and stones over which they flow, an unceasing transportation of matter towards the sea. Again, the moisture of the atmosphere penetrates many hard and solid rocks, and, when frost sets in, expands (as water in freezing always does) with a force sufficient to loosen little fragments, which soon fall off, and crumble into sand. Slowly, yet surely, this sand is washed down by rains, first into the nearest stream, and thence onwards to the sea.

The sea itself, too, is constantly eating up the land. Where the coast is low and sandy, this process goes on rapidly, but it is distinctly visible even on the rockiest shores. The waves, breaking against the bottom of the cliffs, gradually undermine them, so that fragments from time to time tumble down from above. These are exposed to the action of the ever-returning waters, the smaller pieces being dashed backwards and forwards till every corner is rubbed off, and they become as smooth and round as art itself could make them. The sandy matter which this constant wear produces, is deposited, like the detritus of rivers, in the bottom of the sea. There, in the course of long ages, are formed thick layers of sand, mud, stones, and other substances, which gradually consolidate into rocks. Such rocks, being formed by the action of water, are called

aqueous.

This process of disintegration, to which the surface of the earth is everywhere exposed, is slow indeed, but it is unceasing. The dry land is daily crumbling away; and were there no counteracting influence, every continent and island would at length disappear, and again, as of old, when the fountains of the great deep were broken up,

'A shoreless ocean tumble round the globe."

ARRANGEMENT OF ROCKS.

It is to the earthquake and similar agencies, the result of subterranean forces, that we are indebted for counteracting

those disintegrating processes, which tend to reduce the earth's surface to one common level. The influence of earthquakes on the general configuration of a country is prodigious, the whole slope and drainage being sometimes changed. Large tracts of ground are often elevated by these terrible phenomena above their former level, and often depressed below it. Even when there is no sudden or violent convulsion, such elevations and depressions frequently go on gradually and in silence, till what was once the bottom of the sea comes to be the mountain top. So extensive have such changes been, that there is probably no part of the earth's surface which has not been long under water, and no part of it which has not at some time or other been dry land. Thus it is that aqueous rocks are found at great distances from the sea.

It is believed that nearly all the rocks of the earth's crust have been produced by such causes as were described in last lesson, acting through cycles of immense duration, too vast for human minds to grasp. If we keep this in view, and remember also how the earthquake and kindred forces suddenly or gradually disturb levels, and alter the boundaries of land and sea, we shall the more easily comprehend the order in which the various kinds of rock are now actually found to occur.

Lowest and most ancient are igneous rocks, forming, as it were, the groundwork or basis on which the superstructure of the earth's crust is built. Of these the chief varieties have already been mentioned. The aqueous rocks are piled above them in layers or strata, originally horizontal, like courses of stupendous masonry. Hence the aqueous rocks are also called stratified, whilst the igneous, consisting of matter agglomerated without any semblance of order, are said to be unstratified.

The difference in composition and appearance between the successive strata, prove that they have been deposited at different times, and under different circumstances. The lowest are usually known as transition or metamorphic rocks, inasmuch as they correspond in character partly with the

igneous rocks immediately beneath, and partly with those incumbent upon them. They seem to have been originally formed by deposition, but afterwards altered by the action of fire. The chief varieties are gneiss, slate, and mica-schist. Next in order, as we proceed upwards, are the rocks called primary.* They include the Silurian strata, the old red sandstone, the mountain limestone, and the coal measures. Above these are the secondary rocks, comprehending the new red sandstone, oolite (a kind of limestone), and chalk. Last of all come the tertiary rocks, consisting of several later formations, over which are spread various sands, gravels, clays, peat-mosses, and other accumulations of historic times..

It does not necessarily follow that all these strata are found together in every, or even in any locality. One or more of the series may be wanting, but the natural order of those which do occur is never inverted. Thus, if we take the letters of the alphabet to represent the strata in the order of their formation, H may come immediately after A, but can never come before F or G. We may have such a sequence as A, P, T, but never A, T, P. This is a point of great importance. For example, the coal measures lie below the new, but always above the old red sandstone. If, then, in any locality, we have the latter at the surface of the ground, it is hopeless to search for coal there; whereas, if the former be at the surface, there is a chance, but no certainty, that beds of coal will be found below.

It is obvious that all the strata, when originally formed, must have been horizontal; but, instead of this, they are now found with every variety of slope, having been upheaved and disrupted by forces acting from beneath. In many cases the igneous rocks have been forced up through the superincumbent strata, and form huge hills, on whose summits they appear at the surface, while the edges of the broken strata are ranged in order along the flanks. The

*It ought to be mentioned that there is a great want of uniformity among geologists in regard to the names by which different strata are distinguished. The igneous rocks, for example, are often called primary, and those here denominated primary, are then ranked among the secondary. By others, again, a totally different set of names is employed.

actual state of the earth's crust will be best understood by a reference to the accompanying illustration. At a (fig. 5), the igneous rocks are on the surface. The earliest or lowest strata begin to appear at b, and the edges of others are passed over in succession as we proceed towards c. The same strata occur, with a still greater slope, between g and

h; but here they are covered by later formations (f, k), whose horizontal position shows that they must have been deposited after the forces, by which the surrounding rocks were upheaved, had ceased to operate. The eminences at c are formed of volcanic matter, ejected through the opening seen at e; and the dark lines between 6 and d represent fissures in the various rocks, produced by earthquakes and similar convulsions, and then filled up, from subterranean treasures, with metallic ores and other substances in a state of fusion. It is from such fissures, usually called veins, that most of the metals and their ores are obtained.

ORGANIC REMAINS.

THE most remarkable phenomena, which an examination of the earth's crust presents to us, have not yet been referred to. At an early period in the formation of stratified rocks, vegetables had begun to clothe the surface of the earth, and animals to people its waters. Of thèse, and especially of the latter, the remains of successive generations were

buried in the deposits from which rocks were formed, and are still preserved, petrified and encased in the surrounding stone, as memorials of the early inhabitants of our globe. Such remains are known by the general name of fossils.

It is only by examining the fossils which rocks contain, that we can determine accurately to what period the rocks themselves belong. Strata which were unquestionably deposited at the same epoch are often composed of different materials; and, on the other hand, strata composed of the same materials sometimes differ widely in the date of their formation. This would lead to very great uncertainty in the whole science of geology were it not that fossil remains, of which each stratum has its own peculiar collection, afford evidence of a much more exact and reliable character.

In the lower strata of the primary rocks, fossils are not plentiful, and those that do occur represent only the humbler varieties of animal life. But, as we ascend, skeletons of huge fishes begin to be met with, and these are followed by a few species of reptiles, which seem to have been the first living inhabitants of the land. At the remote epoch to which we now refer, the earth must have been covered with a luxurious and splendid vegetation. The layers of coal, which abound in the upper primary formations, consist entirely of fossilized vegetables; and, as these layers are sometimes sixty feet in thickness, it seems difficult to explain how an accumulation of trees, plants, and foliage, could ever be produced in such enormous quantity. More than two hundred species of plants have been distinguished in the British coal measures, and far greater numbers in those of other countries. Thus we see that a beneficent Creator had stored up for the use of man, when as yet the race of man was not, vast supplies of this substance, so necessary to his comfort and progress in civilization.

The fossils of the secondary rocks are extremely varied and numerous, but still mostly limited to the lower orders of the animal creation. The first certain traces of birds are found in the chalk formation. Their number, however, is