METAMORPHIC ROCKS. Few of the deposits forming the crust of the earth remain in the condition in which they were deposited. By infiltration of a cementing fluid, by pressure, or by some other indurating agency, sand has become converted into sandstone, -and clay and mud into shale. In some strata, this operation has been carried still further. There is a class of rocks, including gneiss, mica-schist, clay-slate, marble, and the like, which, while certainly of aqueous or mechanical origin, have, by intense molecu lar action, become more or less crystalline. To them, the convenient name metamorphic (Gr. transformed) rocks has been given by Lyell.
The metamorphic rocks were formerly considered to be the fundamental strata of the -earth's crust. The original incandescent mass, it was said, losing its heat by radiation, a solid uneven crust of granite was formed. As soon as the ordinary atmospheric and aqueous agencies began to operate, a disintegration took place, and the abraded materials, -carried down by the waters, were deposited in the basins which contained the boiling sea. It was thought that this not only accounted for the condition in which the metamorphic rocks now exist, but for the remarkable undulations and contortions so characteristic of these strata. Gneiss and the allied crystalline schists were accordingly placed as the low est sedimentary strata in a division equivalent to the palmozoic penod, and called the azoic, because they were destitute of orgeanic remains, the conditions in which they were formed being opposed to the existence of animals.
It is now, however, known that metamorphic rocks occur as contemporaneou:s deposits in all epochs of the earth's geological history. In Canada and in the Hebrides, they are of Laurentian agre; in the Highlands of Scotland, Cambrian and Silurian; in Devon and Cornwall, old red sandstone and carboniferous; and in the Alps, oolitic and cretaceous, and in some parts even tertiary. Although deposits of such various ages have been thus altered, the resulting rocks are in structure and composition very similar; their ultimate constituents do not differ from those of ordinary clays and sandstones. In all of them silica forms the largest proportion, consisting of about 60 to 70 per cent; alumina follows next, and then other substances in smaller quantities, such as lime, soda, potash, iron, etc. This similarity of composition, and the abundance of clays and sand stones, suggest the supposition that the metamorphic rocks may be nothing more than these deposits greatly altemd; this is confirmed by many observed instances, in which. aqueous strata are continuous with, and gradually change into, metamorphic rocks. The -granite of Dartmoor has intruded itself into the slate and slaty sandstone, twisting and .contorting the strata. Hence some of the slate rocks have become micaceous; others more indurated, having the characters of mica-slate and gneiss; while others, again, -appear converted into a hard-zoned rock, strongly impregnated with feldspar. In some places in the eastern Pyrenees, the chalky limestone becomes crystalline and saccharoid as it approaches the granite, and loses all trace of the fossils which it elsewhere contains in abundance. These illustrations tell of chang,es occurring in the proximity of granite, and it has been consequently somewhat hastily concluded that this rock, coming up in a molten condition from below, has, by the radiation of its heat, produced the metamor phosis. But the observed stratigraphical position of granite, its sometimes passing by
insensible degrees into gneiss, and the experiments of Solly and Bryson on its internal structure, show without doubt that this rock is, at least in many places, an extreme result of metamorphic action, and not the cause of it. To call the energy producing these results metamorphic or molecular action, is simply to hide our ignorance—we get a name, but nothing more. To speak dogmatically on a subject so obscure, is a sign of the same ignorance. The following, however, are the most probable agents that, together or separately, produced these remarkable changes: 1. Heat.—From whatever source derived, heat does exist, either distributed univer sally, or occurring locally in the mass of the earth; and where it exists, thermo-electric influences induce action, which, carried on over immense series of years, might produce in the end great changes. It is generally maintained that granite is the result of crys tallization from perfect fusion, and that the strata converted into gneiss must have been reduced to a state of seng-fusion. But we know of crystallization taking place in the most compact amorphous solids without any approach to fusion, as in ;the axles of rail way carriages; and of metamorphoric action without semi-fusion, as in the highly indurated bottoms of bakers' ovens, in which the clay is subjected to a long-continued though not a great heat; or in the sandstone floor of an iron furnace, which, from long contact with the molten iron, loses its color, becomes white and hard, and breaks with a poreelanic fracture, having, indeed, been. changed into quartz rock. Besides, the fre quent occurrence of cavities in the rock crystals of granite containing a fluid which fills them only when the tempesature raised to at least 94° Fahr., shows that the crystal could not have been formed at a higher temperature. We are therefore safe in main taining, that the heat was not in ail cases so great as to produce fusion 2. Preseure.--This alone is sufficient to effect the consolidation and induration of aqueous deposits, converting clay or sand into solid stone. When heat is added to pres sure, greater activity is likely to be the result. The undulatory movements of the earth's crust, by carrying down to great depths deposits formed on the surface, bring them under the influence of pressure, heat, and thermo-electricity, and at the same tune elevate rocks that have been thus acted upon.
- It is thought that heated water may be also a powerful agent, especially when it is subjected to great pressure.
These and other agents, then, operating through immense intervals of time, set in motion chemical attraction, whereby the various substances which entered into the coin position of the sedimentary deposits rearranged themselves as they are found in the metamorphic rocks.
The description of the various metamorphic rocks will be found under their different names, viz., GNI-Eiss, QUARTZITE, MICA-SCHIST, CLAY-SLATE, and MARBLE.