ACTION OF RUNNING WATER.

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will convince us that the amount is almost beyond estimate. All the rivers on the face of the earth are constantly pouring their waters into the sea, and yet its level is not affected in the slightest degree, hence we infer that the quantity of mositure evaporated from the surface is exactly equal to the sum of all the rivers of the world. If the evaporation and restoration of the waters were all the effect which is produced by the agencies just described, little change would be accomplished upon the face of the country over which the waters might flow in their passage to the sea. But in the more elevated tracts of country, the atmosphere acts powerfully upon the soil, and by the influence of heat and cold, by dampness and dryness, and of frost and rain, loosens the most coherent masses and disintegrates the solid rocks. The mountain streams flow down more or less charged with earthy matter, worn from the soil and rocks over which they flow. In their passages toward the sea, sometimes over an immense tract of country, they often unite and pour their waters along with almost irresistible fury. The solvent power of the water assists very materially in degrading the rocky channels through which it flows, and acts powerfully on the alkaline and calcareous elements of the soil, and especially when it holds carbonic acid in solution, which is almost always the case. When the earthy matter and pebbles are thus intermingled with running water, a new mechanical power is gained, by the attrition as they are borne along, thus sapping and gradually undermining high banks and rocks, until at length the overhanging mass is precipitated into the current and swept away by its waters. In this manner, islands are cut off from the main lands, and shoals, and rich earthy deposits called deltas are formed at the mouths of rivers. There is nothing so very remarkable in the power of currents to transport even heavy masses of stone, for we must remember that the specific gravity of water is much greater than air, and a stone immersed in a stream will loose about half its weight, and many of the lighter particles of the soil will almost float.

Sir George Staunton estimated that the quantity of sediment borne down by the Yellow River in China, in a single day, was equal to forty-eight millions of cubic feet, and late observations

upon the Ganges, at the time of its flood, or in the rainy season, when it is fully charged with sediment, shows that it discharges 6,082,041,600 cubic feet in 122 days, and during the three months of hot weather, and the five months winter, it discharges 286,035,840 cubic feet more, a quantity small compared with the former, the total annual discharge is therefore 6,368,077,440 cubic feet.

"In order" says Mr. Lyell, "to give some idea of the magnitude of this result, we will assume that the specific gravity of the dried mud, is only one-half that of granite (it would. however, be more), in that case the earthy matter discharged in a year, would equal 3,184,038,720 cubic feet of granite. Now, about

12 cubic feet of granite weigh one ton, and it is computed that the great Pyramid of Egypt, if it were a solid mass of granite, would weigh about 6,000,000 tons. The mass of matter therefore carried down annually, would, according to this estimate, more than equal in' weight and bulk forty-two of the great pyramids of Egypt, and that borne down in four months of the rains, would equal forty pyramids. The base of the great Pyramid of Egypt covers eleven acres, and its perpendicular height is about five hundred feet. It is scarcely possible to present any picture to the mind which will convey an adequate conception of the mighty scale of this operation, so tranquilly and almost insensibly carried on by the Ganges. It may however, be stated, that if a fleet of more than eighty Indiamen, each freighted with about 1400 tons weight of mud, were to sail down the river every hour of every day and night for four months continually, they would only transport from the higher country to the sea, a mass of solid matter equal to that borne down by the Ganges in the flood sea

son."

The same effect is observable in the mighty rivers of America. The Mississippi annually bears down upon its swollen stream innumerable quantities of trees and sediment, which are imbedded in the basin of the sea at the mouth of the river. In this manner the remains of animals and vegetables are being continually enveloped, and, should these deltas some day become dry land, the naturalist could determine by a study of the imbedded remains,

EXCAVATION OF A LAVA CURRENT.

201

the character of the country through which the stream had flowed. Below we give a diagram showing the excavation of a lava current by the action of the river Simento, one of the largest of the

Sicilian rivers, which flows at the base of Etna. A A, bed of lava which has flown to a distance of five or six miles; B, bed of the Simento; C, foot of the cone of Etna; D, marine and volcanic strata; E, ancient bed of the river. The lava current in which the channel is eroded is one of the more recent, having been ejected in 1603. In a little more than two centuries the Simento has worn a passage from fifty, to several hundred feet wide, and in some parts from forty to fifty feet deep. The portion of lava cut through, is not porous, or mixed with cinders and scoria, but consists of a compact homogeneous mass of hard blue rock. The Falls of Niagara afford a magnificent example of the progressive excavation of a deep valley in solid rock. It appears from examination that the Falls were once at Queenstown, about seven miles below their present position. It is possible however that a natural chasm may have previously been formed a part of this distance, which the river has since widened, although a careful study of the face of the country, and also the existing proofs, at various places, several miles below the present falls, of fluvatile deposits, seem to show conclusively that the falls have gradually receded from near the present site of Lewiston and Queenstown.

When by the melting of snows and ice, an unusual amount of water is accumulated at some high point, and the barriers which have been restraining it give way suddenly, the flood sweeps onward with a fury which overcomes every obstacle. Such was the flood in the valley of Bagnes, described by Mr. Lyell, in his Principles of Geology. The bed of the river Dranse being blocked

up by the avalanches of snow and ice, a reservoir or lake, was thus formed, about half a league in length, two hundred feet deep, and seven hundred feet wide. To lessen the mischief apprehended from the sudden bursting of this ice barrier, a channel was cut through the ice about 700 feet in length; the flow of the waters deepened this channel until nearly half the contents of the lake were drained off, but on the approach of the hot season, the remaining mass gave way with a tremendous crash, and the residue of the lake was emptied in half an hour. As the mass of waters and floating ice swept through the narrow gorges, it rose sometimes to an immense height, to burst again with increased fury into the next basin, sweeping along rocks, forests, bridges, and cultivated lands. Immense fragments of granite rock were torn from the ancient soil and borne down; one of these, was sixty paces in circumference.

The Deltas, or triangular sedimentary deposits which are formed at the mouths of large rivers, often exhibit distinct marks of stratification, and when they terminate in an extensive estuary, or arm of the sea, the layer of mud brought down by the river is regularly covered by a layer of sand, borne in and deposited upon the mud at each returning tide. It is in this manner that the ripple marks, and tracks of vermes, and molusces, are preserved. Every one must have noticed, in walking along a sandy shore, at low water, the undulating surface of the mud or sand, caused by the little ripples in the water, and also the varied tracks of worms, shell-fish, and birds. When a thin layer of mud happens to be deposited over these before the next return of the waves, a perfect cast is thus obtained. Mr. Lyell, in his travels in North America mentions that he had obtained at Wolfville, on the Bay of Fundy, thin slabs of the dried red mud, which presented perfect impressions, on the upper side, showing the recent foot-prints of a small sandpiper, as it marched over the soft mud, which had afterwards so much hardened in the sun as to become consolidated, and upon the under surface exhibiting a cast of the impressions made in a previous deposit. The red sediment, or mud deposited by the waters of the Bay, is obtained from undermining cliffs of red sandstone, and soft red marl, and whenever the velocity of the

FLUVIATILE FORMATIONS.

203 current is suspended by the rush of the tides, this mud is thrown down, and very large, and widely extended tracts, of rich soil, have thus been formed, and thousands of acres have been excluded from the encroachments of the sea by artificial embankments. At the time of very low tides, this soft mud is sometimes exposed to the sun for several days, and thus becomes sufficiently baked or consolidated, to a depth of several inches, to resist the flow of water, which soon deposits upon it another thickness of mud. We shall see that in a precisely similar manner, footprints, of high antiquity, were formed in the strata of new red sandstone of the valley of the Connecticut, and also in Europe.

The large rivers which flow from south to north in the northern latitudes, having their sources in a much warmer latitude than their mouths, become swollen in their progress northward, on account of the ice which has not yet been broken up, hence they overflow and sweep through the forests of pines and birches, and carry away thousands of the uprooted trees. The timber thus drifted down is often laden with the earthy deposit around the roots, and being deeply sunk in the water, other masses become piled upon it until at length becoming water-logged it sinks and is imbedded in the strata if there be any forming. "As the trees" says Dr. Richardson, "retain their roots, which are often loaded with earth and stones, they readily sink, especially when water soaked; and, accumulating in the eddies, form shoals which ultimately augment into islands. A thicket of small willows covers the new formed island as soon as it appears above the water, and their fibrous roots serve to bind the whole firmly together. Sections of these islands are annually made by the river, assisted by frost, and it is interesting to study the diversity of appearances they present, according to their different ages. The trunks of the trees gradually decay until they are converted into a blackish brown substance, resembling peat, but which still retains more or less of the fibrous structure of the wood; and layers of this often alternate with layers of clay and sand, the whole being penetrated to the depth of four or five yards or more, by the long fibrous roots of the willows." A deposition of this kind, with the aid of a little infiltration of bituminous matter, would produce an excellent