submerged and acted on by heat and pressure for ages, would become coal, in every particular the same as that we daily use for fuel.

Thus, organic life of all kinds is everywhere busy in forming rockmasses, identical in character and appearance with those presented to our investigation in limestone, chalk, and coal.

Agencies in the Alteration of Rocks.

Rocks are changed from their original position, form, and structure by two classes of agents-those that disturb, and those that wear or disintegrate.

Disturbing Agents.-Stratified rocks in their natural state would be horizontal, or only slightly inclined. How, then, are we to account for the tiltings, upheavals, faults, and various dislocations so prevalent among the strata? The igneous forces, just spoken of, furnish the solution. The whole globe is subject to convulsive movements from the motion of the interior molten matter of the earth, which are seen in earthquakes, and by which the ground is torn into fissures, and the solid crust made to move in mighty undulations, that destroy and swallow great cities. Extensive tracts are also sometimes suddenly raised or depressed. Sometimes, too, great yawning craters open where previously volcanic movement was unknown, and continue for a time in active eruption. In these upheavals and subsidences, sudden or gradual, of extensive tracts, we see the causes at work of the dislocations of the rocks of former times, and of the elevations and depressions that occurred throughout the geologic eras.

Again, we know that in order to the deposition of strata of any thickness, the sea-bottom must have gradually subsided: does any such gradual subsidence take place at the present time? It is ascertained, from extended observations, that on the northern shores of the Baltic, for instance, there has been a gradual rise at the rate of 4 feet in a century, and in South America a rise of 85 feet during the human period, and at Valparaiso of 19 feet in 220 years; while over all the world, and even round our own coasts, ancient sea-beaches may be seen at various elevations, marking former sea-levels. On the other hand, the south coast of Sweden, the coast of Greenland over 600 miles, and parts of South America for the last 300 years, have been slowly sinking; nor are the British shores free from such oscillations.

Thus, again, we see that existing causes perfectly explain the gradual subsidences necessary to the formation of the rocks, and to their subsequent elevation into dry land.

Disintegrating Agents.-Every stratified rock in the immense thick

ness of the crust of the globe has been formed of the débris of pre-existing formations, that have been ground down and held in solution till deposited in the layers afterwards hardened into rock. Whence, then, this immense accumulation of matter, and what the disintegrating agents?

1. Atmospheric Agency.—The atmosphere, by its chemical action, and by the combined effects of alternate heat and cold, wetness and dryness, is continually crumbling down all exposed surfaces, forming new soil, and thus increasing the earthy covering of the globe. The wind, also, has an incredible power of drifting and heaping up sandhills along the shore— as in the county of Elgin, where an ancient barony has been entirely reduced to a desert through this means-and in raising the waves of the sea, and wearing the rocks through the mighty force of its swooping billows. Frost, too, is one of the quietest but most powerful disintegrating agents; for when water has percolated a mass of rock, the act of freezing exerts a great expansive force which cracks the rock. frost can work on a grander scale, for to its agency is due the existence of avalanches, glaciers, and icebergs; which, whether sweeping with overwhelming convulsion, or crawling down the mountain side, or floating and grating on the ocean floor, continually and with terrible effect, wear down or dash to pieces every rock that obstructs their irresistible course.

But

2. Aqueous Agency.-The most extensive aqueous agent is rain, which wears, softens, percolates, and gradually wastes away every rock on which it falls. The rain-water also gathers under the ground in large cavities, where springs are formed, which dissolve the interior rocks, and, bursting out, deposit their solutions of lime, iron, sulphur, soda, flint, and bitumen. One of the most powerful degrading agents is, of course, the sea; which, as it beats on its rocky shores, wears, rolls, and grinds to powdery sand the flintiest rocks, and presents as monuments of its mighty power of waste those lofty cliffs that guard its shores. But more powerful, but less obvious agents of destruction than the sea, are the many streams that everywhere traverse the land on their way to this boundless reservoir. The power of rivers in excavating and wearing away the surface of the globe is much greater than at first thought might be supposed. Every valley, however deep, has been mainly worn down by river-action, extending over immense periods of time. When we contemplate the mighty valleys enclosed by towering peaks capped with eternal snows, that lie hid amidst the mountain solitudes of the Alps, the Andes, or the Himalayas, we may well be astonished at such a statement. But that these huge excavations have been mainly produced by the combined action of air, frost, rain, and river, has been demonstrated beyond a doubt by a vast accumulation of facts and reasonings on phenomena in all parts

J

of the globe. Hence, valleys thus excavated are termed valleys of erosion,1 from being ground out by the powerful action of these mighty agents. This being proved to be the case even during the human period, we are at no loss to account for the great denudation everywhere seen; and for the immense accumulations of sedimentary matter that form so much of the solid crust of our globe.

Transporting Agents.-We have finally to account for the deposition of strata in one part of the country, the materials for which have been obtained at great distances; and for the transport of immense boulders hundreds of miles from their original seats, as exhibited in all parts of the globe.

1. Aqueous Agency.-The most obvious agents of transport are rivers, that bear down from every part of their courses the débris deposited at their mouths. Their power of carrying masses of the heaviest materials is immense, as may be seen after a flood in the smallest streams in our neighbourhood. Waves have also a wonderful power of removing and carrying to a distance the blocks on which they daily dash. But the currents that flow through the ocean, which are but mighty ocean rivers, have the greatest influence in this respect. By their means, materials of all kinds, organic and inorganic, are conveyed to incredible distances. The Gulf-stream, for instance, conveys substances from the South African coasts to those of Norway and the far North.

2. Ice Agency.-But the transporting influence of these currents in bearing rock-masses is greatest when icebergs are carried on their surface. These huge frost-mountains have embedded in their mass the largest blocks, which are gradually dropped over wide areas as the ice slowly melts away. The size and number of some of these transported rocks are often almost incredible. Every country exhibits such travelled rocks, which are called erratic boulders; and our own little island presents no mean examples of such ice-borne masses.

But ice also acts as a transporter in the form of glaciers—those great ice-rivers that fill the upland valleys of the Alps, Himalayas, and such mountain systems. In front of every glacier, along its sides, and on its surface, are great collections of rocky fragments of every size, borne down by the ice-stream, and left as evidences of its existence when the glacier has melted away. Such collections of rocks are called moraines, from their mural or wall-like aspect as seen running across a valley. The distance to which such blocks are borne is astonishing, and depends on the size of the glacier. Evidences of extinct glaciers are seen in most countries, and we may trace their remains in our own island, where now not a particle of glacier ice can exist.

1 From Latin e, out, away, and rodo, rosum, to gnaw.

3. Igneous Agency. It is evident that volcanoes have a great power in throwing out masses of different materials to great distances, and of carrying many substances on their mighty lava-streams; and evidences of their power in this respect in geologic times are everywhere apparent. Sometimes the fine ashes that issue from the crater are borne by the wind to great distances, often fifty or a hundred miles, where they are deposited as a layer of finest dust; and this may account for the existence of trap-tuffs in places where no volcanic eruption seems to have taken place.

It thus appears that the agencies now at work on our own globe are adequately sufficient to account for all the phenomena of the forming, disturbing, disintegrating, elevating, depressing, and transporting of the rock-masses that form the crust of the earth. Such being abundantly proved, and the laws of nature being uniform and unchangeable, we are not only warranted, but compelled, to infer that the same influences were at work in these bygone ages, and were the joint causes of the formation of our rock-systems as they are now presented to our eyes and subjected to our investigation.

Examples of Geological Reasoning.

We shall now give some examples of the method of reasoning on the various phenomena presented in the rocks, the formation and history of which it is the province of Geology to unfold.

Reasoning regarding Rock Sections.

1. Section with Igneous Action and Dislocation.-From the section (fig. 81) let us see what we can discover, by the mutual relations of the rocks,

regarding the times of their formation and dislocation. Which rock was first deposited in this section? The lower rocks were of course formed

first, and the various strata were formed in the order of their superposition. You will observe that the strata marked from A to H are inclined, and, as these must have been deposited horizontally, they must therefore have been tilted up afterwards. We also see that the cause of their disturbance is the upheaval of the igneous mass I, which, issuing from beneath, has ruptured the strata previously flat, and overflowed part of the rocks where it rose to the surface. This upheaval has also thrown the strata at different angles, those to the right being more inclined than those to the left, and therefore forming a fault. We observe also that in the rocks to the left there occurs a great slip, for part stands at a different height from the rest. We can, however, easily trace each rock, as, for instance, the coal-seam C, from the one side of the section to the other through both slip and fault.

We also observe that after the trap eruption, the exposed rocks were subjected to long-continued water-action; for their outcrop is hollowed out into two valleys. After this period of denudation, the horizontal strata were deposited, and have not since been disturbed by volcanic action. We see also that a great part of the trap is covered by this new deposit, so that what was once a hill, before this new rock was laid down, is now almost hidden beneath the surface. The horizontal undisturbed rocks have, however, been much worn away by water-action ; for deep valleys have been excavated in them, and the bed of a river still continuing the scooping out is seen at O. By looking at the highest and therefore last-formed deposit, we observe that it consists of gravel or silt. This, therefore, once formed the bottom of a lake or inland sea, where river-débris was deposited. This débris has been allowed to lie undisturbed till elevated above the sea-level, after which it has been worn down by streams. Patches of it still remain on the hill-tops over various parts of the surface, as at the points marked N.

2. Section with Aqueous Action alone, without Dislocation.-This second section (fig. 82) is of a part of the north coast of Norfolk, near the town of Cromer, and it is very striking and instructive. Let us see what we can learn from it regarding the history of the Norfolk shores. This section differs from the first in the entire absence of igneous action, for there is no displacement of any kind, its regularity being due to aqueous causes alone.

The lowest rock (A) is chalk, in which we can see represented regular rows of flints. This chalk consists of the remains of shell-fish and other creatures, whose organisms are found in the flints. After the deposition of the chalk, the land was raised, and a great forest grew over the chalky soil, which is well suited for the growth of trees. The roots of the old oaks, with part of their stems, are still seen erect as they grew !-embedded in the mud that accumulated round them when afterwards submerged.