The Elements of Geology; Adapted to the Use of Schools and Colleges Part 9

Water is, next to heat, the most important geological agent. All the stratified rocks are aqueous deposits, and their total amount is in some respects a measure of the influence which this agent has exerted. The materials have been obtained from the destruction of preexisting rocks, transported by water, and deposited in layers.

When the first strata were formed, the sediment must have been obtained entirely from igneous rocks, because only those rocks existed; but now it is obtained from every kind of rock which is exposed to abrading or decomposing agencies. Hence, many of the later formations contain fragments, and sometimes within the fragments well-characterized fossils, of earlier formations.

The sediment which is ultimately to become stratified rock is deposited on the beds of the ocean, and other great reservoirs of water. The formation of most of the aqueous rocks, therefore, as well as of the igneous rocks, is deep below the surface; and neither of these operations, on the large scale, is directly exposed to our observation.

We may, however, learn by observation, how the sediment is furnished to the waters and transported by them, and we can form some correct ideas of the manner in which it will be laid down on the bed of the ocean, and solidified.

I. _The Furnis.h.i.+ng of Sediment._

1. Almost all the minerals which occur in the geological formations are, to some slight extent, _soluble in water_. Hence, rain water, by pa.s.sing through a stratum of earth or rock and reappearing as a spring, loses the insipidity which it had as pure water, and becomes palatable.

It is then found to hold in solution some small proportion of earthy substances, upon which this change of taste depends. Although the proportion of dissolved matter is very small, yet the surface of earth upon which this distilled water is shed is one-fourth of the surface of the globe, and solution below all that surface is constantly taking place. No inconsiderable amount must thus have been furnished, from the existing rocks of each period, towards the formation of the strata of a later period.

There are some substances which are soluble in water, in large quant.i.ties. _Rock-salt_ is an example. It is not found in any very large proportion in rocks generally, but a very large aggregate amount has been taken up by the waters which have filtered through the strata. The ocean gathers into itself, by degrees, all the soluble substances which are thus taken up. It receives supplies of water charged with these substances from springs, rivers and lakes. It returns as much water as it receives; but it is always in the form of vapor, and is therefore pure water. Hence the saline properties of the ocean, and of those inland seas which have no outlets. There is thus gathered the materials for the rock-salt deposits.

But many substances which are not considered soluble in water become so by some modification of the water. Water of a high temperature is capable of dissolving silex. In Iceland and other volcanic regions, the hot springs are charged with silex, which is deposited as the water cools. Thus, siliceous formations acc.u.mulate around springs of this kind. The various agates may have been deposited from such solutions.

In the decomposition of mica, felspar and volcanic rocks, a considerable amount of pota.s.sa is set free. Pota.s.sa or soda renders the water in which it is dissolved capable of dissolving silex in large quant.i.ty. In these ways water removes, with some degree of rapidity, one of the most insoluble minerals which rocks contain.

In volcanic countries, and in coal districts, carbonic acid is abundant, both in spring-water and in the gaseous form. Water charged with this gas becomes capable of dissolving limestone. Where the water is exposed to the air, the gas gradually escapes, and the calcareous matter is deposited. Many acc.u.mulations of this kind are now taking place. Some have already extended several miles in length, and they are often of great thickness, in one instance, in Italy, two hundred feet (Fig. 72).

It is also probable that many calcareous springs issue below the surface of lakes and seas, and thus, both fresh-water and marine deposits would now be forming. These formations are distinctly stratified, and are white and crystalline, and become solid at the time of deposition.

[Ill.u.s.tration: Fig. 72.]

These dissolved materials are less observed than others, because they do not render the water turbid; but there is reason to believe that several of the aqueous formations, particularly the limestones, have been built up chiefly from them.

2. The _abrading action of rivers_ furnishes considerable detrital matter. The general form of the river courses is determined by other causes than the agency of the river itself, yet a river which has a rapid current is continually deepening its channel. We have proof of this by observing, when the water is low, that irregularity of surface which running water always produces, by wearing away the softer parts of the rock, and leaving the harder in relief. Hence, a river will have its rapids either where the hardest strata occur, and which therefore wear down least rapidly, or where the rock has been hardened by the intrusion or near proximity of dikes.

The abrading power of rivers occasionally becomes greatly increased by waterfalls. The force which the water acquires in its descent is such as to excavate a deep cavity at the foot of the fall, reaching back under the ledge from which the water descends. The ledge is therefore constantly being undermined. The cataract of Niagara is peculiar, in having the rock at its base of a soft and friable texture, so that it is rapidly worn away, while the upper rock is a compact siliceous limestone. If the order of superposition had been the reverse, the falls would have been converted into a series of rapids. It is now preserved as a single fall, and as such it has probably cut the gorge, about two hundred feet deep and seven miles in length, through which its waters now reach Lake Ontario. A few years since, a large ma.s.s, perhaps half an acre in area, fell from the centre of the horse-shoe fall. Another ma.s.s of equal size has recently fallen from the western extremity of the ledge. Thus the fall is gradually receding.

But the foreign substances, such as drift-wood, ice, sand and gravel, with which the waters of a river are occasionally charged, contribute more than everything else to its abrading power. At such times its volume is generally greatest, and its current the most rapid. Its bed is then sometimes perceptibly deepened and widened in a few hours.

Much the greater part, however, of the earthy matter which rivers convey in such quant.i.ty to the ocean, is furnished by other means than the eroding action of the river itself. It is the loose material, the soil and alluvium, to which the solid rocks have been reduced by the imperceptible but incessantly operating atmospheric agencies, from which most of the sediment of rivers is obtained. After a rain, every tributary rivulet is turbid with suspended earthy matter, and it is from these sources that the larger streams receive the most of their sediment.

Some observations have been made for the purpose of ascertaining the quant.i.ty of sediment which rivers annually carry into the sea. The Kennebec furnishes materials which, if spread evenly on an area of one mile square, and consolidated into rock of the specific gravity of granite, would have a thickness of six inches. The Merrimac furnishes about two-thirds as much, the Ganges about two hundred and fifty times as much, and the Mississippi two thousand times as much.

Thus, the tendency is, to reduce the highest parts of the land, and to fill up the depressions of the sea; and though we have not data enough to form any reliable estimate of the total annual discharge of sediment into the ocean by rivers, yet they are sufficient to show that the effects of this kind are on a large scale, and to relieve us from any impression that existing agencies are inadequate to the production of the stratified rocks.

3. _The action of waves_ is another means by which detrital matter is furnished. Wherever the sh.o.r.e consists of loose materials, and is favorably situated to be acted upon by the waves, there is annually a sensible encroachment of the sea. Such encroachments are rapidly making in many places; and thus a large amount of sediment is delivered to the waters of the ocean.

The waves also encroach upon the coast when it consists of rocks, even of the most indestructible kinds. They continually beat upon it, undermine the cliffs, and precipitate them into the sea. The tides increase the power of the waves, by varying the place of their action, so as to present the same surface of rock alternately to the action of water and of the air, frost and sun. During storms, the waves have sufficient force to break off fragments of rock from the escarpment, sometimes in ma.s.ses weighing twenty tons or more, and remove them many rods inland.

A bold, rocky coast always exhibits evidence of a great amount of erosion. The steep escarpments and the high rugged shafts of rock (Fig.

73) against which the waves now beat are the remnants of ma.s.ses of rock which once extended further into the sea, but have been worn away by the waves. It is by such agency that the deep inlets and harbors of the coast of New England and Nova Scotia have been excavated.

[Ill.u.s.tration: Fig. 73.]

This more violent action of the waves is only occasional; but when of less power, they are incessantly rolling the loosened fragments of rock upon each other, and thus wearing them down to particles small enough to be carried away by the water.

4. The action of waves is confined to the coast, and never extends to great depths. But _marine currents_ act princ.i.p.ally on the bed of the sea. The temperature of the ma.s.s of the ocean is much higher in the equatorial than in the polar regions. At the surface, the difference amounts to sixty degrees. The waters of the torrid zone are thus expanded, and flow over the colder waters of the north and south; while these colder waters of the polar seas flow back, in an under current, towards the equator.

For the same reason,--a difference of temperature,--there will be, in the higher regions of the atmosphere, a current of warm and moist air flowing from the equator north and south, while the cold and dry air conies in from the polar regions towards the equator. In this way the equatorial waters are carried, in a state of vapor, towards the poles, where they are condensed, and go to increase the currents of water moving towards the equator.

Such are the general causes of the oceanic movements in a north and south direction; but these currents at once become deflected westward, by the diurnal revolution of the earth, as the trade winds do. Hence there results a Pacific equatorial current, which has a motion of about thirty miles a day, and an Atlantic equatorial current, moving from sixty to seventy miles a day. The princ.i.p.al marine currents are shown in Fig. 74.

The currents moving towards the poles are superficial, and therefore do not produce any marked geological effects. But the polar currents, and those which are produced from them, are of great depth, and there is no reason to suppose that they do not move, from their commencement, along the bed of the ocean. There is also reason to suppose that they exist at great depths, where the opposing superficial currents entirely conceal them.

Wherever these currents come to the surface, their motion is undoubtedly greater than it is at the bottom, where it is r.e.t.a.r.ded by the friction which the moving waters encounter, and by the irregularities of the bed of the ocean. It should, however, be remembered, that they move with the weight of the whole superior body of water; and therefore, though the motion be very slow, it will still possess great power.

Any irregularities in the bed of the ocean beneath such a current must be subject to very rapid abrasion. We shall sea hereafter, that earthquake vibrations often s.h.i.+ver the rocks at the solid surface; and if any of these ridges at the bottom of the ocean were thus acted upon, the loosened portions would be swept away by the current and deposited at lower levels, or where the current subsides. If, in any instance during an earthquake convulsion, a fault should be produced across one of these marine currents, like the great fault of over five hundred feet in England, the abutment thus thrown up would soon be worn down; and if it consisted of unconsolidated matter, it would be swept away almost bodily.

[Ill.u.s.tration: Fig. 74.]

The effect of such currents will be greatest where they are deflected by a continent or island. Thus, a marine current sets from near New Holland in a direct line to the north of the island of Madagascar, where it is arrested by the African coast, and deflected into the narrow Mozambique channel, and there acquires a velocity of four or five miles an hour. It is impossible that any kind of rock should receive the constant force of such a body of water without being rapidly worn away; and, if there should be any difference of texture in this rocky barrier, the softer portions would yield the most rapidly, and thus valleys might be formed.

It is not improbable that the deep indentation on the western coast of Africa may have been due, in a great measure, to the coast current from the Cape of Good Hope; and that the Caribbean Sea and the Gulf of Mexico may have been excavated by the force of the Atlantic equatorial current being thrown into this angle.

We may regard these currents as oceanic rivers; and it is obvious that the volume of the terrestrial rivers would bear no comparison with that of these currents, and their effects would be equally small in the comparison. The Gulf Stream, and the Mozambique and other similar currents, must be wearing down the valleys through which they flow, to such an extent as to furnish an immense amount of detrital matter for the formation of new rocks.

It is princ.i.p.ally to the agency of these deep marine currents that we are to refer those extensive denudations, so abundant on the present continents, such as the wearing out of the intermediate ma.s.ses of rock between the hills already referred to (Fig. 66), the denudation of the Connecticut river sandstone, and, perhaps, the excavations which have formed Lake Erie and Lake Ontario.

II. _The Transportation of Sediment._

The detrital matter obtained in these several ways is swept away by running water. The specific gravity of rocks does not, in general, exceed two and a half. Hence, to keep them suspended in water, will require a force of only three-fifths of what would be necessary to suspend them in the atmosphere. In the case of river currents, the velocity and irregularity of motion are generally sufficient to keep all the finer sediment equally distributed.

There will, however, be a division of the sediment according to the strength of the current. Hence, the bed of a mountain stream, if there is any loose material, always consists of pebbles. As it approaches the alluvial region, the bed is sandy; and when the current becomes very sluggish, it consists of a fine mud.

Rivers never deposit all their sediment, some of them none of it, along their course. Large rivers continue partially distinct from the ocean water to a considerable distance beyond their mouths. The waters of the Amazon have been recognized at a distance of three hundred miles. This depends in part upon the volume and velocity of the river; more, however, upon the fact that river water is lighter than sea water. This extension of a river will, in most cases, be sufficient to deliver a part of its sediment into a marine current. When such a current sweeps very near the mouth of a river, as it does to that of the Niger, the Amazon, or the Mississippi, it is probable that most of its sediment is carried away by it.

The transporting power of a marine current is greater than that of a river, in consequence of the greater specific gravity of its water; but it has scarcely any of that irregular motion of rapid rivers, upon which their transporting power in a great degree depends. The force of the current alone, when it reaches the bottom, is, however, sufficient to remove every form of loose earthy matter. Thus it may be presumed that the Gulf Stream sweeps all the sediment from its bed until it reaches the lat.i.tude of Cape Hatteras, where the cold waters from the north begin to underlie it, and it takes the character of a surface stream.

But the transporting power of marine currents depends mostly upon the depth of water. It is found, by experiment, that ordinary river sediment will sink in water about one foot in an hour. A current, therefore, of a thousand feet in depth, which moves a mile in an hour, would carry its sediment a thousand miles. It is obvious, then, that there is no part of the bed of the sea which may not be receiving sediment.

III. _The Deposition of Sediment._

From what has been said of the weight of sediment, it follows that it will be deposited whenever the water in which it is suspended is at rest. Hence, when a river increases in breadth so as to form a lake, the waters at the outlet are seldom turbid. The earthy matters with which the princ.i.p.al and tributary streams were charged all settle to the bottom, and go to lessen the capacity of the reservoir. Thus lakes are continually diminis.h.i.+ng in depth and area. In many instances, they are already filled with sediment, and are thus converted into alluvial plains, through which the river flows in a narrow channel.

It is frequently the case that a river, as it approaches the sea, has so slow a motion that its sediment is deposited on the bed of the stream.

Thus the bed will be raised, and the banks will also be raised, by the deposition of sediment upon them at periods of overflow. The river will then be raised above the adjacent country. The river Po, for the last part of its course, is from ten to twenty feet above the adjacent lands.

The same is true of the Mississippi, and many other rivers. The streets of New Orleans are several feet below the surface of the river. In an uninhabited country, such a river would soon seek a new and lower channel; but in a populous country, it becomes a matter of interest and safety to confine the river in its old channel, by artificial embankments.

But the princ.i.p.al part of the sediment of rivers is conveyed to the sea.

It here mingles with the debris which the waves have furnished, and a part of it is deposited to form deltas. The remaining part is taken up by marine currents, mingled with the debris which they have furnished, and is spread out on the bed of the ocean.

Of the extent of these deposits we can form no estimate. Those of rivers and lakes are comparatively unimportant, as they are in the older formations. Some of the delta deposits are already of great extent. That of the Ganges contains an area of twenty-six thousand square miles, that of the Niger twenty-five thousand, and that of the Nile twelve thousand.

The delta of the Rhone has increased its area by three hundred square miles in the last thousand years. The Po has encroached upon the Adriatic two thousand square miles in the last two thousand years, and the Mississippi has enlarged its delta by one hundred square miles in the last hundred years. In the deep valleys of the ocean acc.u.mulations may be taking place on as large a scale as they ever have been in former times.

IV. _Character of the Formations thus produced._

Sedimentary matter thus deposited would take the form of _strata_. Thus, a delta deposit may receive at one time from a river a layer of coa.r.s.e gravel and pebbles, and in the course of a few hours the current may be so reduced that it will convey to the same place only fine sand and silt. Or, if a depositing current receive its sediment only at intervals, the heaviest particles would be thrown down first, and the more finely levigated particles would continue to fall, till the water became transparent. Another supply would furnish another similar stratum, and so on. The same arrangement might result from the sediment being furnished by different rivers. Thus, if sediment were furnished to the Gulf Stream by the Merrimac river, and the streams emptying into the Bay of Fundy, the freshets would occur earlier in the season in the Merrimac, and it would furnish a supply of sediment from a region of primary rocks. A later supply would come from the red sandstone region of Nova Scotia, and the stratification would be indicated by the different kinds of rock produced. Thus stratification will result from difference in the color, composition, or size of the particles of which rocks consist. A great variety of causes, both general and local, may therefore give to a deposit this character. Hence, as stratified rocks are produced by the sediment now laid down from water, we may conclude that the older stratified rocks are the sediment deposited in like manner, in former times.