Metamorphic Rocks.
The rocks composing the terrestrial crust have not always remained in their original state. They have frequently undergone changes which have altogether modified their properties, physical and chemical.
When they present these characteristics, we term them Metamorphic Rocks. The phenomena which belong to this subject are at once important and new, and have lately much attracted the attention of geologists. We shall best enlighten our readers on the metamorphism of rocks, if we treat of it under the heads of special and general metamorphism.
When a mass of eruptive rock penetrates the terrestrial crust it subjects the rocks through which it passes to a special metamorphism—to the effects of heat produced by contact. Such effects may almost always be observed near the margin of masses of eruptive rock, and they are attributable either to the communicated heat of the eruptive rock itself, or to the disengagement of gases, of steam, or of mineral and thermal waters, which have accompanied its eruption. The effects vary not only with the rock ejected, but even with the nature of the rock surrounding it.
In the case of volcanic lava ejected in a molten state, for instance, the modifications it effects on the surrounding rock are very characteristic. Its structure becomes prismatic, full of cracks, often cellular and scoriaceous. Wood and other combustibles touched by the lava are consumed or partially carbonised. Limestone assumes a granular and crystalline texture. Siliceous rocks are transformed, not only into quartz like glass, but they also combine with various bases, and yield vitreous and cellular silicates. It is nearly the same with argillaceous rocks, which adhere together, and frequently take the colour of red bricks.
The surrounding rock is frequently impregnated with specular iron-ore, and penetrated with hydrochloric or sulphuric acid, and by divers salts formed from these acids.
At a certain distance from the place of contact with the lava, the action of water aided by heat produces silica, carbonate of lime, aragonite, zeolite, and various other minerals.
From immediate contact with the lava, then, the metamorphic rocks denote the action of a very strong heat. They bear evident traces of calcination, of softening, and even of fusion. When they present themselves as hydrosilicates and carbonates, the silica and associated minerals are most frequently at some distance from the points of contact; and the formation of these minerals is probably due to the combination of water and heat, although this last ceases to be the principal agent.
The hydrated volcanic rocks, such as the basalts and trappean rocks in general, continue to produce effects of metamorphism, in which heat operates, although its influence is inconsiderable, water being much the more powerful agent. The metamorphosis which is observable in the structure and mineralogical composition of neighbouring rocks is as follows:—The structure of separation becomes fragmentary, columnar, or many-sided, and even prismatic. It becomes especially prismatic in combustibles, in sandstones, in argillaceous formations, in felspathic rocks, and even in limestones. Prisms are formed perpendicular to the surface of contact, their length sometimes exceeding six feet. Most commonly they still contain water or volatile matter. These characters may be observed at the junction of the basalts which has been ejected upon the argillaceous strata near Clermont in Auvergne, at Polignac, and in the neighbourhood of Le Puy-en-Velay.
If the vein of Basalt or Trap has traversed a bed of coal or of lignite, we find the combustible strongly metamorphosed at the point of contact. Sometimes it becomes cellular and is changed into coke. This is especially the case in the coal-basin of Brassac. But more frequently the coal has lost all, or part of, its bituminous and volatile matter—it has been metamorphosed into anthracite—as an example we may quote the lignite of Mont Meisner.
Again, in some exceptional cases, the combustible may even be changed into graphite near to its junction with Trap. This is observed at the coal-mine of New Cumnock in Ayrshire.
When near its junction with a trappean rock, a combustible has been metamorphosed into coke or anthracite, it is also frequently impregnated by hydrated oxide of iron, by clay, foliated carbonate of lime, iron pyrites, and by various mineral veins. It may happen that the combustible has been reduced to a pulverulent state, in which case it is unfit for use. Such is the case in a coal-mine at Newcastle, where the coal lies within thirty yards of a dyke of Trap.
When Basalt and Trap have been ejected through limestone rock, the latter becomes more or less altered. Near the points of contact, the metamorphism which they have undergone is revealed by the change of colour and aspect, which is exhibited all around the vein, often also by the development of a crystalline structure. Limestone becomes granular and saccharoid—it is changed into marble. The most remarkable instance of this metamorphism is the Carrara marble, a non-fossiliferous limestone of the Oolite series, which has been altered and the fossils destroyed; so that the marble of these celebrated quarries, once supposed to have been formed before the creation of organic beings, is now shown to be an altered limestone of the Oolitic period, and the underlying crystalline schists are sandstones and shales of secondary age modified by plutonic action.
The action of basalt upon limestone is observable at Villeneuve de Berg, in Auvergne; but still more in the neighbourhood of Belfast, where we may see the Chalk changed into saccharoid limestone near to its contact with the Trap. Sometimes the metamorphism extends many feet from the point of contact; nay, more than that, some zeolites and other minerals seem to be developed in the crystallised limestone.
When sandstone is found in contact with trappean rock, it presents unequivocal traces of metamorphism; it loses its reddish colour and becomes white, grey, green, or black; parallel veins may be detected which give it a jaspideous structure; it separates into prisms perpendicular to the walls of the injected veins, when it assumes a brilliant and vitreous lustre. Sometimes it is even also found penetrated by zeolites, a family of minerals which melt before the blowpipe with considerable ebullition. The mottled sandstones of Germany, which are traversed by veins of basalt, often exhibit metamorphism, particularly at Wildenstern, in Würtemberg.
Argillaceous rocks, like all others, are subject to metamorphism when they come in contact with eruptive trappean rocks. In these circumstances they change colour and assume a varied or prismatic structure; at the same time their hardness increases, and they become lithoidal or stony in structure. They may also become cellular—form zeolites in their cavities with foliated carbonate of lime, as well as minerals which commonly occur in amygdaloid. Sometimes even the fissures are coated by the metallic minerals, and the other minerals which accompany them in their metalliferous beds. Generally they lose a part of their water and of their carbonic acid. In other circumstances they combine with oxide of iron and the alkalies. This has been asserted, for example, at Essey, in the department of the Meurthe, where a very argillaceous sandstone is found, charged with jasper porcellanite, near to the junction of the rock with a vein of basalt.
Hitherto we have spoken only of the metamorphosis the result of volcanic action. A few words will suffice to acquaint the reader with the metamorphism exercised by the porphyries and granites. By contact with granite, we find coal changed into anthracite or graphite. It is important to note, however, that coal has seldom been metamorphosed into coke. As to the limestone, it is sometimes, as we have seen, transformed into marble; we even find in its interior divers minerals, notably silicates with a calcareous base, such as garnets, pyroxene, hornblende, &c. The sandstones and clay-slates have alike been altered.
The surrounding deposit and the eruptive rock are both frequently impregnated with quartz, carbonate of lime, sulphate of baryta, fluorides, and, in a word, with the whole tribe of metalliferous minerals, which present themselves, besides, with the characteristics which are common to them in the veins.
General Metamorphism.
Sedimentary rocks sometimes exhibit all the symptoms of metamorphism where there is no evidence of direct eruptive action, and that upon a scale much grander than in the case of special metamorphism. It is observable over whole regions, in which it has modified and altered simultaneously all the surrounding rocks. This state of things is called general, or normal, metamorphism. The fundamental gneiss, which covers such a vast extent of country, is the most striking instance known of general metamorphism. It was first described by Sir W. E. Logan, Director of the Canadian Geological Survey, who estimates its thickness at 30,000 feet. The Laurentian Gneiss is a term which is used by geologists to designate those metamorphic rocks which are known to be older than the Cambrian system. They are parts of the old pre-Cambrian continents which lie at the base of the great American continent, Scandinavia, the Hebrides, &c.; and which are largely developed on the west coast of Scotland. In order to give the reader some idea of this metamorphism, we shall endeavour to trace its effects in rocks of the same nature, indicating the characters successively presented by the rocks according to the intensity of the metamorphism to which they have been subjected.
Combustibles, which have a special composition, totally different from all other rocks, are obviously the first objects of examination. When we descend in the series of sedimentary deposits, the combustibles are observed completely to change their characters. From the peat which is the product of our own epoch, we pass to lignite, to coal, to anthracite, and even to graphite; and find that their density increases, varying up to at least double. Hydrogen, nitrogen, and, above all, oxygen, diminish rapidly. Volatile and bituminous matters decrease, while carbon undergoes a proportionate increase.
This metamorphism of the combustible minerals, which takes place in deposits of different ages, may also be observed even in the same bed. For instance, in the coal formations of America, which extend to the west of the Alleghany mountains, the Coal-measures contain a certain proportion of volatile matter, which goes on diminishing in proportion as we approach the granite rocks; this proportion rises to fifty per cent. upon the Ohio, but it falls to forty upon the Manon-Gahela, and even to sixteen in the Alleghanies. Finally, in the regions where the strata have been most disturbed, in Pennsylvania and Massachusetts, the coal has been metamorphosed into anthracite and even into graphite or plumbago.
Limestone is one of the rocks upon which we can most easily follow the effects of general metamorphism. When it has not been modified, it is usually found in sedimentary rocks in the state of compact limestone, of coarse limestone, or of earthy limestone such as chalk. But let us consider it in the mountains, especially in mountains which are at the same time granitic, such as the Pyrenees, the Vosges, and the Alps. We shall then see its characters completely modified. In the long and deep valleys of the Alps, for example, we can follow the alterations of the limestone for many leagues, the beds losing more and more their regularity in proportion as we approach the central chain, until they lose themselves in solitary pinnacles and projections enclosed in crystalline schists or granitic rocks. Towards the upper regions of the Alps the limestone divides itself into pseudo-regular fragments, it is more strongly cemented, more compact, more sonorous; its colour becomes paler, and it passes from black to grey by the gradual disappearance of organic and bituminous matter with which it has been impregnated, at the same time its crystalline structure increases in a manner scarcely perceptible. It may even be observed to be metamorphosed into an aggregate of microscopic crystals, and finally to pass into a white saccharoid limestone.
This metamorphism is produced without any decomposition of the limestone; it has rather been softened and half melted by the heat, that is, rendered plastic, so to speak, for we find in it fossils still recognisable, and among these, notably, some Ammonites and Belemnites, the presence of which enables us to state that it is the greyish-black Jurassic limestone, which has been transformed into white saccharoid or granular limestone. If the limestone subjected to this transformation were perfectly pure, it would simply take a crystalline structure; but it is generally mixed with sand and various argillaceous matters, which have been deposited along with it, matters which go to form new minerals. These new minerals, however, are not disseminated by chance; they develop themselves in the direction of the lamination, so to speak, of the limestone, and in its fissures, in such a manner that they present themselves in nodules, seams, and sometimes in veins.
Among the principal 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 constitutes the Cipoline marble, which is found in Corsica, and in the Val Godemar in the Alps. Some white marbles are quarried in France, chiefly at Loubie, at Sost, at Saint-Béat 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, associated 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 lamellæ of mica of brilliant and silky lustre, 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 analogous to the preceding. We can follow them readily through all their gradations when we direct our attention towards such granitic masses as those which constitute 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 principal 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 Cumberland, 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 associated 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 masses 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 principal 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 principal 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-dykes 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 masses. 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 passing long-continued galvanic currents through masses of moistened clay, there is a tendency to produce cleavage and a semi-crystalline arrangement of the particles of matter.[31]
[11] Lyell’s “Elements of Geology,” p. 694.
[12] “Physical Geology and Geography of Great Britain,” by A. C. Ramsay, p. 38, 2nd ed.
[13] At the same time it may be safely assumed (as Professor Ramsay believes to be the case) that granite in most cases is a metamorphic rock; yet are there many instances in which it may with greater truth be considered as a true plutonic rock.
[14] “Elements of Geology,” p. 716, 6th edition.
[15] “Elements of Geology,” p. 717.
[16] Ibid, p. 718.
[17] “Geology of the Island of Arran,” by Andrew C. Ramsay. “Geology of Arran and Clydesdale,” by James Bryce.
[18] See Quarterly Journal of Geological Society, vol. viii., pp. 9 and 10.
[19] For full information in reference to the rocks and geology of this part of France, the reader is referred to the masterly work on “The Geology and Extinct Volcanoes of Central France,” by G. Poulett Scrope, 2nd edition, 1858.
[20] “Volcanoes,” 2nd ed.
[21] “Elements of Geology,” p. 596.
[22] Ibid, p. 677.
[23] “Cosmos,” vol. i., p. 25. Bohn.
[24] “Cosmos,” vol. i., p. 237.
[25] Darwin’s “Journal,” p. 291, 2nd edition.
[26] “Elements of Geology,” p. 732.
[27] Ibid, p. 733.
[28] Lyell’s “Elements of Geology,” p. 617.
[29] Lyell’s “Elements of Geology,” p. 620.
[30] Ibid, p. 620.
[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.