1 through fissures in the ice, to the bottom, where some become firmly frozen into the mass, and are pushed e base of the glacier, abrading, polishing, and grooving y floor below, as a diamond cuts glass, or as emery polishes steel, and the larger blocks are reciprocally and polished by the rocky floor on their lower sides. one becomes included close to the sides of the glacier, tches the adjacent rocks, producing long striæ. The ich are made, and the deep grooves which are scooped his action, are rectilinear and parallel to an extent never those produced on loose stones or rocks, where shingle ed along by a torrent, or by the waves on a sea-beach. ame time a stream of water, produced by the melting of issues from beneath the glacier charged with mud, deot only from the atmospheric waste of the rocks above, part also from the crushing of the fragments of stone, Lave reached the bottom of the glacier, and the abrasion cky floor.

Idition to these polished, striated, and grooved surfaces , another proof of the former action of a glacier is by the 'roches moutonnées,' or projecting eminences which have been smoothed and worn into the shape of I domes by the glacier as it passed over them. They en traced in the Alps to great heights above the present and also to great distances below and beyond them. If er is diminished in length greatly by melting, large fragments, which are called 'perched blocks,' are left. ne blocks on the Jura.-The moraines, erratics, surfaces, domes, perched blocks, and striæ, above dere observed in the great valley of Switzerland, fifty ; and on the Jur ey. The avers i the Alps, and :esents morai

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which lies to the north The Jura is about onedestitute of glaciers ; and grooved surfaces. even to a height of is astonished and perntury. No conclusion ese angular blocks of me from the Alps, and nce of fifty miles and

so that they are now tary formations. The locks retained, after a excited wonder; for and one in particular,

unctuous clay was frequently found covering the sea floor near glaciers.

Transporting and abrading power of glaciers.-In the higher regions of mountains where the amount of snow that falls in winter so far exceeds the loss in summer, through melting and evaporation, an indefinite thickness would accumulate if it were

Switzerland. (Agassiz.)

a a. White streaks or scratches, caused by small grains of flint frozen into the ice. bb. Furrows.

not prevented by the formation of nevé. This becoming gradually converted into ice, the glaciers are fed, and they glide down the principal valleys. On the glaciers' surface are seen long lines or heaps of sand and mud, with angular fragments of rock, which fall in quantities from the steep slopes or precipices on either side, where the rocks are daily exposed to great changes of temperature. These deposits being arranged along the sides of the glacier are termed lateral moraines. When two glaciers meet, unite and continue their course, the right lateral moraine of the one and the left of the other meet together in the centre of the joint glacier, forming what is called a medial moraine. These surface moraines finally fall, or are dropped at the lower end or foot of the glacier, and form the terminal moraine.

Besides the blocks thus carried down on the top of the glacier,

many fall through fissures in the ice, to the bottom, where some of them become firmly frozen into the mass, and are pushed along the base of the glacier, abrading, polishing, and grooving the rocky floor below, as a diamond cuts glass, or as emery powder polishes steel, and the larger blocks are reciprocally grooved and polished by the rocky floor on their lower sides. Some stone becomes included close to the sides of the glacier, and scratches the adjacent rocks, producing long striæ. The striæ which are made, and the deep grooves which are scooped out by this action, are rectilinear and parallel to an extent never seen in those produced on loose stones or rocks, where shingle is hurried along by a torrent, or by the waves on a sea-beach. At the same time a stream of water, produced by the melting of the ice, issues from beneath the glacier charged with mud, derived, not only from the atmospheric waste of the rocks above, but in part also from the crushing of the fragments of stone, which have reached the bottom of the glacier, and the abrasion of its rocky floor.

In addition to these polished, striated, and grooved surfaces of rock, another proof of the former action of a glacier is afforded by the roches moutonnées,' or projecting eminences of rock which have been smoothed and worn into the shape of flattened domes by the glacier as it passed over them. They have been traced in the Alps to great heights above the present glaciers, and also to great distances below and beyond them. If the glacier is diminished in length greatly by melting, large angular fragments, which are called 'perched blocks,' are left.

Alpine blocks on the Jura. The moraines, erratics, polished surfaces, domes, perched blocks, and striæ, above described, are observed in the great valley of Switzerland, fifty miles broad; and on the Jura, a chain which lies to the north of this valley. The average height of the Jura is about onethird that of the Alps, and it is now entirely destitute of glaciers ; yet it also presents moraines, and polished and grooved surfaces. The erratics, moreover, which are upon it even to a height of 2,500 feet, present a phenomenon which has astonished and perplexed the geologist for more than half a century. No conclusion can be more incontestable than that these angular blocks of gneiss and other crystalline formations came from the Alps, and that they have been brought for a distance of fifty miles and upwards, across a wide and deep valley; so that they are now lodged on hills composed of sedimentary formations. The great size and angularity which the blocks retained, after a journey of so many leagues, has justly excited wonder; for many of them are as large as cottages; and one in particular,

composed of gneiss, celebrated under the name of Pierre à Bot, rests on the side of a hill about 900 feet above the lake of Neufchâtel, and is no less than 40 feet in diameter.

The manner in which these erratics were conveyed from the Alps to the Jura was formerly the subject of considerable controversy. M. Venetz proved that the Alpine glaciers must formerly have extended far beyond their present limits, and it was argued that the blocks now found on the Jura had been transported by their agency. Other writers, on the contrary, conjectured that the whole country had been submerged, and that the moraines and erratic blocks must have been transported by floating icebergs, as it was held that the difference in height between the two mountain ranges was not sufficient to have allowed the glaciers to have flowed from the Alps across the wide valley to the Jura. But the definite order in which the Alpine erratics are arranged, and the total absence of marine shells, have gone far to disprove this last hypothesis. Besides, we have no right to assume that the relative heights of the Alps and Jura have remained unaltered since the era of the transportation of the erratics, still less that the change of level which last took place was uniform over a great district, whether in quantity or direction.

In addition to the many evidences of the action of ice in the northern parts of Europe which we have already mentioned, there occur here and there in some of these countries what are wanting in Switzerland, deposits of marine fossil shells, which exhibit so many forms now characteristic of Arctic seas, that they must have led the geologist to infer the former prevalence of a much colder climate, even had he not encountered so many accompanying signs of ice action. The same marine shells demonstrate the submergence of large areas in Scandinavia and the British Isles during part of the Glacial period.

A characteristic feature of the deposits under consideration in all these countries, is the occurrence of large erratic blocks, and sometimes of moraine matter, in situations remote from lofty mountains, and separated from the nearest points where the parent rocks appear at the surface by great intervening valleys, or arms of the sea. We also often observe striæ and furrows, as in Norway, Sweden and Scotland, which deviate from the direction which they ought to follow if they had been connected with the present line of drainage, and they, therefore, imply the prevalence of a very distinct condition of things at the time when the cold was most intense. The actual state of North Greenland seems to afford the best explanation of such abnormal glacial markings.

Greenland continental ice.-Greenland is a vast unexplored area, and much of it is evidently buried under one continuous and colossal mass of ice that is always moving seaward, a very small part of it in an easterly direction, and all the rest westward, or towards Baffin's Bay. All the minor ridges and valleys are levelled and concealed under a general covering of snow, but here and there some steep mountains protrude abruptly from the icy slope, and a few superficial lines of stones or moraines are visible at certain seasons, when no snow has fallen for many months, and when evaporation, promoted by the wind and sun, has caused much of the upper snow to disappear. The height of this continent is unknown, but it must be considerable, for the most elevated lands of the outskirts attain altitudes of 4,000 to 6,000 feet. The icy slope gradually lowers itself towards the outskirts, and then terminates abruptly in a mass about 2,000 feet in thickness, the great discharge of ice taking place through certain large friths, which are often several miles across at their upper end. The ice is protruded in huge masses, several miles wide, down the western coast, which continue their course, grating along the rocky bottom, like ordinary glaciers, long after they have reached the salt water. When at last they reach a depth of water sufficient to buoy up icebergs from 1,000 to 1,500 feet in vertical thickness, broken masses of them float off into Baffin's Bay, carrying with them on their surface not only fine mud and sand but large stones. These fragments of rock are often polished and scored on one or more sides, and, as the ice melts, they drop down to the bottom of the sea, where large quantities of mud are deposited, and may mingle with the remains of the mollusca which inhabit its bottom. This mud is a sticky clay, and closely resembles some boulder clay; taken with the scratched blocks, it is the analogue of the till.

Although the direction of the ice-streams in Greenland may coincide in the main with that which separate glaciers would take, yet the striation of the surface of the rocks of a continent covered with an ice cap would, on the whole, differ considerably in its minor details from that which would be imprinted on rocks constituting a region of separate glaciers. It is probable that the moving ice cap may sometimes cross deep narrow ravines, or the crests of buried ridges, even at right angles; and glacial striæ and polishing of rocks at great elevations and in remarkable directions are thus accounted for.

Dr. Rink mentions that in North Greenland powerful springs of clayey water escape in winter from under the ice, where it descends to the outskirts,' and where, as already stated, it is often 2,000 feet thick-a fact showing how much grinding