to rapid recession. The uppermost rock, over which the water falls, is a hard limestone ninety feet thick; beneath is a soft shale, which is easily worn away by frost and the friction of the water, leaving the limestone jutting over, in table rocks, sometimes forty feet beyond the shale. From time to time these table rocks fall off. River Simeto excavating a channel in solid Lava. 19. The Simeto, one of the principal rivers of Sicily, has excavated its bed from fifty to several hundred feet wide, and forty to fifty feet deep, since A. D. 1603. In the cut given above, A, indicates the lava bed, which descending from the summit of the great volcano, has flowed five or six miles, and usurped the old bed of the Simeto, E; B, the present bed of the river. The lava is not soft and scoriaceous, but is a compact, hard rock. c, indicates the foot of the cone of Etna, and D, marine and volcanic beds. The general declivity of the river is slight, having two falls of six feet each. The abraded materials of the volcanic rock have greatly assisted the attrition.* 20. The mechanical force of rivers is exerted in carrying forward the sedimentary matter contained in there. When the rapidity of the stream is diminished the heavier portions of the sediment are deposited: but when the rate *Lyell's Principles. ocal import Frid on the of the current is increased by freshets, coarse gravel, pebbles, and even large blocks of stone, are borne forward by it. Torrents, produced by the bursting of dams, are very destructive in their effects. A remarkable example of this action occurred in the Valais (Switzerland) in A. D. 1818. A dam four hundred feet high and six hundred feet wide was formed across the valley during the winter, of the rocks and ice brought down from the mountains by avalanches. The water of the river Dranse accumulated in this lake containing more than eight hundred million cubic feet. The inhabitants, anticipating destructive results from its eruption, opened an orifice in the dam near the surface of the water, by which three-fifths of the water was conducted off in three days, without violence. The barrier then gave way, and all the remaining waters were discharged in half an hour. The torrent reached the Lake of Geneva, forty-five miles distant, in six hours, destroying houses and trees that were in its path, and strewing the valley with rocks and earth. The rapidity with which water excavates a channel was illustrated a few years since in the town of Glover, Vermont. A small channel made from Long Pond, which was two and a quarter miles long by one and a quarter wide, with a depth of one hundred and fifty feet, was in a few minutes so enlarged as to discharge all the water into Lake Memphremagog, twenty miles distant, with the usual violent effects of an inundation. water. 21. The transporting power of water is rendered more efficient by the loss of weight which minerals sustain in Most minerals weigh about one-half as much in water as in the air. It is ascertained by experiment that a velocity of six inches per second will raise and transport 20 Italian STRUCTURE OF THE EARTH. on a horizontal surface, fine sand; eight inches, coarse sand; twelve inches, gravel; and twenty-four inches, rounded pebbles. Fresh water moving with a velocity of one and a half mile per day, raises fine clay, and eight and a half miles per day removes sand. Fine mud settles so slowly in moving water that it is often transported hundreds of miles before reaching the bottom. For this reason the sediment of rivers near their mouths consists mostly of mud. 22. Deposits of mud are called silt. The thickness of the alluvial bed of the Mississippi river one hundred miles above its mouth, is two hundred and fifty feet. Rivers in this way sometimes raise their beds higher than the adjoining plains, and forsaking their old beds, flow in new channels. The Po has repeatedly deserted its bed, destroying a number of towns. Several churches have been taken pebbledown and removed to escape its erratic movements. on Clake In one instance, during the fifteenth century, it returned to the bed it had forsaken, but again deserted it during the same century. It is now restrained by artificial banks on an elevated mound, so that its surface at Ferrara is higher than the roofs of the houses in the city. The Mississippi is also confined by levees. 7 23. Deltas are deposits of mud, sand, and gravel, made at the mouths of rivers, triangular in form, with the apex of the triangle up the stream. They are called deltas from their resemblance to the Greek letter A. Deltas are fluviatile when formed at the entrance of one river into another; lacustrine when at the entrance of a river into a lake; and maritime when the river empties into the sea. These deltas often accumulate rapidly, and mountain lakes are sometimes entirely filled up by them. The Rhone and Arve enter the Lake of Geneva as turbid streams, but . the Rhone emerges from the lake perfectly transparent. The delta formed is six miles long—a flat alluvial plain of sand and mud. An ancient town, Port Vallais, (Portus Valesiæ of the Romans,) which anciently stood upon the shore of the lake, has had its harbor silted up, and is now more than a mile and a half inland. Mr. Lyell says, "We may look forward to the period when this lake will be filled up, and then the distribution of the transported matter will be suddenly altered, for the mud and sand brought down from the Alps will thenceforth, instead of being deposited near Geneva, be carried nearly two hundred miles southwards, where the Rhone enters the Mediterranean." The Rhone after leaving the Lake of Geneva again acquires sediment, and has formed a maritime delta at its mouth of three hundred and twenty thousand acres. The mouth of the river has advanced more than eight miles since the commencement of the Christian era. 24. The delta of the Nile commences one hun dred miles from the Mediterranean, and is two hundred and thirty miles broad. It does not at present increase much, because the sediment which reaches the Mediterranean is swept away by an easterly current, and the detritus brought down by Fig. 7. Delta of the Nile. its periodic floods are spread over the plains of Egypt. This fertilizing deposit, consisting of silica, alumina, oxide of iron, and carbonates of lime and magnesia, has accumulated 22 37 28 27.6 230 2.2 1 STRUCTURE OF THE EARTH. to the depth of more than six feet since the commencement 25. The Ganges presents a delta of gigantic dimensions, The whole coast of Northern Central Europe, from Calais to the Baltic, has been covered with mud brought down by the Rhine, which in some places is several hundred feet thick. Experiments show that the quantity yielded by this river, is about four hundred tons weight per hour. 26. The delta of the Mississippi comprises an area of thirteen thousand six hundred square miles, projecting into the Gulf of Mexico, with a depth of 1056 feet, and containing 2720 cubic miles. The quantity of water annually dis- · charged by the river is 14,883,360,636,880 cubic feet; quantity of sediment discharged is 28,188,083,892 cubic feet, being the one five hundred and twenty-eighth part of the water. One cubic mile is formed in five years and eighty-one days. The formation of the whole delta would have required, at this rate, 14,203 years. The quantity annually discharged is sufficient to cover a township five miles square with a layer of mud forty feet thick. 27. The accessions to the land from the sediment brought down by the Po and the Adige, are one hundred miles long, and vary in width from two miles to twenty miles. The town Adria, a seaport in the time of the Em* according later obsuvation by Redcell · 3,702, 758,400 Gauges, it carrers more than андч I as much " |