Principle of reasoning common to all the physical sciences—This principle applicable to Geology—Carbonic acid an agent of denudation—Vast quantity of lime dissolved by the waters of the Rhine and borne away to the German ocean—Disintegration of rocks by frost—Professor Tyndall on the Matterhorn—Running water—Its erosive power—An active and unceasing agent of denudation—Mineral sediment carried out to sea by the Ganges and other great rivers—Solid rocks undermined and worn away—Falls of the Clyde at Lanark—Excavating power of rivers in Auvergne and Sicily—Falls of Niagara—Transporting power of running water—Floods in Scotland—Inundation in the valley of Bagnes in Switzerland.

In the physical sciences it is a common principle of reasoning to account for the phenomena that come before us in nature, by the operation of natural causes which we know to exist. Nay, this principle seems to be almost an instinct of our nature, which guides even the least philosophical amidst us, in the common affairs of life. When we stand amongst the ruins of an ancient castle, we feel quite certain that we have before us, not alone the monument of Time’s destroying power, but also the monument of human skill and labor in days gone by. We entertain no doubt that ages ago the sound of the mason’s hammer was heard upon these walls, now crowned with ivy; that these moss-grown stones were once hewn fresh in the quarry, and piled up one upon another by human hands; and that the building itself was designed by human skill, and intended for the purposes of human habitation and defence. Or, if we see a footprint in the sand, we conclude that a living foot has been there; and from the character of the traces it has left, we judge what was the species of animal to which it belonged, whether man, or bird, or beast. It is true that God is Omnipotent. He might, if it had so pleased Him, have built the old castle at the creation of the world, and allowed it to crumble slowly into ruins: or he might have built it yesterday, and made a ruin begin to be where no castle had stood before; and covered the stones with moss, and mantled the walls in ivy. And as to the footprint in the sand, it were as easy for Him to make the impress there, as to make the foot that left the impress. All this is true: but yet if any one were to argue in this style against us, he would fail to shake our convictions; we should still unhesitatingly believe that human hands once built the castle, and that a living foot once trod the shore.

Now, this principle of reasoning is the foundation on which the ablest modern Geologists claim to build their science. The untiring hand of Nature is ever busy around us: they ask us to come and look at her works, and to judge of what she has done in past ages, by that which she is now doing before our eyes. She is still, they say, building up her strata all over the globe, of limestone, and sandstone, and clay; she is still lifting up in one place the bed of the ocean, and in another submerging the dry land; she is still bursting open the Crust of the Earth by the action of internal fire, disturbing and tilting up the horizontal strata; she is still upheaving her mountains and scooping out her valleys. All these operations are open to our inspection; we may go forth and study them for ourselves; we may examine the works that are wrought, and we may discover, too, the causes by which they are produced. And if it should appear that a very close analogy exists between these works that are now coming into existence, and the long series of works that are piled up in the Crust of the Earth, it is surely not unreasonable to refer the latter class of phenomena to the action of the same natural causes which we know to have produced the former.

It cannot be denied that this argument is deserving of a fair and candid consideration. Let us proceed, then, to examine how far it is founded on fact, and how far it can be justly applied to the various heads of Geological theory. We will commence with the origin and history of Stratified Rocks; for this constitutes, in a manner, the framework on which the whole system of Geology is supported and held together. It is alleged that the elements of which Stratified Rocks are composed are but the broken fragments and minute atoms of pre-existing rocks, carried off by the agents of Denudation, and spread out over some distant area in regular beds or layers; which, in progress of ages, were slowly consolidated into rocks of various quality and texture. With the view of testing this theory by the light of the principle just explained, we purpose, in the first place, to exhibit some examples of the many forms in which the process of Denudation is going on at the present day all over the world; and afterward, to show that out of the materials thus obtained Stratified Rocks of every description—Mechanical, Chemical, Organic—are being regularly built up in sundry places; and that these correspond in every essential feature with the Stratified Rocks in the Crust of the Earth.

Among the chemical agents of Denudation, there is none more widely diffused than Carbonic acid gas. It is everywhere given out by dead animal and vegetable matter during the process of putrefaction; it is plentifully evolved from springs in every country; and it is emitted in enormous quantities from the earth in all volcanic districts, as well those in which the volcanoes are now extinct as those in which they are active. Now, it is well known from observation, that carbonic acid has the property of decomposing many of the hardest rocks, especially those in which felspar is an ingredient. This phenomenon is exhibited on a large scale in the ancient volcanic district of Auvergne, in central France. The carbonic acid, which is abundantly evolved from the earth, penetrates the crevices and pores of the solid granite, which being unable to resist its decomposing action, is rapidly crumbling to pieces. This mysterious decay of hard rock has been happily called by Dolomieu, “la maladie du granite.”15

Again, all the water which flows over the surface of the land is highly charged with carbonic acid. The rain imbibes it in falling through the atmosphere; and the rivers receive still further accessions from the earth as they pursue their course to the sea. In this combination we discover a powerful agent of Denudation; for limestone rock will be dissolved by water which is impregnated with carbonic acid. Thus all the rivers and streams in the world, when they flow through a limestone channel, are constantly dissolving the solid rock and bearing away the elements of which it is composed. A single example will be sufficient to show the magnitude of the results which are thus produced. It has been calculated by Bischof, a celebrated German chemist, that the carbonate of lime which is carried each year to the sea by the waters of the Rhine, is sufficient for the formation of 32,000,000,000 of oyster shells; or, to view the matter in another light, it would be sufficient to produce a stratum of limestone one foot thick, and four square miles in extent.16 If such be the yearly produce of one river, how great must be the accumulated effects of all the rivers in the world since our planet first came from the hand of its Creator!

Passing from the chemical to the mechanical agents of Denudation, it is worth while to notice the immense power which is often generated by the agency of frost, especially in those countries that are subject to great vicissitudes of heat and cold. During a thaw, water finds its way into the clefts and joints by which all rocks are traversed, and when it is afterward converted into ice, it expands with a mechanical force that is almost irresistible. The hardest rocks are burst asunder, great blocks are detached from the mountain side, and sent rolling down its slopes, or tumbling over crags and precipices, until at length they come to rest in shattered fragments at the bottom of the valley. In this condition they await but the coming of the winter’s torrent to be borne still further on their long journey to the sea.

The fearful havoc done in this way by the alternate action of sun and frost contributes in no small degree to the fantastic and picturesque forms assumed by the mountain peaks of Switzerland. Huge masses of rock have been literally hewn away, until nothing has remained behind but those splintered obelisks and tapering pinnacles so familiar to the eye amidst the sublime scenery of the Alps. Indeed one of the greatest perils encountered by the adventurous spirits whose ambition it is to rival one another in the danger of their exploits, and to climb whatever was before regarded as inaccessible, arises from the enormous fragments of rock which are rent almost unceasingly from the overhanging crags and hurled into the abysses below them. The following incident related by Professor Tyndall is very much to the point. “We had gathered up our things, and bent to the work before us, when suddenly an explosion occurred overhead. Looking aloft, in mid-air was seen a solid shot from the Matterhorn describing its proper parabola through the air. It split to pieces as it hit one of the rock-towers below, and its fragments came down in a kind of spray, which fell wide of us, but still near enough to compel a sharp look out. Two or three such explosions occurred afterward, but we crept along the back fin of the mountain, from which the falling boulders were speedily deflected right and left.”

This occurred in 1862, on the occasion of an unsuccessful attempt to reach the highest peak of the Matterhorn. Six years later, when Professor Tyndall at length actually accomplished the object on which he seems to have set his heart, he found the work of destruction still going on. “We were now,” he says in his narrative, “beside a snow-gully, which was cut by a deep furrow along its centre, and otherwise scarred by the descent of stones. Here each man arranged his bundle and himself so as to cross the gully in the minimum of time. The passage was safely made, a few flying shingle only coming down upon us. But danger declared itself where it was not expected. Joseph Maquignas led the way up the rocks. I was next, Pierre Maquignas next, and last of all the porters. Suddenly a yell issued from the leader: ‘Cachez vous!’ I crouched instinctively against the rock, which formed a by no means perfect shelter, when a boulder buzzed past me through the air, smote the rocks below me, and with a savage hum flew down to the lower glacier.”17

Even in our own country, every one is familiar with the efficacy of frozen water in producing landslips. The rain which soaks into the ground in winter, is converted into ice when frost sets in; and upon steep slopes or precipices, its expansive power bursts open the earth, and causes large masses of stones and clay to tumble headlong to the bottom.

But moving water constitutes the most powerful, and, at the same time, the most universal agent of Denudation. And it is chiefly to the effects of moving water that we mean to direct attention; because its action is more striking to the eye, and more easily understood by the general reader. Every one is aware that the waters of the ocean are constantly passing off by evaporation into the higher regions of the atmosphere, and are there condensed into clouds. These clouds in course of time descend upon all parts of the earth, but especially on the high and mountainous districts. Then rivulets are formed which flow smoothly down the gentle slopes of the undulating country, or plunge headlong over the rocky mountain cliffs; and the rivulets uniting form streams, and the streams, receiving new tributaries as they advance, become rivers; and the rivers flow on to the sea, and discharge each day and each hour their enormous volumes of water back again into the ocean from which they came. Thus all the water of the world is constantly in motion, ever hurrying on, as it were, in one unending round of duty. This is the teaching of daily experience and observation. And we may add, it is the teaching of Sacred Scripture as well. The Wise Man said long ago: “All the rivers run into the sea, yet the sea doth not overflow: unto the place from whence the rivers come, thither they return to flow again.”18

Now, the power of this moving water is a mighty wide-spread agent of change in the physical condition of the globe. For wherever water is in motion over the surface of the land, whether it be a rippling stream, or a mountain torrent, or a majestic river, it is surely wearing away the channel through which it flows, and carrying along in its course particles of clay, or sand, or gravel. This subject is illustrated with great force and great simplicity by Mr. Page. “Every person,” he says, “must have observed the rivers in his own district, how they become muddy and turbid during floods of rain, and how their swollen currents eat away the banks, deepen the channels, and sweep away the sand and gravel down to some lower level. And if, during this turbid state, he will have the curiosity to lift a gallon of the water, and allow it to settle, he will be astonished at the amount of sediment or solid matter that falls to the bottom. Now, let him multiply this gallon by the number of gallons daily carried down by the river, and this day by years and centuries, and he will arrive at some faint idea of the quantity of matter worn from the land by rivers, and deposited by them in the ocean. In the same way as one river grinds and cuts for itself a channel, so does every stream and rill and current of water. The rain as it falls washes away what the winds and frosts have loosened; the rill takes it up, and, mingling it with its own burden, gives it to the stream; the stream takes it up and carries it to the river, and the river bears it to the ocean.”19

When the current is feeble, the greater part of this earthy material is thrown down upon the way, and forms a stratum of alluvial soil in the bed of the river, and also in the adjoining lowlands, during the time of temporary floods. But when several streams unite, then the carrying power of the current is enormously increased: huge stones are rolled along, and dashed one against another, and broken into fragments, and the fragments are rounded by friction, and become pebbles, and the pebbles become gravel, and the gravel, mud; and the mud is carried on to the mouth of the river, and there falling to the bottom, it forms a tongue of land which is called a delta; or else perhaps it chances to meet with some great ocean current, and then it begins a new journey, and is borne far away to be deposited in the profound and tranquil depths of the sea. It is not, however, mineral matter alone that is transported by the action of rivers. Trees that once were growing on the banks of the stream, and the bones of animals, and human remains, and works of art, are seen floating down with the current, and are found embedded in the sand and mud of the delta at the river’s mouth.

These are some of the actual realities which all may witness, who will go and study for themselves the history of this wonderful element, from the time when it first soars aloft as vapor to the sky, until it returns to the bosom of its parent ocean laden with the spoils of the land. To some of our readers, perhaps, results of this kind may appear insignificant, when considered in relation to the enormous bulk of the stratified rocks. But it should be remembered that the force of which we speak is unceasing in its operation over the whole surface of the earth; and even though the work were small which is accomplished in each successive year, the accumulated effects produced in a lengthened period of time must be immensely great. Besides, it would be a very serious error to form our ideas on this subject, as many would seem to do, from the examples which are to be found within the narrow limits of our own island. We should rather seek for our illustrations among those mighty rivers that drain the vast continents of the world, and exhibit the erosive and transporting power of running water on the grandest scale.

It happens, fortunately for our purpose, that an attempt has been made by scientific men to compute the amount of matter discharged into the sea, by some particular rivers within a given time. For such a computation it is necessary, in the first place, to calculate the volume of water that passes down the channel during that time; and then, by repeated experiments, to ascertain the average proportion of earthy matter which is held suspended in the water. This has been done with the greatest care by the Rev. Mr. Everest, in the case of the river Ganges; and it appears that during the rainy season, which lasts four months every year, from June to September, about 6,000,000,000 cubic feet of mud are carried along by the stream past the town of Ghazepoor, near which the observations were made. Now this enormous bulk of mineral matter would be sufficient to form a stratum of rock one foot in height, and two hundred and eighteen square miles in extent. Or, to adopt the computation of Sir Charles Lyell, the amount which passes by every day is equal to that which might be transported by 2000 Indiamen, each freighted with a cargo of mud 1400 tons in weight. And it is important to remember that this estimate represents but a portion of the sediment which passes into the sea through the channel of the Ganges; for the observations of Mr. Everest were taken at a point which is 500 miles from the sea, and at which the river has not yet received the contributions of its largest tributaries.

We are able, therefore, with some degree of confidence, to estimate the amount of Denudation which is every year effected by the Ganges. And, although the same calculations have not yet been applied with equal care to other great rivers, there is no reason to suppose that the Ganges is an exception. It is asserted on good grounds that the Brahmapootra, which unites with the Ganges close to the Bay of Bengal, carries with it an equal amount of earthy sediment. According to Sir Charles Lyell, the quantity of solid matter brought down each year by the Mississippi amounts to 3,702,758,400 cubic feet. And it is said that 48,000,000 cubic feet of earth are daily discharged into the sea by the Yellow River in China, called by the natives the Hoang Ho.20 Thus year after year the waste of the land is carried away by rivers, to be spread out over wide areas of the ocean, and perhaps to furnish the materials of future continents.

The effects of running water in wearing away and transporting masses of solid rock are not less deserving of our notice. Every one who has followed the course of a great river when it flows through a rocky channel, must have observed large blocks projecting from the cliffs above, which, having been undermined by the action of the water, seem ready to tumble headlong into the stream; and others lying below, which had fallen before; and others again which had been already carried a considerable distance by the winter’s torrent. Even where the rocks are not displaced, they are gradually being worn away, partly by the friction of the water, but much more by the grinding action of the gravel which the water holds in suspension. Not only is the surface of the rocks thus rounded and polished, but large circular pits, called pot-holes, are formed by the whirling waters of an eddy carrying round and round a few grains of hard sand.

At the falls of the Clyde near Lanark in Scotland, these various phenomena may be seen to great advantage. Good illustrations are to be found also in many volcanic regions. Some of the larger streams in Auvergne have in course of time forced their way through the solid lava rock, cutting out for themselves channels broad and deep. In Sicily too, we are told, the river Simeto, whose course was blocked up by a current of lava about the beginning of the seventeenth century, has since that time eaten its way through this compact and hardened mass, and now flows on to the sea through a rocky passage forty feet in depth and from fifty to several hundred feet in width.21

But there is no part of the world yet explored where these effects are exhibited on the same gigantic scale as at the far-famed Falls of Niagara. The massive limestone rock from which the waters are precipitated is slowly but certainly disappearing. An enormous volume of water, more than a third of a mile in breadth, plunges in a single bound over a sheer precipice of one hundred and sixty-five feet. The soft slaty rocks upon which the limestone rests are soon eaten away by the action of the spray which rises from the pool below; and then the overhanging cliffs, left without any support, topple over, and are carried off by the torrent. The position of the Falls, therefore, is not stationary, but is receding by very sensible degrees in the direction of Lake Erie, from which the river flows. Speaking of this phenomenon, Sir Charles Lyell observes with much show of reason: “The idea of perpetual and progressive waste is constantly present to the mind of every beholder: and as that part of the chasm which has been the work of the last hundred and fifty years resembles precisely in depth, width, and character the rest of the gorge, which extends seven miles below, it is most natural to infer, that the entire ravine has been hollowed out in the same manner, by the recession of the cataract. It must at least be conceded, that the river supplies an adequate cause for executing the whole task thus assigned to it, provided we grant sufficient time for its completion.”22

With a view to enable our readers to understand more fully the prodigious force which rivers have been known to exert in the transportation of rocks, it may be useful to draw attention to one or two principles of physical science. First, we have the well-known law of Archimedes, that a solid body immersed in a liquid loses a part of its weight equal to the weight of the liquid displaced. Now solid rock as compared with water, bulk for bulk, is rarely more than three times, and often not more than twice as heavy. Consequently, according to this law, almost all rocks will lose a third of their weight, and many will lose one-half, when immersed in water. Again, it has been established that the power of water to move bodies that are in it increases as the sixth power of the velocity of the current. Hence, if the velocity of a current is increased two-fold, its moving power will be increased sixty-four fold; if the velocity is increased three-fold, the moving power will be increased seven hundred and twenty fold; and so on.

From these principles it follows, first, that a much smaller power is required to move a block of stone lying in the bed of a river, than if it were lying on the surface of the land; and secondly, that a very slight increase in the velocity of a current effects a very great increase in its moving power. We need not wonder, then, when we hear of the enormous masses of rocks and trees and mason-work which are carried away even by small rivers in times of flood.23

Here are a few examples. In August, 1829, a fragment of sandstone, fourteen feet long, three feet wide, and one foot thick, was carried by the river Nairn, in Scotland, a distance of two hundred yards. On the same occasion the river Dee swept away a bridge of five arches, built of solid granite, which had stood uninjured for twenty years; the whole mass of masonry sunk into the bed of the stream and was seen no more. And the river Don, as we are assured on the authority of Mr. Farquharson, forced a mass of stones four or five hundred tons in weight up a steep inclined plane, leaving them in a great rectangular heap on the summit. A small rivulet called the College, in Northumberland, when swollen by a flood in August, 1827, “tore away from the abutment of a mill-dam a large block of greenstone-porphyry weighing nearly two tons, and transported it to the distance of a quarter of a mile.”24 But it is needless to multiply examples of phenomena which are occurring every day around us, and of which many among our readers have probably been eye-witnesses.

The transporting power of rivers must not always be estimated by the bulk and velocity of the current; for it is often greatly increased by some accidental obstruction, which for a time blocks up the channel through which the river flows. An instructive illustration is afforded by the river Dranse, which flows through the valley of Bagnes, in Switzerland, and empties itself into the Rhone above the lake of Geneva. In the year 1818 the avalanches which fell down from the mountain side formed a barrier across the valley, and thus effectually blocked up the course of the stream. The upper part of the valley was, in consequence, soon converted into a lake which gradually increased in size as the season advanced. When summer came, and the melting of the snows began, the ice barrier suddenly gave way with a tremendous crash, and the lake was emptied in half an hour. The mass of water, thus in a moment disengaged, burst with destructive violence over the lower valley, sweeping away rocks, forests, houses, bridges, and cultivated lands. Thousands of trees were torn up by the roots, fragments of granite as large as houses were rolled along, and the whole flood presented the appearance of a moving mass of ruins.