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Show 28 SIIIFTING OF TilE AREAS [Ch. III. near Geneva, be carried nearly two hundred miles southwards, where the Rhone enters the Mediterranean. The additional matter thus borne down to the lower delta of the Rhone would not only accelerate its increase, but might affect the mineral character of the strata there deposited, and thus give rise to an upper group, or subdivision of beds, having a distinct character. But the fi1Iing up of a lake, and the consequent transfer of the sediment to a new place, may sometimes give rise to a more abrupt transition from one group to another; as, for example, in a gulf like that of the St. Lawrence, where no deposits are now accumulated, the river being purged of all its impurities in its previous course through the Canadian lakes. Should the lowermost of these lakes be at any time filled up with sediment, or laid dry by earthquakes, the waters of the river would thenceforth become turbid, and strata would begin to be deposited in the gulf, where a new formation would immediately overlie the ancient rocks now constituting the bottom. · In this case there would be an abrupt passage from the inferior and more ancient, to the newer superimposed formation. The same sudden coming on of new sedimentary deposits, or the suspension of those which were in progress, must frequently occur in different submarine basins where there are currents which are always liable, in the course of ages, to change their direction. Suppose, for instance, a sea to be filling up in the same manner as the Adriatic, by the influx of the Po, Adige, and other rivers. The deltas, after advancing and converging, may at last come within the action of a transverse current, which may arrest the further deposition of matter, and sweep it away to a distant point. Such a current now appears to prey upon the il.elta of the Nile, and to carry eastward the annual accessions of sediment that once added rapidly to the plains of Egypt. On the other hand, if a current charged with sediment vary its course, a circumstance which, as we have shown, must happen to all of them. in the lapse of ages, the accumulation of Ch. III.] OF SEDIMENTARY DEPOSITION. 29 transported matter will at once cease in one region, and commence in another. Although the causes which occasion the transference of the places of sedimentary deposition are continually in action in every region, yet they are most frequent where subterranean movements alter, from time to time, the levels of land, and they must be immense during the successive elevations and depressions which must be supposed to accompany the rise of a great continent from the deep. A trifling change of level may sometimes throw a current into a new direction, or alter the course of a considerable river. Som~ tracts will be alternately submerged and laid dry by subterranean movements; in one place a shoal will be formed, whereby the waters will drift matter over spaces where they once threw down their burden, and new cavities will elsewhere be produced, both marine and lacustrine, which will intercept the waters bearinO' sediment, and thereby stop the supply once carried to som: distaut basin. We have before stated, that a few earthquakes of moderate power might cause a subsidence which would connect the sea of Azof with a large part of Asia now below the level of the ocean. This vast depression, recently shown by Humboldt to extend over an area of eighteen thousand square leagues, surrounds Lake Aral and the Caspian, on the shores of which seas it sinks in some parts to the depth of three hundred feet below the level of the ocean. The whole area might thus suddenly become the receptacle of new beds of sand and she1Js p~obab1y differing in mineral character from the masses pre-' VIOusly existing in that country, for an exact correspondence could only arise from a precise identity in the whole combination of circumstances which should give rise to formations produced at difterent periods in the same place. Without entering into more detailed explanations the reader will perceive that, according to the laws now go~erning the aqueous and igneous causes, distinct deposits must, at different periods, be thrown down on various parts of the earth,s surface, |