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texture by this force alone. But during this mechanical pressure chemical actions and reactions are perpetually taking place in all rock-masses, and thus its effects are facilitated and rendered much more perceptible. A bed of peat, for example, may be solidified by compression, but during its chemical passage into coal it undergoes a process of softening and bituminisation which enables mere pressure to act with greater uniformity and effect. In the earth's crust, therefore, wherever chemical transformations take place, or heat is induced by chemical actions, mechanical pressure will act with increased efficiency; and as these are almost everywhere present, we may regard the two combined (that is, pressure and chemical action) as amongst the most important agents in the metamorphism or transformation of rock-matters. And those who have witnessed the effects of the hydraulic press-the conversion of the softest pulp into a solid mass, and the loosest powder into a hard and brittle stone-can have little difficulty in conceiving the myriad-fold greater result of thousands of feet of superincumbent strata, and the continuance of their weight and pressure for unknown ages.

But while all rock-substances are thus continually pressed upon and transformed by those that lie above them, the infiltration of chemical solutions as well as chemical reactions among the particles of their own mass are also materially assisting in changing their composition and texture. The loose shelly sand of the sea-shore is often cemented into a compact conglomerate* by the percolation of rain-water, which, dissolving the limy matter of the shells, diffuses it

*This "littoral concrete," as it has been lithologically termed, may be witnessed among the sand-drifts, and along the shores of many parts of the British Islands. Composed of sand, shells, and pebbles, it is often of stony hardness, and might be mistaken for an older rock, were it not for the imbedded shells, which are all recent, and to be found in the neighbouring seas.

through the mass, as a mortar to bind the incoherent particles into a solid rock. Every rock in the earth's crustsedimentary and vulcanic—is rendered more compact and crystalline by this process of chemical infiltration. Water is ever permeating this outer shell, and thus solutions of lime, silica, iron, and other mineral and metallic substances are borne hither and thither-here cementing sand and gravel into grits and conglomerates, and there calcifying a sandstone; here silicifying and hardening some earthy limestone, and there converting volcanic dust and ashes into stony tufas and amygdaloids. Besides simply cementing and hardening the masses through which they percolate, these solutions give rise to new chemical actions, by which certain rocks are rendered more crystalline, and altogether changed in their texture. A solution of silica, for example, may permeate a porous sandstone, and by cementing together its particles render it merely harder and more compact; while a similar solution, in passing through an earthy chalk, might form a chemical union with the mass, and convert it into a tough and flinty chertstone. Those acquainted with the changes that can be artificially induced by chemical action, can have little difficulty in conceiving how vast and complex the metamorphoses that may be wrought within the rocky crust by the same agency, and all the more that heat and moisture are there ever present to facilitate its operations. And even where the changes are slow and gradual-so slow as to be almost imperceptible -we may rest assured they are still going forward, and only require time for their perfect development. recent rocks must necessarily have suffered less metamorphism than the ancient, but still the time and fitting conditions will come when the loose sand will be converted into sandstone, and the sandstone, by further change, into quartzite; the peat be changed into lignite, the lignite

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into coal, and the bituminous coal into a hard and flameless anthracite.

But whatever the amount of rock-change brought about by chemical agency, it is far less noticeable than that induced by the operations of subterranean heat, whether acting in the dry way, as in volcanoes, or in the wet, as in hot-springs and vaporiform exhalations. Whatever be the source of volcanic heat, its effect by contact on all the stratified rocks is at once marked and decisive. Sandstones are frequently converted into crystalline quartzites, shales baked into splintery porcellanite and porcelain jasper, and bituminous coals changed into coke-like anthracites. We every day witness the effects of dry heat in our brickkilns, coke-ovens, glass-houses, and iron-furnaces; and if the effect of such temporary heat be to melt and bake and calcine, how much more the results of those subterranean fires, that may continue to operate in certain areas unabated for ages! In the earth's crust, then, we may rest assured that volcanic heat has effected, and is still effecting, extensive metamorphism in all rock-masses, and that those lying at great depths and under vast pressure will be affected very differently from those occurring near the surface. But beyond this contact with dry heat there is the permeation of heated water and vapours, which greatly facilitate the dissolution and recombination of mineral matters. As the temperature of the crust increases at the rate of one degree Fahr. for every 60 or 65 feet of descent, the transforming effect of this heat at the depth of a few miles must be enormous; and we can readily conceive (though we cannot witness the operation) how ordinary strata may be converted into crystalline schists and igneous rocks, as the traps and lavas, undergo a process of recrystallisation, and assume the character of granites. Heat, in whatever form transformer of mineral

it operates, is indeed the great

matter-here baking and hardening, there calcining and melting; here inducing crystallisation, and there, in combination with other forces, filling veins with veinstuffs and metallic ores.

In connection with this deep-seated heat, we may also believe that magnetic currents and crystallisation (to whatever force or forces it may be due) are also effecting a marked metamorphism on all the older and deeper-seated rocks. The earth is in fact a great magnet, through whose exterior crust currents of varying intensity are ever passing; and these, we may rest assured, are actively instrumental in altering the molecular arrangement of rock-masses, and conferring upon them not only new textures, but also new structures, such as cleavage, crumpling, foliation, and other peculiarities which distinguish the slates and schists of the older formations. Experiment has tried to imitate this mode of metamorphism by passing galvanic currents through masses of moistened pottery clay, and the result was rearrangement of the particles so as to produce cleavage, or fissility, such as occurs in roofing-slate, and at the same time a glistening and semi-crystalline texture. Taken in connection with heat, these subtle forces of chemistry, magnetism, and crystallisation, are obviously important modifiers of mineral and metallic matters; and thus, among the oldest rocks, which have been the longest subjected to their influence, we find crystallisation, cleavage, foliation, and kindred phenomena in their greatest intensity. It is to these older rocks that the term METAMORPHIC is generally applied; and though all rock-matters are continually undergoing metamorphism, and in some localities intensely so, yet these old semi-crystalline and highly-altered slates and schists are so intimately associated in character and position that they have been grouped into a system-the "METAMORPHIC SYSTEM" of Systematic Geology.

Summing up our knowledge of this metamorphism or transformation of mineral matter-by which chalk, for instance, can be changed into crystalline marble, and clay into glistening roofing-slate-it may be safely affirmed that the following are the principal agents concerned in its production, even though we may not be always able to determine their precise modes of operation:-1. Heat by contact, as when an igneous mass, like lava, indurates, crystallises, or otherwise changes the strata over or through which it passes. 2. Heat by transmission, conduction, or absorption, which may also produce metamorphism, according to the temperature of the heated mass, the continuance of the heat, and the conducting power of the strata affected. 3. Heat by permeation of hot water, steam, and other vapours, all of which, at great depths, may produce vast changes among the strata, when it is recollected that steam under sufficient pressure may acquire the temperature of molten lava. 4. Electric and galvanic currents in the earth's crust, which may, as the experiments of Mr W. Fox and Mr R. Hunt suggest (passing galvanic currents through masses of moistened pottery clay), produce cleavage and semi-crystalline arrangement of particles. 5. Chemical actions and reactions, which, both in the dry and moist way, are incessantly producing atomic change, and all the more readily when aided by an increasing temperature among the deeper-seated strata. 6. Mechanical pressure, produced by the mere weight of superincumbent strata, and which is obviously concerned in the solidifying, compacting, and hardening of all rock-matter, whether belonging to the superficial or to the deeper-seated formations. 7. New molecular arrangement by pressure and motion silent but efficient agent of change, as yet little understood, but capable of producing curious alterations in internal structure, especially when accompanied by heat, as we

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