METAMORPH Rocxs.
The Metamorphism of Sediments.— Those most ancient of all sediments which were de rived from the disintegration of the original or primal rocks of the lithosphere, and which were first deposited in the primordial seas, have been so deeply buried in the crust of the earth as to have been affected by the uprising heat of the earth's interior. This heat, united with the tre mendous pressure exerted by the superincum bent mass of sediments and the dissolving effect of the intensity (super-) heated mositure pres ent have changed these old rocks into thor oughly crystalline masses, consisting of distinct mineral aggregates more or less resembling the crystalline rocks of the igneous type, but dif ferent from the latter chiefly in this, that the original stratification (bedding) planes are still visible. Such rocks are utterly changed in every thing except their chemical composition from their original condition and appearance and are said to be metamorphosed. As a class they compose the metamorphic rocks. It is not alone these ancient sediments which have suffered transformation. Sediments of every geologic age have been found in some quarter of the globe in the metamorphosed condition.
There are two kinds of metamorphism: con tact and regional. Contact metamorphism is produced by the heat of intruded masses of igneous material upon the surrounding rocks. The contact zone or area about the intrusion affected by its heat is usually narrow, but it may extend for rods or even miles from the in truded mass. The changes resulting from con tact metamorphism are not so important in the production of altered rock-masses as they are in the formation of new minerals, so that they are of more especial importance to the min eralogist.
Regional metamorphism, on the other hand, as the name implies, has been exerted over wide areas, and has produced the most profound and far-reaching effects. By it the rodo over many thousands of square miles have been completely altered from their original condition. As a result of pressure the older deeper-seated 'rocks (as well as some of the more recent ones) have been intensely folded, plicated and sheared, which folding and shearing alone would be suffi cient to alter them completely. Metamorphic rocks have other structures developed in them, such as crushing; jointing (series of more or less parallel cracks intersecting each other at nearly right angles and breaking the rock masses into blocks with rudely parallel sides) ; faults (produced by the slipping of the walls of rock on either side of a fissure over each other) ; slaty cleavage (produced by great pres sure exerted at right anifles to the cleavage sur faces) ; fissility (the visible separation of rocks into thin plates, also due to pressure, and at tended by slight movement of the plates on each other) ; schistosity or foliation (the property of splitting into plates with rough undulating sur faces, to the parallel orientation of the mineral crystals). This may be cleavage,
fissility, or both.
The same processes of metamorphism, so potent in the transformation of true sediments, are just as efficacious in producing changes in the igneous rocks themselves, by virtue of which processes the igneous rocks frequently become so profoundly altered as to be utterly unrecognizable.
The metamorphic rocks as a class are com pletely crystalline., and in this respect resemble the igneous rocks of plutonic origin. Their chief varieties will now be considered.
Disintegrated or residual gran ite consists of disaggregated particles of quartz and more or less thoroughly kaolinized feld spar, with considerable true clayey residue re sulting from the complete decomposition of some of the feldspar, all somewhat tinged by the oxide of iron. It is easy to see how a similar assemblage of materials could be brought together upon a sea-beach. By a subsidence of the area and a continued deposition of other sedimentary ma terials above them, that first assemblage of sand and clay might become so deeply buried as to come within the sphere of metamorphic action. If subjected to heat and pressure, the elements present would recrystallize as quartz, feldspar and mica. In being so altered it would not necessarily have its original bedding-planes obliterated, and the rock would have the min eralogical composition of granite, but obviously would be of sedimentry origin. A metamor phic rock of such a character would be called gneiss. In the same manner a variety of mix tures might be altered with the same result. Conglomerates are known thus to pass by im perceptible gradations over into gneisses. An arkose might alter to gneiss. If the material originally contained much iron, then horn blende, or some other ferromagnesian mineral, such as biotite or augite, would result and the rock would accordingly be called hornblende-, biotite- or augite-gneiss. Garnets might result from the metamorphism and the gneiss would then be called garnetiferous.