chida, are not discovered in either the primary or secondary strata. Lastly, as to

Man, the perfection, so far as we yet know, of all organized life-where does he first appear? The answer is, nowhere but in the loose surface soil, in mud, gravel, and caverns, and generally accompanied by pottery, bones, and other relics of the early industry of our kind. Can any more decisive evidence be afforded of the lateness of the period when man first trod the earth, of which he was to become supreme master, or of the incalculable ages the unfathomable abysses of time, as we might rather call them— that must have elapsed before his home was deemed sufficiently prepared for him?

CHAPTER IV.

AGENCIES STILL AT WORK ON THE EARTH'S CRUST.

BEFORE We enter into the study of the respective strata, systems, and formations described in our last chapter, we will pause awhile to consider how far the crust of the earth is being changed or modified by existing influences. Obviously we must do this before we can consider ourselves in a right position for the study of its ancient history. Five grand divisions occur under which all these influences may be ranged ::

I. External, or Astronomical phenomena.

II. Subterranean, or Igneous.

III. Atmospheric.

IV. Aqueous.

V. Organic being.

The External Influences may be reduced to the effects of light and heat. Heat.-All the variations of corpuscular and mechanical phenomena that are every where ceaselessly exhibited, both in organic and inorganic substances, may be ascribed to the unequal accession of heat from the sun unto our globe, which is constantly varying in distance, and whose parts are variously presented to the calorific rays, and to the unequal abstraction of heat by the cold ethereal spaces through which our planet revolves.

Light.—In light we recognize the chief element of change in the animal and vegetable creations.

Subterranean, or Igneous Influences.—Among the subterranean influences we may first mention one special effect of the distribution of the heat that rises from the interior of the earth-that is, the gradual change of level of certain parts of the land as compared with the general level of the ocean; as, for instance, on the shores of the Baltic, where certain parts are understood to be slowly rising above the sea. Whether this elevation is counterbalanced by corresponding depression elsewhere is not at present known. Sir Charles Lyell thinks the sum of the depressions from this cause greater than the elevations, but no proof is given. On this important but obscure point we shall quote the words of the author of the article "Geology" in the Penny Cyclopædia, who says, "If there be in the earth a pervading high temperature, which diminishes from the interior toward the surface, it appears, from

*

Sir John Herschel's reasoning (given in Babbage's Ninth Bridgewater Treatise), that along the shores of the sea, the isothermal lines of the interior of the globe should rise, because of the continual deposition of imperfectly conducting sediments there. For then the radiation of heat along these lines would be diminished until the interior heat had come nearer to the surface. By the consequent expansion of the subjacent earthy substances the sea-shore should rise, and thus the addition of sediment from watery action, and the effect of the effort to restore equilibrium in the disposition of the interior temperature, would, upon the whole, coincide in minutely raising the surface of the sea." This comparatively regular action of subterranean heat must be carefully distinguished from the highly irregular one, to which we owe earthquakes and volcanoes.

Earthquakes form the most terrible of all natural phenomena. They make the solid globe itself tremble and quiver beneath our feet, and sometimes to appear to the eye to undulate like the waves of the sea when agitated by the wind. They break up the crust of the earth, elevating it here into hills, depressing it there into valleys; seaming it with rents and fissures, from which often arise products never before known in the district; altering the course of rivers; producing new shores and beaches; raising the sea-bottom up to become dry land, and depressing the richly-wooded land to become henceforth the bottom of the sea; leaving cities that overhung the ocean several miles inland, and submerging other cities again below the waters; altering the distribution of animal life, and occasionally destroying it to a vast extent. Let us mention a case or two by way of illustration. In 1822, a tract of territory on the Chili coast, above one hundred miles in extent, was raised from two to six feet; and the sea-bottom, thus laid bare, emitted for a long time the most intolerable odour from the decay of dead fish, &c. In 1596, on the other hand, several Japanese towns were covered by the sea.

Volcanoes and earthquakes are doubtless but manifestations of the same subterranean fires, operating with different degrees of force, and perhaps, also, under somewhat different circumstances as regards the superincumbent masses. Thus when, in 1759, the new volcano of Jornollo was formed on the plains west of Mexico, it was what we should call an earthquake that caused the ground to swell upward like a bladder to the extent of two or three miles, and which then, bursting, became a volcano, and ejected such masses of materials that a mountain 1,695 feet high was formed by them. But the effects of volcanoes in modifying the earth's crust are even yet more extensive than such events would suggest. The burning lava emitted by them has been known to issue (as from a volcano in Iceland) in such profusion as to form a slow-moving river of melted rock, fifteen miles broad, from 100 to 600 feet deep, and extending, before it finally rested, to fifty miles from the place of its issue. It is chiefly near the sea-coast that the volcanic phenomena of modern times are found-a fact which seems to show that the admission of water to the buried fires beneath is necessary to rouse them into such a state of terrible activity.

Effects of Volcanic Action on the Bed of the Sea.-The bed of the sea is supposed to be materially affected by volcanic forces. Thus islands are

* Curves traced on a map or globe, so that each shall pass through a series of localities, where the mean annual temperature is the same.

raised, as in the South Seas, which then become centres of aggregation for all sorts of matter floating in the waters or in the air, are soon covered with vegetation, and lastly, with organic life, both of which alternately decay and spring up in never-ending sequence, and all the while increase the magnitude of the parent soil, while encroaching upon, and thereby decreasing, the size and depths of the waters around. But this kind of action may be going on invisibly to us in innumerable parts of the seas and oceans, without being sufficiently powerful to raise the lifted rocks above the surface. Of course here again corresponding depressions take place; but whether to an equal amount is not known. Even in that case we perceive that the general result of volcanic action beneath the seas and oceans must be a partial deepening and contracting: therefore still change ceaseless change. The constancy of the earth's dimensions, as indicated by the unvarying length of the solar day, may be supposed sufficient to determine the fact of an equality in these results; but that is by no means the case, for such changes, however important in the main, if we think of them as operating through a million or two of years, become apparently insignificant when thought of only in connection with our historical period.

Atmospheric Agencies include the air itself, rain, frosts, winds, and electrical phenomena; and their combined effect is to wear down, mechanically and chemically, the surface of all rocks, and to create soil, part of which is gradually strewed over the land, and becomes the means of vegetation and organic life, while the remainder is carried away by rivers and running waters towards the oceans, seas, or lakes, into which they flow.

Air acts potently upon all substances, even the hardest, that are exposed to it, through the chemical action of its oxygen and carbonic acid. Oxygen eats away the metals-carbonic acid bites into the substance of rocks—and both together at last reduce the surfaces upon which they act to a mere powder. As one layer, as it were, is worn away, another is ready to be attacked; and so the process goes on without cessation. Thus iron is reduced to rust; thus granite is pulverized to soilan operation which, it is said, has been effected to the depth of three inches within twice as many years.

Rain enters the fissures of rocks, softening and dissolving them, both by its chemical and its mechanical powers, and so preparing the way for the still more destructive agents that follow, as frosts, &c. It forms floods and inundations when it continues for a long time, which may sweep whole villages before it. On the other hand, it exerts a most beneficial effect in promoting the growth of vegetation. Its very floods, indeed, in certain parts of the world, produce the same effect on a large scale as in the annual flooding of the Nile, for which the people of Egypt look always with such welcoming anticipations. The fall of rain varies greatly in amount in different parts and different seasons. Thus, in Bombay, the monthly depth of rain in June has been given as twenty-four inches, and in October as only between one and two inches. In London the depth varies from between eight and nine inches in the half-year from January to July, to between twelve and thirteen inches in the remaining half-year. Whatever the influence exercised by rain at present, that influence was, in all probability, much greater in remote geological periods, when the heat was so much higher, and when, therefore, more liquid matter was drawn up in vapours into the sky, to descend again in rains.

Frost.-Wherever rain can insinuate itself, as already described, into the crevices of rocks, there is left an opportunity for the evolution of a highly destructive principle the expansion of the liquid particles by freezing, and the consequent rending asunder of the rocky surfaces, which, thus enlarged, can receive still larger quantities of rain, to be again frozen and expanded, and so on endlessly. This is one of the various modes of operation in which frost exhibits its power of modifying the earth's crust. Avalanches and icebergs show it in its grander manifestations of the same force.

Avalanches originate in the higher regions of mountains, and are formed of gradually accumulating masses of snow, which at last become so ponderous, that the inclined planes on which they rest can no longer support them, and they are hurled down into the valley beneath, often destroying villages with all their inhabitants, filling up rivers so as to change their course suddenly, and scattering abroad the rocky débris which they have brought down with them.

Icebergs are immense bodies of ice, extending occasionally two miles long by one broad, and some hundreds of feet high, which are found floating in the polar seas, and are formed in two ways-in the sea itself, by the accumulation of snow and ice, or on precipitous shores, in glaciers, which are ultimately broken off by their own weight, and often carry with them enormous pieces of rock. These, as the icebergs melt, when floated into warmer regions, are dropped, with all their lesser earthy contents of gravel, &c., to the bottom of the ocean, and so help to raise its bed. The erratic masses of stone called boulders, found scattered in various parts of the world on the surface, without any apparent connection with the rocks in the crust beneath, are supposed to have been thus deposited in some remote time, when the locality was covered with the deep waters.

Winds raise waves-which again act upon the rocky shores-uproot forests, cover green valleys with barren sand-drift, and form extensive Downs, as we call those tracts of land which extend, generally at a high level, by the sea-shores.

The Electrical Phenomena, which exert a sensible action on the surface of the earth's crust, are as yet but imperfectly studied. We are impressed by thunder-storms, for we can at once appreciate the strength of the power which sets fire to extensive forests, and shivers houses and rocks, and which not unfrequently reduces the human form in an instant to a mere blackened cinder; but it is probable that the slow, imperceptible effects of electricity are of infinitely greater importance in the production of specific geological effects. How intimately connected with all chemical and vital action that power is which we know under the various names of electricity, galvanism, and magnetism, is a recognized truth; but we are unable, as yet, to measure its precise effects or mode of operation. One single fact may illustrate sufficiently for present purposes the influence this power must exert in continually modifying and changing the earth's crust:-The hardest, and, to all other powers, most intractable of substances, can be artificially dissolved and re-organized by the chemist with its aid.

Aqueous Influences.-What the atmospheric agencies thus break down, the aqueous, to a great extent, carry away; and in so doing still further help on the grand operations of modification and change of the earth's

surface.

Springs open out channels, which may ultimately become river-courses.

They dissolve the rocks and minerals between, over, or around which they pass. Sometimes they exercise a petrifying power; and if heated to a high temperature, their ordinary chemical and mechanical forces are greatly increased. Unless very pure, they also carry down to the rivers into which they discharge themselves the débris collected in their course.

Rivers perform this latter operation on a great scale, often carrying down towards their mouths such quantities of mud, sand, gravel, &c., as to form vast plains, called deltas, like those of the Ganges and the Nile. But rivers are destroyers as well as carriers. We refer to the process known in geology

as

Denudation a word meaning to lay bare, and devoted to the expression of the effects of running water in the removal of solid matters on the surface, and thus of laying bare some rock beneath, which is then said to be denuded. The power of rivers in this way has been reduced to the following calculation-If the speed be three inches per second, fine clay will be torn up; if six inches per second, fine gravel will be raised; if twenty-four inches, rounded pebbles an inch in diameter will yield to the momentum; whilst a speed of thirty-six inches in the second is sufficient to drive along angular stones as large as a hen's egg. The effect of such forces, when operating through a long period, is almost incredible. There are gorges in the valley of the Alps 600 to 700 feet deep, which have been thus scooped out. The great cataract of Niagara has receded, under the operation of this power, fifty yards in less than as many years of the present century. These facts show the ordinary action of rivers; but when swollen so as to overflow their banks, a new class of effects are produced on the surrounding land, and which are often of a very serious character.

Effects of River Sediments on the Sea-bottoms.-The sediments thus formed by all the foregoing influences, and borne along by springs and rivers, are deposited by the sea-shores, and are therefore steadily diminishing the depths of the sea. Now, as the quantity of water on the globe is supposed to be constant, this change in the sea must be accompanied by an increase of the whole watery or oceanic area, or the surface must rise. The former is probably the truth. As much land is probably worn down in one part by the action of the waves as is wasted in another by the deposition of sediments. For such waves, by their restless agitation, undermine the cliffs that are above their level, grind away the rocks that are covered and uncovered during every ebb and flow of the tide, and form out of the materials at its disposal, here a dangerous sandbank-there a cultivatable piece of land, out of what was merely the sea-shore.

Organic Influences are, perhaps, the least important of all those we have named in their effects upon the crust of the globe. They are not, however, to be passed over in silence. Their effects may be summed up generally thus:-They increase the superficial soil-or that which, at some time or other, has been the superficial soil-by their decay after death, and the fresh luxuriance of organic life to which the decay gives rise by the increase of its food; such organic life again decaying, and so on perpetually.

Plants living in the sea do not probably materially affect the crust, except by their support of animal life, which, as we shall presently observe, has a noticeable effect on the sea-bottom. But terrestrial plants play a more conspicuous part: witness the formation of great bogs, which we often find (as in Ireland) to cover a very extensive space, and to sink to a considerable

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