hundreds of miles in quantities sufficient to darken the air, and have fallen in quantities sufficient to make a layer of some inches at that distance, and of many feet in the immediate neighbourhood of the mountain.

Streams of molten lava, many hundred yards in breadth, many feet in depth, and many miles in length, have flowed from volcanoes into the lowlands around them, or into the sea near their foot. These streams of lava, when cooled, form masses of dark heavy rock, the upper part of which is often cindery and porous, but the inside and lower portion close-grained or compact, hard, solid stone.

5. The showers of ashes, in like manner, when they accumulate to any thickness, become sometimes compressed below, by the pressure of their own weight, into a compact stone, of greater or less hardness and solidity according to circumstances. Those parts that fall into the sea of course become saturated by its water, and gradually sink to the bottom, forming first mud and afterwards stone. Very often springs, containing the substance called carbonate of lime or perhaps some other mineral matter in solution, and which are therefore often called petrifying springs, gain access to these beds of ashes, and aid in binding them together into a firm and solid rock.

On the flanks and in the neighbourhood of volcanoes alternations of these deposits of lava and ashes take place through the lapse of ages, until they form very large masses. The thickness of such accumulations over some parts of Herculaneum, for instance, is more than 100 feet.

6. On examining any great volcanic district, we find that whole mountains, and even great chains and ranges of mountains, are either entirely or in very large part made up of these accumulations; so that we must conclude that these ranges of mountains have been gradually formed by the process of the successive eruption and outpouring, and the consequent piling up, of materials proceeding from the interior of the earth. In other words, we arrive at the conviction that there was a time when these particular mountain masses did

not exist, when their materials were buried in the interior of the earth instead of being heaped up on its surface. Compared with that time therefore all the rocks comprising these volcanic mountains are newly formed, and fresh ones are even now being occasionally added to them before our eyes.

7. When, moreover, we come to travel over the globe, we find many other mountainous regions which are not now volcanic, or at least from which no eruptions nor outbursts of fiery matter have proceeded within the periods of history or tradition, which are yet so obviously made up of lava and ashes that we have no hesitation in attributing to them a volcanic origin.

There are therefore large masses of rock and stone on various parts of the earth's surface which have been formed by the agency of Fire. We shall see presently that we shall have to attribute to the same agency, in rather a different form, still larger and more extensive

masses.

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8. AQUEOUS ACTION.-Let us now examine those rocks and stones that have been formed by the agency of Water. Water may act in two ways. It may either dissolve mineral matter, as it does salt for instance, or it may wear it away bit by bit. We all know that constant dropping wears away stones,” and we have all seen under spouts or pumps, where the dripping of water has taken place for any considerable time, hollows worn in stones placed underneath them. There are some minerals, like salt, that are easily dissolved by water, some, like carbonate of lime, that can only be dissolved slowly and partially, and when the water contains some acid: others again, like silica and alumina, on which water may be said generally to exert no dissolving power. When, then, water wears away stones, consisting wholly or chiefly of siliceous or aluminous particles, it is clear that it does so not by dissolving but by removing them. Its weight, and motion, and friction gradually loosen and tear asunder the little particles of which such stones are composed, and when loose wash them away.

This action may be called a mechanical action, be

cause the water acts like a machine, the dissolving action may, in like manner, be called a chemical action, being one of the many processes familiar to us in chemistry.

9. CHEMICAL.-Now suppose we follow these processes out to their results. The water which has dissolved mineral matter in one place, and under one set of circumstances, may of course be made by an opposite set of circumstances to redeposit it, just as salt dissolved in water may be recovered by causing the water to evaporate. Water, therefore, containing carbonate of lime in solution may deposit it ultimately as limestone. It does this continually in many caverns, on the sides of many springs, in the arches of aqueducts, and along the banks of brooks, in either pendant masses like icicles, or encrusting masses on the ground, or round sticks, stones, and other substances.

10. It is obvious that what we see thus taking place on land, may be still more abundantly occurring at the bottoms of seas and lakes, especially when the purely chemical forces are modified by the actions of life in solidifying mineral matter. All the corals and shells, and multitudes of starfishes, crabs, lobsters, and other marine animals, whose hard parts consist chiefly of carbonate of lime, must derive it from the waters of the sea. Some coral reefs, as those on the northeast coast of Australia, are a thousand miles in length, with an average breadth of fifteen or twenty miles, and a maximum thickness at their outer part of probably, at least, two thousand feet. Every part of this huge mass consists of carbonate of lime, separated particle by particle from the waters of the sea, by the processes of life going on within the bodies of marine animals. All the adjacent seas for hundreds of miles, wherever any particles have been brought up by the soundinglead from the bottom, are found to be strewed with fine calcareous mud, derived from the waste of these reefs.

We know, then, that in many parts of the globe a purely chemical precipitation of carbonate of lime is taking place, producing varieties of limestone, and we know that at the bottoms of our present seas this precipitation is aided by means of the organic chemistry

of animal life in producing vast deposits of the same substance.

11. Now, all rocks and stones thus formed from mineral matter that has been held in solution by water may be called chemically-formed rocks. These may be said to consist of:

1st. All limestones, or rocks composed of carbonate of lime, including stalactite, stalagmite, tufa, travertine, chalk, oolite, common limestone, statuary marble, and most other marbles. When limestone contains a considerable proportion of carbonate of magnesia, mingled with the carbonate of lime, it is called a magnesian limestone, or sometimes a dolomite.

2nd. Rocks composed of sulphate of lime, commonly called gypsum, and locally alabaster or plaster-stone. This substance occurs either compact, when it looks like a white, soft, earthy limestone; or crystalline, when it is often fibrous, pure transparent crystals of it being called selenite.

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3rd. Rocks composed of chloride of sodium, known as rock-salt;" these occur generally as thick beds associated with clay or marl, the salt being in a rudely crystalline and semi-transparent form, often stained to the colour of the marls with which it is associated.

All these "chemically-formed rocks" have one character in common, namely, that they have frequently a crystalline structure, being made up of an assemblage of crystals of their respective minerals, all interlocked one with the other. This crystalline structure is always a proof of the rock having been once fluid, either by its having been dissolved in water or melted by heat.

LESSON II.

MECHANICAL ACTION OF WATER.

"The waters wear the stones."

1. THAT the mechanical action of water is no less prevalent over the globe, may be 'shown at once by the abundance of mud, sand, gravel, shingle, or boulders which are found in all rivers, and along all sea-shores. It would be impossible for any one to live for many

years in the neighbourhood of any exposed sea-cliff, however hard the rock of which it is composed, without observing the occasional fall of some mass of rock which had been undermined by the breakers of the sea. Along some coasts, as, for instance, along all the eastern coast of England, this action becomes so strong and rapid that whole fields are carried away within the lives of one generation, and the sites of towns and villages, whose names are mentioned in history, are in some instances as much as three or four miles out at sea. All this waste of land has taken place in consequence of the wearing and tearing action of the breakers, and the carrying off by strong currents of the broken and pounded fragments in the shape of small pebbles, sand, or mud, according to the nature of the material. Any one who will watch the action of breakers at high water, especially during a storm, at the foot of a cliff at which are shingle and round pebbles, will observe how the sea washes and dashes these to and fro, pounding and grinding them together, and often hurling them against the cliffs and rocks, using them like geological hammers to knock corners off them.

2. If we were to trace a large river from its source to its mouth, we should see among the mountain torrents whence it takes its rise, many evidences of its action in the shape of blocks that had fallen from the sides of its ravines, and cuts made, even in hard rocks, by its waters. This wearing action would be especially obvious about cascades and waterfalls, which can often be proved to have cut back their own beds, even miles from the place where they once were. The eating or erosive action of a few pebbles, or even a little sand aided by the moving of water, can often be observed to have acted on a small scale in the beds of rivers or on the sea-shore. Circular holes, like wells, sometimes as much as a yard or two in diameter and three or four yards in depth, may be seen in the hardest rocks, with just a few little pebbles or a handful of sand at the bottom. These holes were formed, by the action of a small whirlpool setting the fragments into circular motion, and thus wearing a hollow in the