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term "mechanically formed rocks: " they have been produced by the simple mechanism of moving water transporting earthy matter from one position to another.

There is, however, another set of rocks, which we may with equal reason call "chemically formed rocks." Of these limestone is the principal and most abundant, but there are some others, such as gypsum and rock-salt.

Almost every one must have heard of "petrifying wells and springs,"-places where birds'-nests, twigs, sticks, and other articles, being placed in the water, become in the course of a few weeks incrusted over with stone, which, when tested, is found to be limestone, or what the chemists call carbonate of lime. The waters having this property are often quite pure-looking and transparent, holding this mineral substance in perfect solution, as if it were melted salt or sugar,-not in mechanical suspension, or in the state of mud or sand. The chemist, indeed, tells us that perfectly pure water, that is, water having no mineral substances whatever dissolved in it,-is a very great rarity on the face of the earth, much rarer than are now-a-days the black swans the poet made such a fuss about.

We can easily understand that water, which under one set of circumstances can dissolve mineral substances, will under another set of circumstances precipitate them or deposit them at its bottom. Without attempting to puzzle ourselves with any of the recondite mysteries of chemistry, we may recollect that lime is nothing more than an oxide of a metal called calcium, or might be called the rust of calcium, just as ironrust is an oxide of iron. Salt, again, is made of the union of another metal, sodium with another simple substance called chlorine, and is therefore a chloride of sodium.

Lime, again, combines with carbonic acid to make carbonate of lime, or calc spar, or pure chalk and limestone, and with sulphuric acid to make sulphate of lime or gypsum.

Now, simple carbonate of lime is only sparingly soluble in water; but if to the lime a double dose of carbonic acid be added, or it become what chemists call a bicarbonate, it is altogether soluble in water, bicarbonate of lime never having been known to exist in a solid state.

If, therefore, the waters of a brook or spring contain any quantity of carbonic acid mingled with them, which almost all natural water does, and they come in contact with any lime or limestone in their passage, they will dissolve a portion of it and hold it in solution, till, either from the evaporation of the water or the abstraction of some of the carbonic acid from some cause

or other, the carbonate of lime becomes solid again, and incrusts whatever happens to be nearest at hand. Hence, the phenomena of petrifying springs, and of the stalactites and stalagmites so often found in limestone caverns.

Now, as in all these cases we see solid calcareous rock deposited from water under our very eyes, and can trace the method of its formation, we are naturally the more disposed to believe that the waters of our present seas and lakes may contain in solution the elements of similar rocks, and that the calcareous rocks which enter so largely into the composition of all existing lands may have been deposited from those waters at the time when those portions of the earth's surface were covered by them.

If, however, our belief in the aqueous origin of arenaceous and argillaceous rocks was strengthened and confirmed by finding them so full of the remains of animals or plants that had inhabited the water, or whose remains had been swept into it, much more will this be the case as regards the calcareous rocks. All calcareous rocks, whether they be chalk, limestone, or marble, abound in fossils, being for the most part full of shells, corals, starfishes, sea-urchins, crustacea (such things as crabs and lobsters), teeth and bones and scales of fish, bones of seareptiles, &c. In some instances, whole mountain masses are entirely made up of the fragments of sea-creatures, and sometimes of only one kind of sea-creature, such are the crinoidal * marbles, often used for chimney-pieces.

It would be impossible for any one attentively to examine any limestone country, and see the way in which the shells and other fossils, often of the most delicate, fragile, and beautiful structure, lie in bed over bed, and layer over layer, with the utmost order and regularity, over large areas, without being convinced that the whole mass of rock had been gradually accumulated at the bottom of the sea. J. Beete Jukes.

SUBSTANCE OF ROCK: SEDIMENTARY AND VOLCANIC. IN approaching any large mountain range, the ground over which the spectator passes, if he examine it with any intelli

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*So called because they are almost entirely composed of the fragments of the stems of crinoidal animals, the sea lilies, animals resembling a starfish, growing on a stem, with the fingers split up into a multitude of jointed filaments.

gence, will almost always arrange itself in his mind under three great heads. There will be, first, the ground of the plains or valleys he is about to quit, composed of sand, clay, gravel, rolled stones, and variously mingled soils; which, if he has any opportunity, -at the banks of a stream, or the sides of a railway cutting, to examine to any depth, he will find arranged in beds exactly resembling those of modern sand-banks or sea-beaches, and appearing to have been formed under such natural laws as are in operation daily around us... At the outskirts of the hill district he may, perhaps, find considerable eminences formed of these beds of loose gravel and sand; but, as he enters into it farther, he will soon discover the hills to be composed of some harder substance, properly deserving the name of rock, sustaining itself in picturesque forms, and appearing, at first, to owe both its hardness and its outlines to the action of laws such as do not hold at the present day ... He can easily explain the nature, and account for the distribution of the banks which overhang the lowland road, or of the dark earthy deposits which enrich the lowland pasture; but he cannot so distinctly imagine how the limestone hills of Derbyshire and Yorkshire were hardened into their stubborn whiteness, or raised into their cavernous cliffs... Still, if he carefully examines the substance of these more noble rocks, he will, in nine cases out of ten, discover them to be composed of fine calcareous dust, or closely united particles of sand; and he will be ready to accept as possible, or even probable, the suggestion of their having been formed by slow deposit at the bottom of deep lakes and ancient seas, under such laws of Nature as are still in operation.

But as he advances yet farther into the hill district, he finds the rocks around him assuming a gloomier and more majestic condition. Their tint darkens; their outlines become wild and irregular; and whereas before they had only appeared at the roadside in narrow ledges among the turf, or glancing out from among the thickets above the brooks in white walls and fantastic towers, they now rear themselves up in solemn and shattered masses far and near; softened, indeed, with strange harmony of clouded colors, but possessing the whole scene with their iron spirit; and rising, in all probability, into eminences as much prouder in actual elevation than those of the intermediate rocks, as more powerful in their influence over every minor feature of the landscape.

And when the traveller proceeds to observe closely the materials of which these nobler ranges are composed, he finds

also a complete change in their internal structure. They are no longer formed of delicate sand or dust, each particle of that dust the same as every other, and the whole mass depending for its hardness merely on their closely cemented unity; but they are now formed of several distinct substances, visibly unlike each other; and not pressed, but crystallised into one mass,crystallised into a unity far more perfect than that of the dusty limestone, but yet without the least mingling of their several natures with each other... Such a rock, freshly broken, has a spotty, granulated, and, in almost all instances, sparkling appearance; it requires a much harder blow to break it than the limestone or sandstone; but, when once thoroughly shattered, it is easy to separate from each other the various substances of which it is composed, and to examine them in their individual grains or crystals; of which each variety will be found to have a different degree of hardness, a different shade of color, and a different character of form.

But this examination will not enable the observer to comprehend the method either of their formation or aggregation, at least by any process such as he now sees taking place around him ; he will at once be driven to admit that some strange and powerful operation has taken place upon these rocks, different from any of which he is at present cognisant; and farther inquiry will probably induce him to admit, as more than probable, the supposition that their structure is in a great part owing to the action of enormous heat prolonged for indefinite periods. Ruskin.

VOLCANIC ROCK.

WE have glanced at that important class of rocks which, having been deposited in regular layers or strata under water, are therefore termed sedimentary or aqueous rocks; let us now turn to the other leading type of rocks, those which have cooled down and consolidated from a state of fusion, and are, therefore, termed igneous rocks.

Lava, as the immediate product of the volcano, is the best known of these rocks. It has many varieties, called trachyte, dolerite, basalt, &c. There are also other igneous rocks, which, having not been poured out at the surface, but cooled and consolidated at greater or less depths in the interior, have become more crystalline than lava usually is, and differ from it in texture, and to a certain extent in composition. Granite, sye

nite, green-stone, felstone, are the best known of these rocks, and their porphyritic varieties which are sometimes spoken of simply as porphyry.

A volcano is generally a more or less perfectly conical mountain, composed very largely of pumice, ashes, and cinders, with streams of rough cooled lava running down its sides, and spreading here and there about its base... There is commonly at its summit a basin-shaped hollow, or crater, which, during periods of eruption, is open either entirely or partially; and from this orifice are belched forth showers of red-hot stones, ashes, steam, gases of various kinds sometimes inflammable. Melted rock or lava is sometimes forced up over the lip of the crater, and rushes like a torrent down the sides of the cone, or sometimes breaks out lower down.

The effects of single eruptions, such as the fall of ashes and powder in sufficient quantities to darken the sky, have been known to extend even hundreds of miles from the orifice whence they were blown. Great streams of lava some miles in width, and fifty or a hundred feet in depth, have been known to run for twenty or thirty miles in length, filling up the hollows of lakes and the valleys of rivers, and totally obliterating the old features of large tracts of country... The sides of great volcanic mountains have been frequently rent by long fissures, which have been filled with molten rock; and these, when cooled and consolidated, act the part of great ribs and bars, supporting the framework of the structure. These stand out of the sides on the cliffs or ravines, or on the worn flanks of the mountain, like walls, projecting by reason of their superior hardness, having resisted the erosive action which formed the cliff or ravine, in a greater degree than the softer materials about them.

When the volcano has grown to a considerable magnitude by the successive additions of materials ejected from one focus, it often happens that in some following eruption it breaks out on the side or near the foot of the mountain; a new cone is then formed on a smaller scale, which is itself ultimately covered up and buried by accumulations derived from the old or from still newer vents... If therefore we could take any large volcanic mountain, such as Etna or Teneriffe, and dissect it, so as to observe the successive steps of its formation from the commencement, we should find it possessing a most curiously complicated structure. Its roots would probably consist of hard compact or crystalline igneous rocks, interstratified with aqueous ones; its upper part, of coat upon coat of ejected matters deposited first upon one side, then upon another; the great cone

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