Part 6 (1/2)
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 s.h.i.+ne 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 s.h.i.+ne 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.