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The Denudation of Rocks in Staen Switzerland.

In a former lesson, I intimated that the formation of valleys was a difficult problem in geology. It is evident that rivers, m general, have not excavated their own beds, but flow in valleys which have been formed, for the most part, by other agents. In the majority of instances, rivers are filling up,

VOL. III.

| produced in the strata of the earth's crust, either when tracting, or when suddenly elevated from the bed of the ocean. They are longitudinal, following the direction of the mountain chain; or they are transverse, running across that direction Their sides are generally rugged, mostly steep, and their edge 53

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scores of miles square, to have been, at an early epoch, @vered with horizontal beds deposited by water, and that the present outline of the surface is like that represented in fig. 64.

While looking at this figure, you cannot doubt that at one time the beds cdefg were as continuous as those of a b. If the section in this diagram were a brick wall, and if the series of beds were layers of different coloured bricks, and you found that, after some tremendous hurricane or earthquake, the wall presented the omissions and gaps that now appear, you would never infer that the wall had been originally built in this manner. You must apply the same method of reasoning to the aspect of these layers of rock.

What, then, has become of all the materials marked by the dotted lines from A to B? They have all been worn away and removed by the denuding power of the ocean. Suppose the thickness of each bed to be 200 feet, and the distance from-A to B to be forty miles. Then, strata of 1,000 feet thick, and of many miles in extent, have been removed by denudation.

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action of sea currents.

lines have been removed by denudation. Sections in South | It is not to be supposed that what are called valleys of Wales and Gloucestershire show that enormous beds of old red sandstone and mountain limestone, have been thus removed from the surface of the underlying rocks by the denuding In mountainous countries, these denudations are sometimes marked by rocks many thousand feet in height, which are separated from each other by intervals or valleys many miles, and even leagues in breadth. Of this there is a grand specimen on the north-west coast of Ross-shire, in Scotland. Those three stupendous mountains, Suil Veinn, Coul Beg, and Coul More, which consist of nearly horizontal strata of red sandstone repose on gneiss, the fundamental rock of the

country.

denudation have been produced by the streams which flow through them now. In many parts of the world there are valleys without any water at all running in them. It is obvious that these valleys are not the result of river action. England abounds with examples of these dry valleys, especially in the combes of chalk districts, and in the numerous depressions found in the slate districts of Devonshire. Even in Jamaica, where heavy tropical rains are frequent, there are valleys in which the waters are immediately swallowed up by subterraneous cavities or sink-holes. On the west coast of Peru, where rain never falls, there are remarkable instances of dry valleys, which much resemble the lowland valleys of Europe. These dry valleys appear as if they had been scooped out by the materials which offered the least resistance. This denudation might have been effected by great disturbances beneath long range of mountains from the sea, or by a disruption of an ocean, such as would be caused either by the elevation of a the strata of which the crust of the earth in that region was composed. quake, or they may have been formed beneath agitated waters, They were produced either by a marine earthin which there were strong currents moving with great velocity. The rocks which were thus worn and denuded beneath such currents were afterwards protruded above the level of the sea, One of the most splendid generalisations in geological science is presented in the "Survey of Great Britain," by these currents are constant, some are periodical, and some The ocean abounds in currents strong and deep. Some of Professor Ramsey. He shows that the missing beds which have been washed away from the summits of the Mendip Hills, bodies of water, moving with different velocities, and in are only occasional. These currents consist of immense between Wells and Bristol, must have been originally about a mile in thickness. In considerable districts of Monmouthshire these currents we have a well-known specimen in the Gulf streams of different breadths and depths. Of the character of and Breconshire on the west, and Gloucestershire and Here-Stream of the Atlantic Ocean. High winds and heavy gales fordshire on the east, he shows that a series of ancient sedimentary rocks, no less than eleven thousand feet in thickness have been stripped off by denudation.

These red sandstone mountains consist of an immense suc-moving masses of water passing over them, and carrying off cession of thin layers, forming mere flags, with their surfaces distinctly ripple marked. They rise up at once, like pyramids, from the gneiss to the height of about 2000 feet, and to an average elevation of about 3000 feet above the level of the sea. It is impossible to look at these three high mountains, now rising in scattered and detached portions without inferring that, at one time, the whole country was covered with a great body of sandstone, and that enormous masses, from 1000 to more than 3000 feet in thickness, have been washed away by the denuding action of water.

The stupendousness of this generalisation is in the two facts -that all these materials have been removed and transported to some other regions to compose rocks of a new formation; and that these paleozoic rocks are from twenty to thirty thousand feet thick. It is evident that whatever has been contributed to one area on the face of the globe, must have

been derived and taken from another.

One of the most magnificent, and, at the same time, most clear and palpable specimens of denudation, is furnished by Saxon Switzerland, a district of Germany about ten miles beyond Dresden. The rock of the district is what the German geologists call quadersandstein, corresponding to the green sand formation of England. The rocks on each side of the river Elbe are cut in all directions into chasms, gorges, and passages, as if mechanical tools had been used to hew them into particular shapes. Some of these passages among the rocks look like narrow lanes, so narrow are the openings and so smoothly perpendicular do the gigantic walls of rock rise on both sides. The walls of these rocks are cut vertically into separate masses by narrow passages, reaching from the summit to the very bottom, as if a cement that once united them had been washed away.

The perpendicular masses of these separate rocks or cliffs are divided horizontally into distinct layers, like blocks regularly laid upon each other in a massive work of artificial masonry. The terminations or perpendicular extremities of these masses or columns are very rarely sharp or angular, but are almost invariably well rounded, which is a clear proof of subaqueous action. Some of them appear as if two sugarloaves were put together, the small end of one resting on the small end of the other.

From what is called the Bastei (on the right of the engraving, fg. 66), and 600 feet above the Elbe, the country looks as an amphitheatre, studded with lofty and rounded ranges of mountains. From the bosom of this amphitheatre, huge columnar hills start up at once from the ground, at a considerable distance from each other. All these are monuments of the denuding agency of water-not that of the present Elbe, but of the sea-at a time when this part of Germany was slowly rising from the ocean. The identity of structure and of composition in all these columnar eminences prove that they once formed one body, and that all the softer parts of their beds tave been removed by denudation,

greatly affect the velocity and the strength of these currents sometimes diminishing their breadth and augmenting their currents are supposed to be certain prevalent winds-such as velocity, and vice versa. The principal causes of all ocean the trade winds, the monsoons, &c.

Among the periodical currents of the ocean we must place the tides. These have great power in scooping shallow banks, and in abrading our coasts. The general velocity of the tide is about a mile and a-half in an hour. When any obstacles are presented to its currents, its abrading and transporting power becomes much augmented, and the process of denuda tion takes place very rapidly and extensively.

It is often found that at greater or less distances from the shore, a great discoloration of the sea frequently takes place. This discoloration is produced by heavy gales and powerful hurricanes, and is due to the action of the sea on the rocks beneath, and not to the sands and mud which the ebbing tide brings with it.

Since strong winds are generally the causes of currents in the ocean, it is obvious that the ocean streams thus produced will not extend deeper than the depth to which the propelling power of the wind or gale extends. All hydrographers have demonstrated that the waters of the ocean vary in density according to their depth. A wind, then, sufficient to agitate and propel the surface water to a certain depth beneath, will reach a point below at which it can produce no change or movement, as all water beneath that depth would, as far as surface causes are concerned, be immoveable, and would consequently exert no denuding agency. Hence, the denuding power of ocean currents depends on the depth of the sea. The smaller the depth, the greater is their denuding power, and consequently the greatest amount of denudation has, in every geological era, taken place on shoals and near coasts.

Sea currents have always their greatest velocity and force in shallow water and in contracted channels. It is, therefore, in these situations that, among ancient rocks, we always discover the greatest effects of their denuding power. Their geological importance depends on two things: on the relative depth of the sea which they traverse, and on their proximity to land. By their shallowness, their velocity is increased; and by their nearness to coasts, they wear away and remove the rocks that resist them. At very great depths in the bed of the ocean we have no reason to suppose that this denuding power exists. If it does exist, its cause must differ from all surfa influences.

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