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are crowned with lofty pinnacles. Ravines, gorges, and cleuchs, are bounded by more or less perpendicular walls of rock, and are due to the cutting power of running water, or to early rifts in the rock produced by some violent convulsion. Lowland valleys differ from both in being of a rounded form, as if a large body of water had passed over them, rounding their inequalities.
Accurate soundings have demonstrated that hill and dale, mountain and valley, exist now in the ocean, and must have existed in the crust of the earth from the earliest geological periods. Over many of these inequalities, sedimentary rocks have deen deposited, which have filled up and covered their numerous hollows and fissures.
In our early lessons on volcanic agency, your attention was directed to that violent disruption of strata, by which stratified beds have been in some instances contorted, and in others thrown up on their edges. It would depend on the force of the upheaving power, whether the beds of the earth's crust would be merely bent and curved, or whether large rents and fractures would be produced in them, which would be longitudinal and transverse fissures.
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.
If you suppose this disruption of the crust of the earth to take place beneath the waters of an ocean, the sea would be greatly agitated, and would react upon the rocks, rushing into D the cracks, sweeping around them, rounding off their angles, and accumulating detritus at the bottom of hollows. Should the land which was in this process of being elevated or cracked, be partly in the ocean, and partly above the surface of the waves, the reaction of the sea would operate chiefly on the lower portion of the upraised strata, and the abrasion and the excavation would present a rounded or curvilinear form. Had the elevated rock been already in the atmosphere, the modification of the original cracks in the crust would be affected only by such atmospheric agents as rain, frost, and heat.
Horizontal Beds exposed by Denudation.
Its appearance now presents a broken surface of a country, with a series of hills, cliffs, valleys, and mountains in the landscape-one of the mountains rising to a height of 1,400 feet above the level of the sea.
Imagine, again, that instead of these beds being horizontal, they have been contorted by some powerful disturbing force
Horizontal Bede, first cracked by Upheaval, and then laid bare from below, as is represented in fig. 65.
Fig. 63 represents the upheaving power acting against the beds of the crust A B. Ate it has so swelled up the lower rocks as to produce at dà crack, which was probably very small at first. On this the currents and waves began to act immediately, until they denuded all the beds down to the bed. When the same subterranean force acted on the crust at f, it produced a fissure or a fault throughout the whole of the beds, and tilted a part of them on their edges. The rupture thus produced at a would be widened and deepened by the denuding agency of the ocean.
When this district would be raised to the elevation of several hundred or thousand feet, and appear as high plateaus, they would present the appearance of a wild and desolate landscape of broken and shattered hills, separated by deep and gloomy ravines, that speak distinctly of a period of convulsion, when upheaving fires from the abyss, and ocean currents above, had contended in sublime antagonism-the subterranean power slowly elevating the entire district, and the ocean grinding down the rocks and sweeping them away. As the region of this crust first presented its broad back over the waves, the upper surface consisted exclusively, from one extremity to the other, of a continuous tract, say of old red sandstone, though, ere the land fully emerged, the ocean currents of ages had swept it away, all except in the lower and last-raised beds, and in the detached localities where it still remains in pyramidal hills, to show the amazing depth to which it had once overlaid the inferior rocks.
To understand this, imagine a large district of land several
We are sure that the beds of axy are continuous under B, for
What, again, has become of all the materials of the rocks between a and o? The whole masses marked by the dotted
Synclinical means having the same inclination, or rather inalining together.
LESSONS IN GEOLOGY.
lines have been removed by denudation. Sections in South 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 action of sea currente.
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.
It is not to be supposed that what are called valleys of In many parts of the world there are denudation have been produced by the streams which flow through them now. that these valleys are not the result of river action. England valleys without any water at all running in them. It is obvious 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, These dry valleys appear as if they had been scooped out by which much resemble the lowland valleys of Europe. the materials which offered the least resistance. This denuan ocean, such as would be caused either by the elevation of a dation might have been effected by great disturbances beneath long range of mountains from the sea, or by a disruption of the strata of which the crust of the earth in that region was composed. They were produced either by a marine earthquake, or they may have been formed beneath agitated waters, in which there were strong currents moving with great velocity. currents were afterwards protruded above the level of the sea, The rocks which were thus worn and denuded beneath such The ocean abounds in currents strong and deep. Some of these currents are constant, some are periodical, and some are only occasional. streams of different breadths and depths. Of the character of bodies of water, moving with different velocities, and in these currents we have a well-known specimen in the Gulf Stream of the Atlantic Ocean. High winds and heavy gales sometimes diminishing their breadth and augmenting their The principal causes of all ocean velocity, and vice versa. greatly affect the velocity and the strength of these currents currents are supposed to be certain prevalent winds-such as the trade winds, the monsoons, &c.
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.
One of the most splendid generalisations in geological science is presented in the "Survey of Great Britain," by Professor Ramsey. He shows that the missing beds which have been washed away from the summits of the Mendip Hills, between Wells and Bristol, must have been originally about a mile in thickness. In considerable districts of Monmouthshire and Breconshire on the west, and Gloucestershire and Herefordshire 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.
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, & 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
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,
These currents consist of immense
Among the periodical currents of the ocean we must place The general velocity of the tide the tides. These have great power in scooping shallow banks, is about a mile and a-half in an hour. When any obstacles and in abrading our coasts. are presented to its currents, its abrading and transporting power becomes much augmented, and the process of denudation 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 according to their depth. A wind, then, sufficient to agitate demonstrated that the waters of the ocean vary in density 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 geological era, taken place on shoals and near coasts. consequently the greatest amount of denudation has, in every
Sea currents have always their greatest velocity and force in From what is called the Bastei (on the right of the engraving, 6g. 66), and 600 feet above the Elbe, the country looks as an shallow water and in contracted channels. It is, therefore, in amphitheatre, studded with lofty and rounded ranges of moun- these situations that, among ancient rocks, we always discover tains. From the bosom of this amphitheatre, huge columnar the greatest effects of their denuding power. Their geological hills start up at once from the ground, at a considerable dis- importance depends on two things: on the relative depth of All these are monuments of the the sea which they traverse, and on their proximity to land. tance from each other. At very great depths in the bed of the ocean we denuding agency of water-not that of the present Elbe, but By their shallowness, their velocity is increased; and by their of the sea-at a time when this part of Germany was slowly nearness to coasts, they wear away and remove the rocks that rising from the ocean. The identity of structure and of com-resist them. position in all these columnar eminences prove that they once have no reason to suppose that this denuding power exists. formed one body, and that all the softer parts of their beds If it does exist, its cause must differ from all surfa bare been removed by draudation.