Among the princ.i.p.al minerals of the saccharoid limestone we may mention graphite, quartz, some very varied silicates, such as andalusite, disthene, serpentine, talc, garnet, augite, hornblende, epidote, chlorite, the micas, the felspars; finally, spinel, corundum, phosphate of lime, oxide of iron and oligiste, iron pyrites, &c. Besides these, various minerals in veins figure among those which exist more commonly in the saccharoid limestone.

When metamorphic limestone is sufficiently pure, it is employed as statuary marble. Such is the geological origin of Carrara marble, which is quarried in the Apuan Alps on a great scale; such, also, was the marble of Paros and Antiparos, still so celebrated for its purity. On examination, however, with the lens the Carrara marble exhibits blackish veins and spangles of graphite; the finest blocks, also, frequently contain nodules of ironstone, which are lined with perfectly limpid crystals of quartz. These accidental defects are very annoying to the sculptor, for they are very minute, and nothing on the exterior of the block betrays their existence. In the marble of Paros, even when it is strongly translucent, specks of mica are often found. In the ancient quarries the nodules are so numerous as to have hindered their being worked, up even to the present time.

When the mica which occurs in granular limestone takes a green colour and forms veins, it const.i.tutes the Cipoline marble, which is found in Corsica, and in the Val G.o.demar in the Alps. Some white marbles are quarried in France, chiefly at Loubie, at Sost, at Saint-Beat in the Pyrenees, and at Chippal in the Vosges. In our country, and especially in Ireland, there are numerous quarries of marble, veined and coloured of every hue, but none of a purity suitable for the finest statuary purposes. All these marbles are only metamorphosed limestones.

The white marbles employed almost all over the world are those of Carrara. They result from the metamorphism of limestone of the Lias.

They have not been penetrated by the eruptive rocks, but they have been subjected upon a great scale to a general metamorphism, to which their crystalline structure may be attributed.

It is easily understood that the calcareous strata have not undergone such an energetic metamorphism without the beds of sandstone and clay, a.s.sociated with them, having also undergone some modification of the same kind. The siliceous beds accompanying the saccharoid limestone have, in short, a character of their own. They are formed of small grains of transparent quartz more or less cemented one to the other in a manner strongly resembling those of the saccharoid limestone. Between these grains are usually developed some lamellae of mica of brilliant and silky l.u.s.tre, of which the colour is white, red, or green; in a word, it has produced a _quartzite_. Some veins of quartz frequently traverse this quartzite in all directions. Independent of the mica, it may contain, besides, the different minerals already mentioned as occurring in the limestone, and particularly silicates--such as disthene, andalusite, staurotide, garnet, and hornblende.

The argillaceous beds present a series of metamorphisms a.n.a.logous to the preceding. We can follow them readily through all their gradations when we direct our attention towards such granitic ma.s.ses as those which const.i.tute the Alps, Pyrenees, the Bretagne Mountains, or our own Grampians. The schists may perhaps be considered the first step towards the metamorphism of certain argillaceous rocks; in fact, the schists are not susceptible of mixing with water like clay; they become stony, and acquire a much greater density, but their chief characteristic is a foliated structure.

Experiment proves that when we subject a substance to a great pressure a foliated structure is produced in a direction perpendicular to that in which the pressure is exercised. Everything leads us, therefore, to believe that pressure is the princ.i.p.al cause of the schistous texture, and of the foliation of clay-slates, the most characteristic variety of which is the roofing-slate which is quarried so extensively in North Wales, in c.u.mberland, and various parts of Scotland in the British Islands; in the Ardennes; and in the neighbourhood of Angers, in France.

In some localities the slate becomes siliceous and is charged with crystals of felspar. Nevertheless, it still presents itself in parallel beds, and contains the same fossil remains still in a recognisable state. For example, in the neighbourhood of Thann, in the Vosges, certain vegetable imprints are perfectly preserved in the metamorphic schist, and in their midst are developed some crystals of felspar.

Mica-schist, which is formed of layers of quartz and mica, is found habitually a.s.sociated with rocks which have taken a crystalline structure, proceeding evidently from an energetic metamorphism of beds originally argillaceous. Chiastolite, disthene, staurotide, hornblende, and other minerals are found in it. Mica-schists occur extensively in Brittany, in the Vosges, in the Pyrenees. In all cases, as we approach the ma.s.ses of granite, in these regions, the crystalline structure becomes more and more marked.

In describing the various facts relating to the metamorphism of rocks, we have said little of the causes which have produced it. The causes are, indeed, in the region of hypothesis, and somewhat mysterious.

In what concerns special metamorphism, the cause is supposed to admit of easy explanation--it is heat. When a rock is ejected from the interior of the earth in a state of igneous fusion, we comprehend readily enough that the strata, which it traverses, should sustain alterations due to the influence of heat, and varying with its intensity. This is clear enough in the case of _lava_. On the other hand, as water always exists in the interior of the earth's crust, and as this water must be at a very high temperature in the neighbourhood of volcanic fires, it contributes, no doubt, largely to the metamorphism. If the rocks have not been ejected in a state of fusion, it is evidently water, with the different mineral substances it holds in solution, which is the chief actor in the special metamorphism which is produced.

In general metamorphism, water appears still to be the princ.i.p.al agent.

As it is infiltered through the various beds it will modify their composition, either by dissolving certain substances, or by introducing into the metalliferous deposits certain new substances, such as may be seen forming, even under our eyes, in mineral springs. This has tended to render the sedimentary deposits plastic, and has permitted the development of that crystalline structure, which is one of the princ.i.p.al characteristics of metamorphic rocks. This action has been seconded by other causes, notably by heat and pressure, which would have an immense increase of power and energy when metamorphism takes place at a great depth beneath the surface. Dr. Holl, in an able paper descriptive of the geology of the Malvern Hills, read before the Geological Society in February, 1865, adopts this hypothesis as explanatory of the vast phenomena which are there displayed. After describing the position of this interesting and strangely-mingled range of rocks, he adds: "These metamorphic rocks are for the most part highly inclined, and often in a position nearly vertical. Their disturbance and metamorphism, their being traversed by granitic veins, and still later their invasion by trap-d.y.k.es and their subsequent elevation above the sea-level, were all events which must have occupied no inconsiderable period, even of geological time. I presume," he adds, "that it will not be maintained in the present day that the metamorphism of rocks over areas of any but very moderate extent is due to the intrusion of veins and erupted ma.s.ses. The insufficiency of such agency becomes the more obvious when we consider the slight effects produced by even tolerably extensive outbursts, such as the Dartmoor granite; while in the case of the Malverns there is an absence of any local cause whatever. The more probable explanation in the case of these larger areas is, that they were faulted down, or otherwise depressed, so as to be brought within the influence of the earth's internal heat, and this is the more likely as they belong to an epoch when the crust is believed to have been thinner." When it is considered that, according to the doctrine of modern geology, the Laurentian rocks, or their equivalents, lie at the base of all the sedimentary deposits; that this, like other systems of stratified rocks, was deposited in the form of sand, mud, and clay, to the thickness of 30,000 feet; and that over an area embracing Scandinavia, the Hebrides, great part of Scotland, and England as far south as the Malverns, besides a large proportion of the American continent, with certain forms of animal life, as recent investigations demonstrate--can the mind of man realise any other cause by which this vast extent of metamorphism could have been produced?

Electric and galvanic currents, circulating in the stratified crust, are not to be overlooked. The experiments of Mr. R. W. Fox and Mr. Robert Hunt suggest that, in pa.s.sing long-continued galvanic currents through ma.s.ses of moistened clay, there is a tendency to produce cleavage and a semi-crystalline arrangement of the particles of matter.[31]

[31] Report of the Royal Cornwall Polytechnic Society for 1837. Robert Hunt, in "Memoirs of the Geological Survey of Great Britain,"

vol. i., p. 433.

THE BEGINNING.

The theory which has been developed, and which considers the earth as an extinct sun, as a star cooled down from its original heated condition, as a nebula, or luminous cloud, which has pa.s.sed from the gaseous to the solid state--this fine conception, which unites so brilliantly the kindred sciences of astronomy and geology, belongs to the French mathematician, Laplace, the immortal author of the "Mecanique Celeste."

The hypothesis of Laplace a.s.signs to the sun, and to all bodies which gravitate in what Descartes calls his _tourbillon_, a common origin. "In the primitive state in which we must suppose the sun to be," he says, "it resembles one of those nebulae which the telescope reveals to us, consisting of a more or less brilliant central _nucleus_, surrounded by luminous clouds, which clouds, condensing at the surface, become transformed into a star."

It has been calculated that the centre of the earth has a temperature of about 195,000 Cent., a degree of heat which surpa.s.ses all that the imagination can conceive. We can have no difficulty in admitting that, at a heat so excessive, all the substances which now enter into the composition of the globe would be reduced to the state of gas or vapour.

Our planet, then, must have been originally an aggregation of aeriform fluids--a ma.s.s of matter entirely gaseous; and if we reflect that substances in their gaseous state occupy a volume eighteen hundred times larger than when solid, we shall have some conception of the enormous volume of this gaseous ma.s.s. It would be as large as the sun, which is fourteen hundred thousand times larger than the terrestrial sphere. In Fig. 12 we have attempted to give an idea of the vast difference of volume between the earth in its present solid state and in its primitive gaseous condition. One of the globes, A, represents the former, B the latter. It is simply a comparison of size, which is made the more strikingly apparent by means of these geometrical figures--one the twentieth part of an inch in diameter, the other two inches and three quarters.

[Ill.u.s.tration: VI.--The Earth circulating in s.p.a.ce in the state of a gaseous star.]

[Ill.u.s.tration: Fig. 12.--Comparative volume of the earth in the gaseous and solid state.]

At this excessive temperature the gaseous ma.s.s, which we have described, would shine in s.p.a.ce as the sun does at the present day; and with the same brilliancy as that with which, to our eyes, the fixed stars and planets shine in the serenity of night, as represented on the opposite page (PLATE VI.). Circulating round the sun in obedience to the laws of universal gravitation, this incandescent gaseous ma.s.s was necessarily regulated by the laws which govern other material substances. As it got cooler it gradually transferred part of its warmth to the glacial regions of the inter-planetary s.p.a.ces, in the midst of which it traced the line of its flaming orbit. Consequent on its continual cooling (but at the end of a period of time of which it would be impossible, even approximately, to fix the duration), the star, originally gaseous, would attain a liquid state. It would then be considerably diminished in volume.

The laws of mechanics teach us that liquid bodies, when in a state of rotation, a.s.sume a spherical form; it is one of the laws of their being, emanating from the Creator, and is due to the force of attraction. Thus the Earth takes the spheroidal form, belonging to it, in common with the greater number of the celestial bodies.

The Earth is subject to two distinct movements; namely, a movement of translation round the sun, and a movement of rotation on its own axis--the latter a uniform movement, which produces the regular alternations of days and nights. Mechanics have also established the fact, which is confirmed by experiment, that a fluid ma.s.s in motion produces (as the result of the variation of the centrifugal force on its different diameters), a swelling towards the equatorial diameter of the sphere, and a flattening at the poles or extremities of its axis. It is in consequence of this law, that the Earth, when it was in a liquid state, became swollen at the equator, and depressed at its two poles; and that it has pa.s.sed from its primitive spherical form to the spheroidal--that is, has become flattened at each of its polar extremities, and has a.s.sumed its present shape of an oblate spheroid.

This bulging at the equator and flattening towards the poles afford the most direct proofs, that can be adduced, of the original liquid state of our planet. A solid and non-elastic sphere--a stone ball, for example--might turn for ages upon its axis, and its form would sustain no change; but a fluid ball, or one of a pasty consistence, would swell out towards the middle, and, in the same proportion, become flattened at the extremities of its axis. It was upon this principle, namely, by admitting the primitive fluidity of the globe, that Newton announced _a priori_ the bulging of the globe at the equator and its flattening at the poles; and he even calculated the amount of this depression. The actual measurement, both of this expansion and flattening, by Maupertuis, Clairaut, Camus, and Lemonnier, in 1736, proved how exact the calculations of the great geometrican were. Those gentlemen, together with the Abbe Outhier, were sent into Lapland by the Academy of Sciences; the Swedish astronomer, Celsius, accompanied them, and furnished them with the best instruments for measuring and surveying. At the same time the Academy sent Bouguer and Condamine to the equatorial regions of South America. The measurements taken in both these regions established the existence of the equatorial expansion and the polar depression, as Newton had estimated it to be in his calculations.

It does not follow, as a consequence of the partial cooling down of the terrestrial ma.s.s, that all the gaseous substances composing it should pa.s.s into a liquid state; some of these might remain in the state of gas or vapour, and form round the terrestrial spheroid an outer envelope or _atmosphere_ (from the Greek words at??, _vapour_, and sfa??a, _sphere_). But we should form a very inexact idea of the atmosphere which surrounded the globe, at this remote period, if we compared it with that which surrounds it now. The extent of the gaseous matter which enveloped the primitive earth must have been immense; it doubtless extended to the moon. It included, in short, in the state of vapour, the enormous body of water which, as such, now const.i.tutes our existing seas, added to all the other substances which preserve their gaseous state at the temperature then exhibited by the incandescent earth; and it is certainly no exaggeration to place this temperature at 2,000 Centigrade. The atmosphere would partic.i.p.ate in this temperature; and acted on by such excessive heat, the pressure that it would exert on the Earth would be infinitely greater than that which it exercises at the present time. To the gases which form the component parts of the present atmospheric air--namely, nitrogen, oxygen, and carbonic acid--to enormous ma.s.ses of watery vapour, must be added vast quant.i.ties of mineral substances, metallic or earthy, reduced to a gaseous state, and maintained in that state by the temperature of this gigantic furnace.

The metals, the chlorides--metallic, alkaline, and earthy--sulphur, the sulphides, and even the silicates of alumina and lime; all, at this temperature, would exist in a vaporous form in the atmosphere surrounding the primitive globe.

It is to be inferred that, under these circ.u.mstances, the different substances composing this atmosphere would be ranged round the globe in the order of their respective densities; the first layer--that nearest to the surface of the globe--being formed of the heavier vapours, such as those of the metals, of iron, platinum, and copper, mixed doubtless with clouds of fine metallic dust produced by the partial condensation of their vapours. This first and heaviest zone, and the thickest also, would be quite opaque, although the surface of the earth was still at a red heat. Above it would come the more vaporisable substances, such as the metallic and alkaline chlorides, particularly the chloride of sodium or common salt, sulphur and phosphorus, with all the volatile combinations of these substances. The upper zone would contain matter still more easily converted into vapour, such as water (steam), together with others naturally gaseous, as oxygen, nitrogen, and carbonic acid.

This order of superposition, however, would not always be preserved. In spite of their differences of density, these three atmospheric layers would often become mixed, producing formidable storms and violent ebullitions; frequently throwing down, rending, upheaving, and confounding these incandescent zones.

As to the globe itself, without being so much agitated as its hot and shifting atmosphere, it would be no less subject to perpetual tempests, occasioned by the thousand chemical actions which took place in its molten ma.s.s. On the other hand, the electricity resulting from these powerful chemical actions, operating on such a vast scale, would induce frightful electric detonations, thunder adding to the horror of this primitive scene, which no imagination, no human pencil could trace, and which const.i.tutes that gloomy and disastrous chaos of which the legendary history of every ancient race has transmitted the tradition.

In this manner would our globe circulate in s.p.a.ce, carrying in its train the flaming streaks of its multiple atmosphere, unfitted, as yet, for living beings, and impenetrable to the rays of the sun, around which it described its vast orbit.

The temperature of the planetary regions is infinitely low; according to Laplace it cannot be estimated at less than 100 below zero. The glacial regions traversed in its course by the incandescent globe would necessarily cool it, at first superficially, when it would a.s.sume a pasty consistence. It must not be forgotten that the earth, on account of its liquid state, would be obedient in all its ma.s.s to the action of flux and reflux, which proceeds from the attraction of the sun and moon, but to which the sea alone is now subject. This action, to which all its liquid and movable particles were subject, would singularly accelerate the commencement of the solidification of the terrestrial ma.s.s. It would thus gradually a.s.sume that sort of consistence which iron attains, when it is first withdrawn from the furnace, in the process of puddling.

As the earth cooled, beds of concrete substances would necessarily be formed, which, floating at first in isolated ma.s.ses on the surface of the semi-fluid matter, would in course of time come together, consolidate, and form continuous banks; just as we see with the ice of the present Polar Seas, which, when brought in contact by the agitation of the waves, coalesces and forms icebergs, more or less movable. By extending this phenomenon to the whole surface of the globe, the solidification of its entire surface would be produced. A solid, but still thin and fragile crust, would thus envelop the whole earth, enclosing entirely its still fluid interior. The entire consolidation would necessarily be a much slower process--one which, according to the received hypothesis, is very far from being completed at the present time; for it is estimated that the actual thickness of the earth's crust does not exceed thirty miles, while the mean radius or distance from the centre of the terrestrial sphere, approaches 4,000 miles, the mean diameter being 7,912409 miles; so that the portion of our planet, supposed to be solidified, represents only a very small fraction of its total ma.s.s.

[Ill.u.s.tration: Fig. 13.--Relative volumes of the solid crust and liquid ma.s.s of the globe.]

We say thirty miles, for such is the ordinary estimated thickness of the earth's crust, usually admitted by savants; and the following is the process by which this result has been obtained.

We know that the temperature of the earth increases one degree Centigrade for every hundred feet of descent. This result has been borne out by a great number of measurements, made in many of the mines of France, in the tin mines of Cornwall, in the mines of the Erzgeberge, of the Ural, of Scotland, and, above all, in the soundings effected in the Artesian wells of Grenelle and Pa.s.sy, near Paris, of St. Andre de Iregny, and at a great number of other points.

The greatest depth to which miners have hitherto penetrated is about 973 yards, which has been reached in a boring executed in Monderf, in the Grand Duchy of Luxembourg. At Neusalzwerk, near Minden, in Prussia, another boring has been carried to the depth of 760 yards. In the coal-mines of Monkwearmouth the pits have been sunk 525 yards, and at Dukinfield 717 yards. The mean of the thermometic observations made, at all these points, leads to the conclusion that the temperature increases about one degree Fahrenheit for every sixty feet (English) of descent after the first hundred.

In admitting that this law of temperature exists for all depths of the earth's crust, we arrive at the conclusion that, at a depth of from twenty-five to thirty-five miles--which is only about five times the height of the highest mountains--the most refractory matter would be in a state of fusion. According to M. Mitscherlich, the flame of hydrogen, burning in free air, acquires a temperature of 1,560 Centigrade. In this flame platinum would be in a state of fusion. Granite melts at a lower temperature than soft iron, that is at about 1,300; while silver melts at 1,023. In imagining an increase of temperature equal to one degree for every hundred feet of descent, the temperature at twenty-five miles will be 1,420 C. or 2,925 F.; thirty miles below the surface there will be a probable temperature of 1,584 C. or 3,630 F.; it follows, if these arguments be admitted, and the calculation correct, that the thickness of the solid crust of the globe does not much exceed thirty miles.

This result, which gives to the terrestrial crust a thickness equal to 1/190 of the earth's diameter, has nothing, it is true, of absolute certainty.

Prof. W. Hopkins, F.R.S., an eminent mathematician, has much insisted upon the fact, that the conductibility of granite rocks, for heat, is much greater than that of sedimentary rocks; and he argues that in the lower stratum of the earth the temperature increases much more slowly than it does nearer the surface. This consideration has led Mr. Hopkins to estimate the probable thickness of the earth's solid crust at a minimum of 200 miles.

In support of this estimate Mr. Hopkins puts forward another argument, based upon the precession of the equinoxes. We know that the terrestrial axis, instead of always preserving the same direction in s.p.a.ce, revolves in a cone round the pole of the ecliptic. Our globe, it is calculated, will accomplish its revolution in about 25,000 years. In about this period it will return to its original position. This balancing, which has been compared to that of a top when about to cease spinning, produces the movement known as the _precession of the equinoxes_. It is due to the attraction which the sun and moon exercise upon the swelling equatorial of the globe. This attraction would act very differently upon a globe entirely solid, and upon one with a liquid interior, covered by a comparatively thin crust. Mr. Hopkins subjected this curious problem to mathematical a.n.a.lysis, and he calculated that the precession of the equinoxes, observed by astronomers, could only be explained by admitting that the solid sh.e.l.l of the earth could not be less than from about 800 to 1,000 miles in thickness.

In his researches on the _rigidity of the earth_, Sir William Thomson finds that the phenomena of precession and nutation require that the earth, if not solid to the core, must be nearly so; and that no continuous liquid vesicle at all approaching 6,000 miles in diameter can possibly exist in the earth's interior, without rendering the phenomena in question very sensibly different from what they are.

The calculations of Mr. Hennessey are in direct opposition to those of Sir William Thomson, and show that the earth's crust cannot be less than eighteen miles, or more than 600 miles in thickness.

Admitting, for the present, that the terrestrial crust is only thirty miles in thickness, we can express in a familiar, but very intelligible fashion, the actual relation between the dimensions of the liquid nucleus and the solid crust of the earth. If we imagine the earth to be an orange, a tolerably thick sheet of paper applied to its surface will then represent, approximately, the thickness of the solid crust which now envelopes the globe. Fig. 13 will enable us to appreciate this fact still more correctly. The terrestrial sphere having a mean diameter of 7,912 miles, or a mean radius of 3,956 miles, and a solid crust about thirty miles thick, which is 1/260 of the diameter, or 1/130 of the radius, the engraving may be presumed to represent these proportions with sufficient accuracy.

To determine, even approximately, the time such a vast body would take in cooling, so as to permit of the formation of a solid crust, or to fix the duration of the transformations which we are describing, would be an impossible task.

[Ill.u.s.tration: Fig. 14.--Formation of primitive granitic mountains.]

The first terrestrial crust formed, as indicated, would be incapable of resisting the waves of the ocean of internal fire, which would be depressed and raised up at its daily flux and reflux in obedience to the attraction of the sun and moon. Who can trace, even in imagination, the fearful rendings, the gigantic inundations, which would result from these movements! Who would dare to paint the sublime horrors of these first mysterious convulsions of the globe! Amid torrents of molten matter, mixed with gases, upheaving and piercing the scarcely consolidated crust, large crevices would be opened, and through these gaping cracks waves of liquid granite would be ejected, and then left to cool and consolidate on the surface. Fig. 14 represents the formation of a primitive granitic mountain, by the eruption of the internal granitic matter which forces its way to the surface through a fracture in the crust. In some of these mountains, Ben Nevis for example, three different stages of the eruption can be traced. "Ben Nevis, now the undoubted monarch of the Scottish mountains," says Nicol, "shows well the diverse age and relations of igneous rocks. The Great Moor from Inverlochy and Fort William to the foot of the hill is gneiss. Breaking through, and partly resting on the gneiss is granite, forming the lower two-thirds of the mountain up to the small tarn on the shoulder of the hill. Higher still is the huge prism of porphyry, rising steep and rugged all around." In this manner would the first mountains be formed.

In this way, also, might some metallic veins be ejected through the smaller openings, true injections of eruptive matter produced from the interior of the globe, traversing the primitive rocks and const.i.tuting the precious depository of metals, such as copper, zinc, antimony, and lead. Fig. 15 represents the internal structure of some of these metallic veins. In this case the fracture is only a fissure in the rock, which soon became filled with injected matter, often of different kinds, which in crystallising would completely fill the hollow of this cleft, or crack; but sometimes forming cavities or geodes as a result of the contraction of the ma.s.s.

[Ill.u.s.tration: Fig. 15.--Metallic veins.]