CHAP. XXI.

ON THE DESTRUCTION OF MOUNTAINS, AND THE FORMATION OF SOILS; AND ON ALLUVIAL AND DILUVIAL DEPOSITIONS.

Erroneous Opinions respecting the Growth of Stones, supported by the Authority of John Locke. On the Causes in present Operation that wear down Rocks.- Rapid Destruction of Mountains dependent on their Structure. - Fall of Mont Grenier in Savoy. - Breaking down of the Barriers of Mountain Lakes. Scattered Masses of Rock.- Increase of Land by Alluvial Depositions in Lakes, and the Deltas of large Rivers. On the Formation of productive Soils. - Recent Strata formed in Lakes.— Peat and Peat Moors. - Inundations of Sand. Remains of Elephants and other large Animals, found in the Diluvial Beds in England, and the frozen Regions of Europe and Asia.

FEW persons can have travelled a hundred miles through any country without having seen beds of gravel, or of rounded stones, or fragments of rock scattered in different directions, which were evidently never brought into their present situation by the labour of man. In some instances these masses of loose stones, or large fragments of rock, occur on the summits of hills, or on elevated ground, and the stones are altogether unlike any rocks or strata in the adjacent districts. Among the hundreds of travellers to whom such objects are familiar, it is surprising how few have ever raised the enquiry-" How did these masses of rock, or beds of loose stones, come here?" One great reason for this indifference arises from a cause that may surprise geologists. Many well-educated persons, who possess much information on various subjects, still entertain the belief that stones grow in the places where they are now found: this belief excludes the necessity for further enquiry. They can also refer to the authority of the ablest

philosopher this country ever possessed, for a confirmation of their opinion, should it be controverted.

The celebrated John Locke states, in his "Elements of Natural Philosophy," that "all stones, metals, and minerals, are real vegetables; that is, grow organically from proper seeds, as well as plants."

If any one should think it superfluous to notice this extraordinary passage, in the present age of general information, let him enquire among his friends whether stones grow? and he will be somewhat surprised by the answers he may receive.

These scattered fragments of rock, or beds of loose stones, together with beds of sand and gravel, present objects of enquiry of the most interesting kind. From what districts were they transported? What were the causes by which they were removed? What was the epoch of their removal?

A farther enquiry also presents itself, as some of the beds of loose stone are rounded, or water-worn, like the shingles on the sea beach, but are now raised many hundred feet above the high-water mark. By what agents were these beds raised to their present elevation? Satisfactory solutions to all these enquiries will probably long remain desiderata in geology, though, in some instances, we can now arrive at a high degree of probability, by referring to causes in present operation. These scattered fragments or masses of rock, with beds of loose stones and gravel, or of superficial sand or clay, are comprised by French geologists under the appropriate name of terreins de transport; a name, however, which cannot well be introduced into our language. We shall, therefore, divide them into three groups, adopting the names generally received. Scattered blocks of rock; diluvial beds or diluvium; and alluvial beds or alluvium ; using the two latter without any reference to theory. Alluvial beds, consist of the sand, soil, or stones brought down by rivers, and deposited in their beds, or scattered upon their banks, or carried into the sea or into lakes, forming deltas at the mouths of rivers.

Diluvium, or diluvial beds, comprise both the scattered blocks of rock, and the beds of stone or gravel, that are carried into distant districts. They are called diluvial, on the supposition that they were transported during some great convulsion, by deluges or inundations; or, in other words, that they were removed by causes more powerful than any which are seen in constant operation.

In order to form a more distinct idea of the causes which have transported the beds and fragments of stone into their present situation, we shall first consider the causes that are daily wearing down the loftiest mountains and cliffs, or undermining the solid ground on the sea shore. The disintegration of rocks and mountains is constantly taking place, by the incessant operation of atmospheric causes. 'The infiltration of water into the fissures of rocks, and its expansion by frost, often produces sudden falls of immense masses of rock. The slow operation of descending currents, excavates the soft beds in the lower parts of mountains; and the upper rocks, being undermined, fall, with a tremendous crash, into the vales below. Instances of this kind have occurred in our own times. By both these causes, the process of disintegration is rapidly going on in the Alps; but such is the immensity of these enormous mountain ranges, that ages pass away, before any diminution of their bulk is perceived.

In Alpine districts of great elevation, there is also another cause, more exposed to observation, which is ever in action during the summer months. The snow upon the mountains below the line of eternal congelation, when it begins to dissolve, forms numerous rivulets, that unite into large streams, and descend in cataracts with impetuous force, excavating deep ravines in the lower rocks. To use the words of Professor Playfair, they are "Nature's saws, incessantly at work, cutting down the mountains."

The vignette in the titlepage of this volume represents the upper part of the valley of Sext, in

Savoy, in which the water, descending from the Alpine snow on the Buet and other mountains, is seen rushing in numerous cascades to the lower valley. But the most powerful effects of these cataracts may be observed during thunder storms, or after an unusually rapid thaw, when the upper rivulets overflow their accustomed boundaries, and carry with them the loose stones or masses of rock they meet in their descent, and dash them with inconceivable violence into the lower waterfalls, breaking down the solid rocks on each side, and deepening and enlarging the ravines into which they fall. The operation of this cause will be again referred to in the following chapter.

We need not indeed travel to the Alps to prove, that the mountains have been, and are still wearing down. The rocky fragments in Borrowdale; the deep ravines made by torrents in the sides of Skiddaw; the immense blocks of granite torn from Wastdale Crag, in Westmoreland, and scattered many miles over the adjacent counties, offer striking proofs of this. The central parts of England have once had a greater elevation than at present; pebbles formed of the Charnwood Forest rocks, are spread all over the midland counties. Masses of the rocks of Cumberland and Wales, more or less water-worn, occur almost every where under the alluvial plains of Cheshire and Lancashire. Beds of flint gravel, formed by the disintegration of chalk rocks in which flints were imbedded, occur in many parts of England at a considerable distance from the sea, or from the chalk districts.

The transportation of these masses of rock, or beds of stones and gravel, cannot have been effected by any thing like the present action of rivers in England, and is generally referred to the more extensive operation of deluges, during great convulsions of the globe; but if we return to the Alps, and view the effects now taking place, we must admit, that it is not always easy to make the distinction between alluvial and diluvial depositions.

Innumerable blocks of granite and other primary

rocks, torn from the central range of the Alps, are scattered over the calcareous mountains at a great distance from this range, or are spread in heaps in many of the distant valleys. All the great rivers that issue from the Alps, where the valleys open into the plains, have made deep sections in beds composed of the ruins of the mountains, and exhibit proofs of the vast destruction that has taken place. The river Doire, where it enters the plains of Piedmont, has cut through a mass of fragments more than 1500 feet in depth; these fragments consist of irregular blocks of granite, mica slate, and serpentine, frequently more than thirty cubic yards in extent, covered by smaller fragments, and by earthy matter from the decomposition of the softer rocks: the fragments decrease in size as their distance increases from the parent mountain.

Whoever has ascended the lofty eminences immediately below the highest pinnacles of the Alps, can scarcely fail to have received sensible proofs, of the daily and hourly disintegration of the mountains. Here, placed nearly above the region of vegetable or animal existence, and surrounded by the sublimest objects in nature, the deep silence which prevails around is truly solemn and impressive; but it is broken from time to time, by sounds like the rolling of distant thunder, or by a nearer and louder crash, which is repeated by the echoes from rock to rock. These sounds proceed from the falling of avalanches, or from glaciers splitting and discharging the loose rocks upon their surface, or from éboulements of rock, detached from the bare and exposed sides of the pinnacles and aiguilles. The fragments generally fall into the elevated mountain valleys, and are scattered over the surface of the higher glaciers, which extend from thence into the lower Alpine valleys. As the glaciers in these valleys are gradually melting during summer, the ice above progressively moves downward, bearing with it the cargoes of stones on its surface, which it discharges in heaps at its feet and sides.