust. Even in the middle of summer I have seen these avalanches of stone precipitated from the highest ridges of the Schreckhorn, forming upon the immaculate snow a long black train, consisting of enormous blocks and an immense number of smaller fragments. In the spring a rapid thawing of the winter snows often causes accidental torrents of extreme violence. If the melting is slow, water insinuates itself into the smallest fissures of the rocks, freezes there, and rends asunder the most refractory masses. The blocks detached from the mountains are sometimes of gigantic dimensions: we have found them sixty feet in length, and those measuring thirty feet each way are by no means rare in the Alps.”[106]

Thus, the action of aqueous infiltrations followed by frost, the chemical decomposition which granite undergoes under the influence of a moist atmosphere, degrade and disintegrate the rocks which constitute the mountains enclosing the glacier. Blocks, sometimes of very considerable dimensions, often fall at the foot of these mountains on to the surface of the glacier. Were it immovable the débris would accumulate at its base, and would form there a mass of ruins heaped up without order. But the slow progression, the continuous displacement of the glacier, lead, in the distribution of these blocks, to a certain kind of arrangement: the blocks falling upon its surface participate in its movement, and advance with it. But other downfalls take place daily, and the new débris following the first, the whole form a line along the outer edge of the glacier. These regular trains of rocks bear the name of “moraines.” When the rocks fall from two mountains, and on each edge of the glacier, and two parallel lines of débris are formed, they are called lateral moraines. There are also median moraines, which are formed when two glaciers are confluent, in such a manner that the lateral moraine, on the right of the one, trends towards the left-hand one of the other. Finally, those moraines are frontal, or terminal, which repose, not upon the glacier, but at its point of termination in the valleys, and which are due to the accumulation of blocks fallen from the terminal escarpments of glaciers there arrested by some obstacle. In [Plate XXXI.] we have represented an actual Swiss glacier, in which are united the physical and geological peculiarities belonging to these enormous masses of frozen water: the moraines here are lateral, that is to say, formed of a double line of débris.

XXXI.—Glaciers of Switzerland.

Transported slowly on the surface of the glacier, all the blocks from the mountain preserve their original forms unaltered; the sharpness of their edges is never altered by their gentle transport and almost imperceptible motion. Atmospheric agency only can affect or destroy these rocks when formed of hard resisting material. They then remain nearly of the same form and volume they had when they fell on the surface of the glacier; but it is otherwise with blocks and fragments enclosed between the rock and the glacier, whether it be at the bottom or between the glacier and its lateral walls. Some of these, under the powerful and continuous action of this gigantic grinding process, will be reduced to an impalpable mud, others are worn into facets, while others are rounded, presenting a multitude of scratches crossing each other in all directions. These scratched pebbles are of great importance in studying the extent of ancient glaciers; they testify, on the spot, to the existence of pre-existing glaciers which shaped, ground, and striated the pebbles, which water does not; on the contrary, in the latter, they become polished and rounded, and even natural striations are effaced.

Thus, huge blocks transported to great distances from their true geological beds, that is, erratic blocks, to use the proper technical term, rounded (moutonnées), polished, and scratched surfaces, moraines; finally, pebbles, ground, polished, rounded, or worn into smooth surfaces, are all physical effects of glaciers in motion, and their presence alone affords sufficient proof to the naturalist that a glacier formerly existed in the locality where he finds them. The reader will now comprehend how it is possible to recognise, in our days, the existence of ancient glaciers in different parts of the world. Above all, wherever we may find both erratic blocks and moraines, and observe, at the same time, indications of rocks having been polished and striated in the same direction, we may pronounce with certainty as to the existence of a glacier during geological times. Let us take some instances.

Fig. 196.—Erratic Blocks in the Alps.

At Pravolta, in the Alps, going towards Monte Santo-Primo, upon a calcareous rock, we find the mass of granite represented in [Fig. 196]. This erratic block exists, with thousands of others, on the slopes of the mountain. It is about fifty feet long, nearly forty feet broad, and five-and-twenty in height; and all its edges and angles are perfect. Some parallel striæ occur along the neighbouring rocks. All this clearly demonstrates that a glacier existed, in former times, in this part of the Alps, where none appear at the present time. It is a glacier, then, which has transported and deposited here this enormous block, weighing nearly 2,000 tons.

In the Jura Mountains, on the hill of Fourvières, a limestone eminence at Lyons, blocks of granite are found, evidently derived from the Alps, and transported there by the Swiss glaciers. The particular mode of transport is represented theoretically in [Fig. 197]. A represents, for example, the summit of the Alps, B the Jura Mountains, or the hill of Fourvières, at Lyons. At the glacial period, the glacier A B C extended from the Alps to the mountain B. The granitic débris, which was detached from the summit of the Alpine mountains, fell on the surface of the glacier. The movement of progression of this glacier transported these blocks as far as the summit B. At a later period the temperature of the globe was raised, and when the ice had melted, the blocks, D E, were quietly deposited on the spots where they are now found, without having sustained the slightest shock or injury in this singular mode of transport.

Fig. 197.—Transported blocks.

Every day traces, more or less recognisable, are found on the Alps of ancient glaciers far distant from their existing limits. Heaps of débris, of all sizes, comprehending blocks with sharp-pointed angles, are found in the Swiss plains and valleys. Blocs perchés (Perched blocks), as in [Pl. XXXI.], are often seen perched upon points of the Alps situated far above existing glaciers, or dispersed over the plain which separates the Alps from the Jura, or even preserving an incredible equilibrium, when their great mass is taken into consideration, at considerable heights on the eastern flank of this chain of mountains. It is by the aid of these indications that the geologist has been able to trace to extremely remote distances signs of the former existence of the ancient glaciers of the Alps, to follow them in their course, and fix their point of origin, and where they terminated. Thus the humble Mount Sion, a gently-swelling hill situated to the north of Geneva, was the point at which three great ancient glaciers had their confluence—the glacier of the Rhône, which filled all the basin of Lake Leman, or Lake of Geneva; that of the Isère, which issued from the Annecy and Bourget Lakes; and that of the Arve, which had its source in the valley of Chamounix, all converged at this point. According to M. G. de Mortillet, who has carefully studied this geological question, the extent and situation of these ancient glaciers of the Alps were as follows:—Upon its northern flank the glacier of the Rhine occupied all the basin of Lake Constance, and extended to the borders of Germany; that of the Linth, which was arrested at the extremity of the Lake of Zurich—this city is built upon its terminal moraine—that of the Reus, which covered the lake of the four cantons with blocks torn from the peaks of Saint-Gothard;—that of the Aar, the last moraines of which crown the hills in the environs of Berne;—those of the Arve and the Isère, which, as we have said, debouched from Lake Annecy and Lake Bourget respectively;—that of the Rhône, the most important of all. It is this glacier which has deposited upon the flanks of the Jura, at the height of 3,400 feet above the level of the sea, the great erratic blocks already described. This mighty glacier of the Rhône had its origin in all the lateral valleys formed by the two parallel chains of the Valais. It filled all the Valais, and extended into the plain, lying between the Alps and the Jura, from Fort de L’Écluse, near the fall of the Rhône, up to the neighbourhood of Aarau.

The fragments of rocks transported by the ice-sea which occupied all the Swiss plain follow, in northerly direction, the course of the valley of the Rhine. On the other hand, the glacier of the Rhône, after reaching the plain of Switzerland, turned off obliquely towards the south, received the glacier of the Arve, then that of the Isère, passed between the Jura and the mountains of the Grande-Chartreuse, spread over La Bresse, then nearly all Dauphiny, and terminated in the neighbourhood of Lyons.

Upon the southern flank of the Alps, the ancient glaciers, according to M. de Mortillet’s map, occupied all the great valleys from that of the Dora, on the west, to that of the Tagliamento, on the east. “The glacier of the Dora” says de Mortillet, whose text we greatly abridge, “debouched into the valley of the Po, close to Turin. That of the Dora-Baltéa entered the plain of Ivréa, where it has left a magnificent semicircle of hills, which formed its terminal moraine. That of the Toce discharged itself into Lake Maggiore, against the glacier of the Tessin, and then threw itself into the valley of Lake Orta, at the southern extremity of which its terminal moraines were situated. That of the Tessin filled the basin of Lake Maggiore, and established itself between Lugano and Varèse. That of the Adda filled the basin of Lake Como, and established itself between Mendrizio and Lecco, thus describing a vast semicircle. That of the Oglio terminated a little beyond Lake Iseo. That of the Adige, finding no passage through the narrow valley of Roveredo, where the valley became very narrow, took another course, and filled the immense valley of the Lake of Garda. At Novi it has left a magnificent moraine, of which Dante speaks in his ‘Inferno.’ That of the Brenta extended over the plain of that commune. The Drave and the Tagliamento had also their glaciers. Finally, glaciers occupied all the valleys of the Austrian and Bavarian Alps.”[107]

Similar traces of the existence of ancient glaciers occur in many other European countries. In the Pyrenees, in Corsica, the Vosges, the Jura, &c., extensive ranges of country have been covered, in geological times, by these vast plains of ice. The glacier of the Moselle was the most considerable of the glaciers of the Vosges, receiving numerous affluents; its lowest frontal moraine, which is situated below Remiremont, could not be less than a mile and a quarter in length.

But the phenomenon of the glacial extension which we have examined in the Alps was not confined to Central Europe. The same traces of their ancient existence are observed in all the north of Europe, in Russia, Iceland, Norway, Prussia, the British Islands, part of Germany, in the north, and even in some parts of the south, of Spain. In England, erratic blocks of granite are found which were derived from the mountains of Norway. It is evident that these blocks were borne by a glacier which extended from the north pole to England. In this manner they crossed the Baltic and the North Seas. In Prussia similar traces are observable.

Thus, during the Quaternary epoch, glaciers which are now limited to the Polar regions, or to mountainous countries of considerable altitude, extended very far beyond their present known limits; and, taken in connection with the deluge of the north, and the vast amount of organic life which they destroyed, they form, perhaps, the most striking and mysterious of all geological phenomena.

M. Edouard Collomb, to whom we owe much of our knowledge of ancient glaciers, furnishes the following note explanatory of a map of Ancient Glaciers which he has prepared:—

“The area occupied by the ancient Quaternary glaciers may be divided into two orographical regions:—1. The region of the north, from lat. 52° or 55° up to the North Pole. 2. The region of Central Europe and part of the south.

“The region of the north which has been covered by the ancient glaciers comprehends all the Scandinavian peninsula, Sweden, Norway, and a part of Western Russia, extending from the Niemen on the north in a curve which passed near the sources of the Dnieper and the Volga, and thence took a direction towards the shores of the glacial ocean. This region comprehends Iceland, Scotland, Ireland, the isles dependent on them, and, finally, a great part of England.

“This region is bounded, on all its sides, by a wide zone from 2° to 5° in breadth, over which is recognised the existence of erratic blocks of the north: it includes the middle region of Russia in Europe, Poland, a part of Prussia, and Denmark; losing itself in Holland on the Zuider Zee, it cut into the northern part of England, and we find a shred of it in France, upon the borders of the Cotentin.

“The ancient glaciers of Central Europe consisted, first, of the grand masses of the Alps. Stretching to the west and to the north, they extended to the valley of the Rhône as far as Lyons, then crossing the summit-level of the Jura, they passed near Basle, covering Lake Constance, and stretching beyond into Bavaria and Austria. Upon the southern slopes of the Alps, they turned round the summit of the Adriatic, passed near to Udinet, covered Peschiera, Solferino, Como, Varèse, and Ivréa, extended to near Turin, and terminated in the valley of the Stura, near the Col de Tenda.

“In the Pyrenees, the ancient glaciers have occupied all the principal valleys of this chain, both on the French and Spanish sides, especially the valleys of the centre, which comprehend those of Luchon, Aude, Baréges, Cauterets, and Ossun. In the Cantabrian chain, an extension of the Pyrenees, the existence of ancient glaciers has also been recognised.

“In the Vosges and the Black Forest they covered all the southern parts of these mountains. In the Vosges, the principal traces are found in the valleys of Saint-Amarin, Giromagny, Munster, the Moselle, &c.

“In the Carpathians and the Caucasus the existence of ancient glaciers of great extent has also been observed.

“In the Sierra Nevada, in the south of Spain, mountains upwards of 11,000 feet high, the valleys which descend from the Picacho de Veleta and Mulhacen have been covered with ancient glaciers during the Quaternary epoch.”

There is no reason to doubt that at this epoch all the British islands, at least all north of the Thames, were covered by glaciers in their higher parts. “Those,” says Professor Ramsay, “who know the Highlands of Scotland, will remember that, though the weather has had a powerful influence upon them, rendering them in places rugged, jagged, and cliffy, yet, notwithstanding, their general outlines are often remarkably rounded and flowing; and when the valleys are examined in detail, you find in their bottoms and on the sides of the hills that the mammillated structure prevails. This rounded form is known, by those who study glaciers, by the name of roches moutonnées, given to them by the Swiss writers. These mammillated forms are exceedingly common in many British valleys, and not only so, but the very same kind of grooving and striation, so characteristic of the rocks in the Swiss valleys, also marks those of the Highlands of Scotland, of Cumberland, and Wales. Considering all these things, geologists, led by Agassiz some five or six and twenty years ago, have by degrees come to the conclusion, that a very large part of our island was, during the glacial period, covered, or nearly covered, with a thick coating of ice in the same way that the north of Greenland is at present; and that by the long-continued grinding power of a great glacier, or set of glaciers nearly universal over the northern half of our country, and the high ground of Wales, the whole surface became moulded by ice.”

Whoever traverses England, observing its features with attention, will remark in certain places traces of the action of ice in this era. Some of the mountains present on one side a naked rock, and on the other a gentle slope, smiling and verdant, giving a character more or less abrupt, bold, and striking, to the landscape. Considerable portions of dry land were formerly covered by a bluish clay, which contained many fragments of rock or “boulders” torn from the old Cumbrian mountains; from the Pennine chain; from the moraines of the north of England; and from the Chalk hills—hence called “boulder” clay—present themselves here and there, broken, worn, and ground up by the action of water and ice. These erratic blocks or “boulders” have clearly been detached from the parent rock by violence, and often transported to considerable distances. They have been carried, not only across plains, but over the tops of mountains; some of them being found 130 miles from the parent rocks. We even find, as already hinted, some rocks of which no prototypes have been found nearer than Norway. There is, then, little room for doubting the fact of an extensive system of glaciers having covered the land, although the proofs have only been gathered laboriously and by slow degrees in a long series of years. In 1840 Agassiz visited Scotland, and his eye, accustomed to glaciers in his native mountains, speedily detected their signs. Dr. Buckland became a zealous advocate of the same views. North Wales was soon recognised as an independent centre of a system which radiated from lofty Snowdon, through seven valleys, carrying with them large stones and grooving the rocks in their passage. In the pass of Llanberis there are all the common proofs of the valley having been filled with glacier ice. “When the country was under water,” says Professor Ramsay, “the drift was deposited which more or less filled up many of the Welsh valleys. When the land had risen again to a considerable height, the glaciers increased in size: although they never reached the immense magnitude which they attained in the earlier portion of the icy epoch. Still they became so large that such a valley as the Pass of Llanberis was a second time occupied by ice, which ploughed out the drift that more or less covered the valley. By degrees, however, as we approach nearer our own days, the climate slowly ameliorated, and the glaciers began to decline, till, growing less and less, they crept up and up; and here and there, as they died away, they left their terminal and lateral moraines still as well defined in some cases as moraines in lands where glaciers now exist. Frequently, too, masses of stone, that floated on the surface of the ice, were left perched upon the rounded roches moutonnées, in a manner somewhat puzzling to those who are not geologists.

“In short, they were let down upon the surface of these rocks so quietly and so softly, that there they will lie, until an earthquake shakes them down, or until the wasting of the rock on which they rest precipitates them to a lower level.”

It was the opinion of Agassiz, after visiting Scotland, that the Grampians had been covered by a vast thickness of ice, whence erratic blocks had been dispersed in all directions as from a centre; other geologists after a time adopted the opinion—Mr. Robert Chambers going so far as to maintain, in 1848, that Scotland had been at one time moulded by ice. Mr. T. F. Jamieson followed in the same track, adducing many new facts to prove that the Grampians once sent down glaciers in all directions towards the sea. “The glacial grooves,” he says, “radiate outward from the central heights towards all points of the compass, although they do not strictly conform to the actual shape and contour of the minor valleys and ridges.” But the most interesting part of Mr. Jamieson’s investigations is undoubtedly the ingenious manner in which he has worked out Agassiz’ assertion that Glenroy, whose remarkable “Parallel Roads” have puzzled so many investigators, was once the basin of a frozen lake.

Glenroy is one of the many romantic glens of Lochaber, at the head of the Spey, near to the Great Glen, or the valley of the Caledonian Canal, which stretches obliquely across the country in a northwesterly direction from Loch Linnhè to Loch Ness, leaving Loch Arkaig, Loch Aich, Glen Garry, and many a highland loch besides, on the left, and Glen Spean, in which Loch Treig, running due north and south, has its mouth, on the south. Glenroy opens into it from the north, while Glen Gluoy opens into the Great Glen opposite Loch Arkaig. Mr. Jamieson commenced his investigations at the mouth of Loch Arkaig, which is about a mile from the lake itself. Here he found the gneiss ground down as if by ice coming from the east. On the hill, north of the lake, the gneiss, though much worn and weathered, still exhibited well-marked striæ, directed up and down the valley. Other markings showed that the Glen Arkaig glacier not only blocked up Glen Gluoy, but the mouth of Glen Spean, which lies two miles or so north of it on the opposite side.

At Brackletter, on the south side of Glen Spean, near its junction with Glen Lochy, glacial scores pointing more nearly due west, but slightly inclining to the north, were observed, as if caused by the pressure of ice from Glen Lui. The south side of Glen Spean, from its mouth to Loch Treig, is bounded by lofty hills—an extension of Ben Nevis, the highest of these peaks exceeding 3,000 feet. Numerous gullies intersect their flanks, and the largest of these, Corry N’Eoin, presents a series of rocky amphitheatres, or rather large caldrons, whose walls have been ground down by long-continued glacial action: the quartz-veins are all shorn down to the level of the gneiss, and streaked with fine scratches, pointing down the hollows and far up the rocks on either side. During all these operations the great valley was probably filled up with ice, which would close Glen Gluoy and Glen Spean, and might also close the lowest of the lines in Glenroy. But how about the middle and upper lines?

A glacier crossing from Loch Treig, and protruding across Glen Spean, would cut off Glens Glaibu and Makoul, when the water in Glenroy could only escape over the Col into Strathspey, when the first level would be marked.

Now let the Glen Treig glacier shrink a little, so as to let out water to the level of the second line by the outline at Makoul, and the theory is complete. When the first and greatest glacier gave way, Glenroy would be nearly in its present state.

The glacier, on issuing from the gorge at the end of Loch Treig, would dilate immensely, the right flank spreading over a rough expanse of syenite, the neighbouring hills being mica-schists, with veins of porphyry. Now the syenite breaks into large cuboidal blocks of immense size. These have been swept before the advancing glacier along with other débris, and deposited in a semicircle of mounds having a sweep of several miles, forming circular bands which mark the edges of the glacier as it shrunk from time to time under the influence of a milder climate.

This moraine, which was all that was wanting to complete the theory laid down by Agassiz, is found on the pony-road leading from the mouth of Loch Treig towards Badenoch. A mile or so brings the traveller to the summit-level of the road, and beyond the hill a low moor stretches away to the bottom of the plain. Here, slanting across the slope of the hill towards Loch Treig, two lines of moraine stretch across the road. At first they consist of mica-schists and bits of porphyry, but blocks of syenite soon become intermingled. Outside these are older hillocks, rising in some places sixty and seventy feet high, forming narrow steep-sided mounds, with blocks fourteen feet in length sticking out of the surface, mixed with fragments of mica-schist and gneiss. The inner moraine consists, almost wholly, of large blocks of syenite, five, ten, fifteen, and five-and-twenty feet long.

Fig. 198.—Parallel roads of Glenroy; from a sketch by Professor J. Phillips.

The present aspect of Glenroy is that of an upper and lower glen opening up from the larger Glen Spean. The head-waters of Lochaber gather in a wild mountain tract, near the source of the Spey. The upper glen is an oval valley, four miles long, by about one broad, bounded on each side by high mountains, which throw off two streams dividing the mica-schist from the gneissic systems; the former predominating on the west side, and the latter on the east. The united streams flow to the south-west for two miles, when the valley contracts to a rocky gorge which separates the upper from the lower glen. Passing from the upper to the lower glen, a line is observed to pass from near the junction of the two streams, on a level with a flat rock at the gorge, and also with the uppermost of the three lines of terraces in the lower glen. This line girdles the sides of the hills right and left, with a seemingly higher sweep, and is followed by two other perfectly parallel and continuous lines till Glenroy expands into Glen Spean, which crosses its mouth and enters the great glen a little south of Loch Lochy. At the point, however, where Glenroy enters Glen Spean, the two upper terraces cease, while the lower of the three appears on the north and south side of Glen Spean, as far as the pass of Glen Muckal, and southward a little way up the Gubban river and round the head of Loch Treig.

In Scotland, and in Northern England and Wales, there is distinct evidence that the Glacial Epoch commenced with an era of continental ice, the land being but slightly lower than at present, and possibly at the same level, during which period the Scottish hills received their rounded outlines, and scratched and smoothed rock-surfaces; and the plains and valleys became filled with the stiff clay, with angular scratched stones, known as the “Till,” which deposit is believed by Messrs. Geikie, Jamieson, and Croll to be a moraine profonde, the product of a vast ice-sheet.

In Wales, Professor Ramsay has described the whole of the valleys of the Snowdonian range as filled with enormous glaciers, the level of the surface of the ice filling the Pass of Llanberis, rising 500 feet above the present watershed at Gorphwysfa. In the Lake District of Cumberland and Westmorland, Mr. De Rance has shown that a vast series of glaciers, or small ice-sheets, filled all the valleys, radiating out in all directions from the larger mountains, which formed centres of dispersion, the ice actually pushing over many of the lesser watersheds, and scooping out the great rock-basins in which lie the lakes Windermere, Ullswater, Thirlmere, Coniston Water, and Wastwater, the bottoms of which are nearly all below the sea-level. The whole of this district, he has shown, experienced a second glaciation, after the period of great submergence, in which valley-glaciers scooped out the marine drift, and left their moraines in the Liza, Langdale, and other valleys, and high up in the hills, as at Harrison’s Stickle, where a tarn has been formed by a little moraine, acting as a dam, as shown by Professor Hull.

In Wales, also, valley-glaciers existed after the submergence beneath the Glacial sea. Thus in Cwm-llafar, under the brow of Carnedd Dafydd, and Carnedd Llewelyn, Professor Ramsay has shown that a narrow glacier, about two miles in length, has ploughed out a long narrow hollow in the drift (which “forms a succession of terraces, the result of marine denudation, during pauses in the re-elevation of its submersion) to a depth of more than 2,000 feet.”[108]

The proofs of this great submergence, succeeding the era of “land-ice,” are constantly accumulating. Since 1863, when Professor Hull first divided the thick glacial deposits of Eastern Lancashire and Cheshire into an Upper Boulder Clay, and Lower Boulder Clay divided by a Middle Sand and Gravel, the whole of which are of marine origin, these subdivisions have been found to hold good, by himself and Mr. A. H. Green, over 600 square miles of country around Manchester, Bolton, and Congleton; by Mr. De Rance over another 600 square miles, around Liverpool, Preston, Blackpool, Blackburn, and Lancaster, and also in the low country lying between the Cumberland and Welsh mountains and the sea.

In Ireland, also, the same triplex arrangement appears to exist. Professors Harkness and Hull have identified the “Limestone and Manure Gravels” of the central plain, as referable to the “Middle Sand and Gravel,” and the “Lower Boulder Clay” rests on a glaciated rock-surface along the coasts of Antrim and Down, and is overlain by sand, which, in 1832, was discovered by Dr. Scouler to be shell-bearing. At Kingstown the three deposits are seen resting on a moutonnéed surface of granite, scored from the N.N.W.

In Lancashire and on the coast of North Wales, between Llandudno and Rhyl, Mr. De Rance has shown that these deposits often lie upon the denuded and eroded surface of another clay, of older date, which he believes to be the product of land-ice, the remnant of the moraine profonde, and the equivalent of the Scotch “Till.” He also shows that the Lower Boulder Clay never rises above an elevation of fifty or eighty feet above the sea-level; and that the Middle Sand and Shingle rests directly upon the rock, or on the surface of this old Till.

Near Manchester the Lower Boulder Clay occasionally rests upon an old bed of sand and gravel. It is extremely local, but its presence has been recorded in several sections by Mr. Edward Binney, who was the first to show, in 1842,[109] that the Lancashire Boulder Clays were formed in the sea, and that the erratic pebbles and boulders, mainly derived from the Cumberland Lake Districts, were brought south by means of floating ice.

Most of the erratic pebbles and boulders in the Lancashire clays are more or less scratched and scored, many of them (though quite rounded) in so many directions that Mr. De Rance believes the Cumberland and Westmoreland hills to have been surrounded by an ice-belt, which, occasionally thawing during summer or warm episodes, admitted “breaker action” on the gradually subsiding coast, wearing the fragments of rocks brought down by rivers or by glaciers into pebbles that, with the return of the cold, became covered with the “ice-belt,” which, lifted by the tides, rolled and dinted the pebbles one against another, and gradually allowed them to be impressed into its mass, with which they eventually floated away.

The Middle Sands and Shingles in England have also afforded a great number of shells of mollusca. At Macclesfield they have been described by Messrs. Prestwich and Darbishire as occurring at an elevation of 1,100 to 1,200 feet above the level of the sea.[110]

Among other proofs of glacial action and submersion in Wales may be mentioned the case of Moel Tryfaen, a hill 1,400 feet high, lying to the westward of Caernarvon Bay, and six or seven miles from Caernarvon. Mr. Joshua Trimmer had observed stratified drift near the summit of this mountain, from which he obtained some marine shells; but doubts were entertained as to their age until 1863, when a deep and extensive cutting was made in search of slates. In this cutting a stratified mass of loose sand and gravel was laid open near the summit, thirty-five feet thick, containing shells, some entire, but mostly in fragments. Sir Charles Lyell examined the cutting, and obtained twenty species of shells, and in the lower beds of the drift, “large heavy boulders of far-transported rocks, glacially polished and scratched on more than one side:” underneath the whole, the edges of vertical slates were exposed to view, exhibiting “unequivocal marks of prolonged glaciation.” The shells belonged to species still living in British or more northern seas.

From the gravels of the Severn Valley, described by Mr. Maw, thirty-five forms of mollusca have been identified by Mr. Gwyn Jeffreys. In the Shingle beds of Leyland, Euxton, Chorley, Preston, Lancaster, and Blackpool,[111] Mr. De Rance has obtained nearly thirty species.

In Eastern Yorkshire, Mr. Searles V. Wood, Jun., has divided the glacial deposits into “Purple Clay without Chalk,” “Purple Clay with Chalk,” and “Chalky Clay,” the whole being later than his “Middle Glacial Sands and Gravel,” which, in East Anglia, are overlain by the “Chalky Clay,” and rest unconformably upon the “Contorted Drift” of Norfolk, the Cromer Till, and the Forest Bed. His three Yorkshire clays are, however, considered by most northern geologists to be the representatives of the “Upper Boulder Clay” west of the Pennine Chain, the “Chalky Clay” having been formed before the country had sufficiently subsided to allow the sandstones and marls, furnishing the red colouring matter, to have suffered denudation; while the “Purple Clay without Chalk, and with Shap Granite,” was deposited when all the chalk was mainly beneath the sea, and the granite from Shap Fell, which had been broken up by breaker-action during the Middle Sand era, was floated across the passes of the Pennine Chain and southwards and northwards. A solitary pebble of Shap granite has been found by Mr. De Rance at Hoylake, in Cheshire; and many of Criffel Granite, in that county, and on the coast of North Wales, by Mr. Mackintosh, who has also traced the flow of this granite in the low country lying north and south of the Cumberland mountains.

At Bridlington, in Yorkshire, occurs a deposit at the base of the “Purple Clay,” with a truly Arctic fauna. Out of seventy forms of mollusca recorded by Mr. S. V. Wood, Jun., nineteen are unknown to the Crag—of these thirteen are purely arctic, and two not known as living.

Shells have been found in the Upper Boulder Clay of Lancashire, at Hollingworth Reservoir, near Mottram, by Messrs. Binney, Bateman, and Prestwich, at an elevation of 568 feet above the sea, consisting of Fusus Bamffius, Purpura lapillus, Turritilla terebra, and Cardium edule. The clay is described by Mr. Binney as sandy, and brown-coloured, with pebbles of granite and greenstone, some rounded and some angular. All the above shells, as well as Tellina Balthica, have been found in the Upper Clay of Preston, Garstang, Blackpool, and Llandudno, by Mr. De Rance, who has also found all the above species (with the exception of Fusus), as well as Psammobia ferroensis, and the siliceous spiculæ of marine sponges, in the Lower Boulder Clay of West Lancashire. He has described the ordinary red Boulder Clay of Lancashire as extending continuously through Cheshire and Staffordshire into Warwickshire, gradually becoming less red and more chalky, everywhere overlying intermittent sheets of “sands and shingle-beds,” one of which is particularly well seen at Leamington and Warwick, where it contains Pectens from the Crag, Gryphæa from the Lias, and chalk fossils and flints. The latter have also been found by Mr. Lucy in the neighbourhood of Mount Sorrel, associated with bits of the Coral Rag of Yorkshire. The gravels of Leicester, Market Harborough, and Lutterworth were long ago described by the Rev. W. D. Conybeare as affording “specimens of the organic remains of most of the Secondary Strata in England.”

The Rev. O. Fisher, F.G.S., has paid much attention to the superficial covering usually described as “heading,” or “drift,” as well as to the contour of the surface, in districts composed of the softer strata, and has published his views in various papers in the Journal of the Geological Society and in the Geological Magazine. He thinks that the contour of the surface cannot be ascribed entirely to the action of rain and rivers, but that the changes in the ancient contour since produced by those changes can be easily distinguished. He finds the covering beds to consist of two members—a lower one, entirely destitute of organic remains, and generally unstratified, which has often been forcibly indented into the bed beneath it, sometimes exhibiting slickenside at the junction.

There is evidence of this lower member having been pushed or dragged over the surface, from higher to lower levels, in a plastic condition; on which account he has named it “The Trail.”

The upper member of the covering beds consists of soil derived from the lower one, by weathering. It contains, here and there, the remains of the land-shells which lived in the locality at a period antecedent to cultivation. It is “The Warp” of Mr. Trimmer.

Neither of these accumulations occur on low flats, where the surface has been modified since the recent period. They both alike pass below high-water mark, and have been noticed beneath estuarine deposits.

Mr. Fisher is of opinion that land-ice has been instrumental in forming the contour of the surface, and that the trail is the remnant of its moraine profonde. And he has given reasons[112] for believing that the climate of those latitudes may have been sufficiently rigorous for that result about 100,000 years ago. He attributes the formation of the superficial covering of Warp to a period of much rainfall and severe winter-frosts, after the ice-sheet had disappeared.

The phenomena which so powerfully affected our hemisphere present themselves, in a much grander manner, in the New World. The glacier-system appears to have taken in America the same gigantic proportions which other objects assume there. Nor is it necessary, in order to explain the permanent existence of this icy mantle in temperate climates, to infer the prevalence of any very extraordinary degree of cold. On this subject M. Ch. Martins thus expresses himself: “The mean temperature of Geneva is 9° 5 Cent. Upon the surrounding mountains the limit of perpetual snow is found at 8,800 feet above the level of the sea. The great glaciers of the valley of Chamounix descend 5,000 feet below this line. Thus situated, let us suppose that the mean temperature of Geneva was lowered only 4°, and the average became 5° 5; the decrease of temperature with the height being 1° c. for every 600 feet, the limit of perpetual snow would be lowered by 2,437 feet, and would be 6,363 feet above the level of the sea. We can readily admit that the glaciers of Chamounix would descend below this new limit, to an extent at least equal to that which exists between their present limit and their lower extremity. Now, in reality, the foot of these glaciers is 5,000 feet above the ocean; with a climate 4° colder, it would be 2,437 feet lower; that is to say, at the level of the Swiss plain. Thus, the lowering of the line of perpetual snow to this extent would suffice to bring the glacier of the Arve to the environs of Geneva.... Of the climate which has favoured the prodigious development of glaciers we have a pretty correct idea; it is that of Upsala, Stockholm, Christiana, and part of North America, in the State of New York.... To diminish by four degrees the mean temperature of a country in order to explain one of the grandest revolutions of the globe, is to venture on an hypothesis not bolder than geology has sometimes permitted to itself.”[113]

In proving that glaciers covered part of Europe during a certain period, that they extended from the North Pole to Northern Italy and the Danube, we have sufficiently established the reality of this glacial period, which we must consider as a curious episode, as well as certain, in the history of the earth. Such masses of ice could only have covered the earth when the temperature of the air was lowered at least some degrees below zero. But organic life is incompatible with such a temperature; and to this cause must we attribute the disappearance of certain species of animals and plants—in particular, the Rhinoceros and the Elephant—which, before this sudden and extraordinary cooling of the globe, appear to have limited themselves, in immense herds, to Northern Europe, and chiefly to Siberia, where their remains have been found in such prodigious quantities. Cuvier says, speaking of the bodies of the quadrupeds which the ice had seized, and in which they have been preserved, with their hair, flesh, and skin, up to our own times: “If they had not been frozen as soon as killed, putrefaction would have decomposed them; and, on the other hand, this eternal frost could not have previously prevailed in the place where they died; for they could not have lived in such a temperature. It was, therefore, at the same instant when these animals perished that the country they inhabited was rendered glacial. These events must have been sudden, instantaneous, and without any gradation.”[114]

Fig. 199.—Fissurella nembosa.
(Living shell.)

How can we explain the glacial period? We have explained M. Adhémar’s hypothesis, to which it may be objected that the cold of the glacial period was so general throughout the Polar and temperate regions on both sides of the equator, that mere local changes in the external configuration of our planet and displacement of the centre of gravity scarcely afford adequate causes for so great a revolution in temperature. Sir Charles Lyell, speculating upon the suggestion of Ritter and the discovery of marine shells spread far and wide over the Sahara Desert by Messrs. Escher von der Linth, Desor, and Martins—which seem to prove that the African Desert has been under water at a very recent period—infers that the Desert of Sahara constituted formerly a wide marine area, stretching several hundred miles north and south, and east and west. “From this area,” he adds, “the south wind must formerly have absorbed moisture, and must have been still further cooled and saturated with aqueous vapour as it passed over the Mediterranean. When at length it reached the Alps, and, striking them, was driven into the higher and more rarefied regions of the atmosphere, it would part with its watery burthen in the form of snow; so that the same aërial current which, under the name of the Föhn, or Sirocco, now plays a leading part with its hot and dry breath, sometimes, even in the depth of winter, in melting the snow and checking the growth of glaciers, must, at the period alluded to, have been the principal feeder of Alpine snow and ice.”[115] Nevertheless, we repeat, no explanation presents itself which can be considered conclusive; and in science we should never be afraid to say, I do not know.