GRANITE, a well known igneous rock, composed of the three minerals, quartz, feldspar, and mica, united in a confused crystallization ; that is, without a regular arrange ment of the crystals. The feldspar is the most abundant ingredient, and the proportion of quartz is greater than that of mica. The name has been given to it on account of its granular structure.
Granite differs from greenstonc and the later igneous rocks, in the large quantity of quartz that enters into its composition. In the trappean and other igneous rocks, the silica or silicic acid is only sufficient for union with the bases to form feldspar and horn blende. the constituents of these rocks, none remained free to crystallize as pure quartz; while in granite, so great is the excess of silex, that in its pure state, as quartz, it forms a considerable bulk of the rock. Granite is always a compact rock, it never passes into or alternates with tuffs or breccias. This peculiarity, associated with the crystalline structure of the rock, and the absence of cellular cavities, such as are produced in trap /man and volcanic rocks by the expansion of the contained gases, have led to the belief that granite has been formed at considerable depths in the earth,. and has crystallized slowly under great pressure either from superimposed strata or deep seas. On this account the granitic rocks have been called "Plutonic rocks;" and Lyell has applied to them the term "hypogene," from upo, under, and ginomai, to be born. It was formerly supposed that all granitic rocks were formed before the deposition of any of the sedi mentary strata, and hence they are named "Primitive rocks." But it having been found that granite is associated with formations of various ages, and that even since the begin ning of the tertiary epoch its intrusion among the eocene strata of central Europe has raised the Alps more than 10,000 ft. above the level of the sea, this name has been entirely dropped. Although granite is not absent from the secondary or tertiary strata, it is more frequently associated with the palmozoic formations; indeed, it appears to be the funda mental rock of the earth's crust.. Wherever we reach the base of the stratified rocks, we find them resting upon granite; and whatever the age of the strata thus lying on this igneous rock, we have no reason to suppose that below the granite there occur beds of older date, for, although granite penetrates the stratified rocks, it has not been noticed to spread over them like greenstone, so that wherever it presents itself in a large mass, it is believed that no other rock is beneath it. Some granites, however, occur inter stratified with undoubted sedimentary rocks, and it is argued that, as the transition from these sedimentary rocks to the crystalline granite canbe traced by gradual stages through mica schist and gneiss, the granite is only the final stage of these metamorphic changes. Many of the granites of Scotland are accordingly believed to be older palmozoic sedi ments greatly altered. It is not maintained that all granites have such an origin; but no lithological character has yet been observed whereby the igneous granite can be dis tinanished from that produced by metamorphic action.
large extents of the earth's surface are covered, with granites; occa0onally, it is the superficial rock in flat undulating plains, but it most frequently makes its appearance in mountainous regions. It seems probable that sometimes igneous granite has been
raised from below as a solid indurated rock; it has, however, generally been in a fluid condition, as is evidenced by the number of veins which are protruded from it into the adjacent rocks.
Time varieties of granite depend upon the number and quantity of its mineral con stituents, and upon the state of aggregation of these materials. Ordinary granite is com posed of feldspar, mica, and quartz. The feldspar may be either the flesh-colored potash variety, orthoclase, or the pure white soda variety, albite, or both potash and soda may enter into its composition, The mica varies in color from a pure silvery white, through time more comthon brown, into black. The quartz is generally white, seldom dark-gray or brown. The predominance of one or other of the ingredients, or of a particular variety, gives the peculiar color to the mass, which is generally either red, gray, or white. The red is produced from time predominance of orthoclase; the white, of albite; and the intervening grhy from the mica, or sometimes from the quartz. The feldspar forms gen erally a half, and sometimes even more of the bulk of the rock; the mica in one variety, and the quartz in another, are so minute as to be scarcely visible. Sometimes the feldspar separates into large and distinct crystals forming a porphyritic granite. Time substitu tion of hornblende for mica produces that variety called syenite (q.v.); and if talc takes the place of the mica, the rock is called protogene (q.v.). When the ingredients exist in a compact and finely granular condition, the compound is known as mite. Some times, especially in veins, feldspar and dark quartz are arranged so as to produce an imperfect laminar structure, which, when broken at right angles to the !aniline, presents numerous broken and angular lines that have a faint resemblance to Hebrew characters, whence it is called graphic granite.
Granite is largely used as a building material in bridges and engineering-works, and also in public buildings and dwellings. The difficulty of working it makes it expensive, but this is counterbalanced by its great durability. It cannot be cut, like the majority of building-stones, with saws, but is first worked with large hammers, and then with pointed chisels. The success with which the Egyptians operated upon this refractory stone is very extraordinary. They worked and polished it in a way which we cannot excel, if, indeed, we can come up to it, with all time appliances of modern science; and not content with polishing, they covered some of the blocks with the most delicate and sharply cut hieroglyphics! The granites best known for ornamental purposes are the gray Aberdeen granite and the reddish colored Peterhead granite. Of this last-mentioned variety, handsome polished columns for public halls have been constructed.
The soil produced by the weathering of granitic rocks should be fertile, as their chemical composition contains the necessary elements. The great hardness of the rock, and its resistance to atmospheric influences, prevents a soil of any thickness being formed; and even where it exists, at least in our temperate regions, it is generally so high and exposed, that it is unfavorable to vegetation; in warmer climates, such soils are frequently very fertile.