Granite , a hard firm rock, made up essentially of crystalline grains of feldspar and quartz, deriving its name from its granular structure. The typical granites are generally described as composed of a potash feldspar (orthoclase), quartz, and mica; but there are similar rocks which entirely lack the mica, and others in which it is replaced by hornblende. To this latter combination some writers give the name of syenite, but this term appears to have been originally employed to designate a rock composed of hornblende with a soda feldspar (albite, oligoclase, or labradorite), and without quartz, being identical with what by other authors is called diorite. It seems better therefore to follow the example of certain German lithologists, who define granite as a binary aggregate of orthoclase feldspar and quartz, in which mica and hornblende may be present as accidental minerals, giving rise to micaceous and hornblendic granite, while the variety from which they are both absent is termed normal or binary granite. In some cases a chloritic mineral, often confounded with talc, takes the place of mica, and gives rise to what has been called protogine or talcose granite.
The color of the feldspar of granite is generally white, gray, or reddish, while the quartz is either colorless or somewhat smoky, the hornblende greenish black, and the mica varies in color from nearly white to brownish or blackish. Associated with the orthoclase, some granites contain portions of a soda feldspar, which may be either albite or oligoclase, distinguished from the former by its white or greenish-white color, which often contrasts with the reddish tint of the orthoclase. There are various degrees of fineness in the texture of granites, and some of them, which have large crystals of orthoclase imbedded in a finely granular mixture of the constituent minerals, are called porphyritic granites. Geologically granite is described as an unstratified rock, from the fact that it wants the banded or stratified structure which characterizes gneiss, a granular rock made up of the same mineral species arranged in layers, which are generally supposed to represent planes of deposition. Moreover, granite appears in irregular masses, breaking through gneiss and various crystalline stratified rocks, and often sending out veins or dikes into the midst of these.
All the relations of the true granites to the stratified rocks are in fact such as to suggest the notion that the former have been extruded in a more or less pasty condition from below the latter. From the microscopic study of the minute cavities often found in the quartz of granites, which are filled with water or saline solutions, Sorby has concluded that this rock must have consolidated at a temperature in some cases approaching a red heat, and under a degree of pressure which implies that it was at that time buried beneath a very great weight of rock. There is a popular notion that granite is the oldest of all rocks, and is in fact the substratum which underlies all others; but this idea rests upon certain misconceptions, and is probably incorrect. It is true that it is found breaking up through the newer crystalline stratified rocks, the primitive slate formations of some geologists; but these are seen to rest upon an older formation composed in great part of highly crystalline gneiss, which, though often granite-like in its aspect, is clearly stratified, and includes beds of quartzite, limestone, and iron ores.
This oldest known series, to which the name of Laurentian is given, was by some of the earlier geologists mistaken for the granite which was supposed to underlie the newer series, and it has been suggested with much probability that it is the gneisses of this old series, which in a softened condition have been forced upward among the overlying formations, where they take the form of unstrati-fied granites. The primitive rock, which is supposed to have been before all stratified deposits, is everywhere concealed by these, and from chemical analogies may be supposed to have been very unlike granite. The so-called granites of the Alps are now shown to be stratified rocks of eozoic age, which by great and profound folds have been brought up and made in some cases to overlie the newer strata. (See Alps.) - The dikes of a fine-grained granite, which appear as offshoots from the great eruptive masses, are not to be confounded with the granite veinstones, which appear to have been formed by a process of gradual deposition from aqueous solution in fissures or cavities in the rocks. Such veins, although often made up in great part of feldspar and quartz with mica, are closely related to the veinstones of quartz and calc spar, which are so often the gangue of metallic ores.
They frequently present a banded structure parallel with the walls of the enclosing rock, and are remarkable for containing in many cases large and beautiful crystals, not only of the constituent minerals of granite, but of rarer species. Among the most common of these are garnet, tourmaline, beryl, topaz, columbite, and cassiterite. The gneisses and mica schists of what has been called the Montalban or White mountain series of the Appalachians are noted for the abundance of these veins, and for the fine minerals which these contain. Some of these granite veins are mined for the mica which they afford, and others for the pure and abundant white orthoclase which is sought for the manufacture of porcelain. These veins are of very various sizes, sometimes 100 feet or more in breadth, and often traverse the enclosing rocks at right angles. A peculiar aggregate is. sometimes found in these veins, in which plates and imperfect skeleton crystals of quartz are so scattered through the masses of cleavable orthoclase, that a section across the ends of these plates presents the appearance of written characters or hieroglyphics on a ground of feldspar; hence the name of graphic granite. - Granite is very strong and durable, and resists the atmospheric influences; but in the southern and western parts of the United States and in South America, as also in central France, it is found to be softened and decayed to considerable depths.
This softening, which Dolo-mieu called the maladie du granit, and ascribed to the evolution of carbonic acid from the interior of the earth, depends upon a chemical decomposition of the feldspar, which loses its alkali and a part of the silica in a soluble form, leaving a hydrated silicate of alumina, which in its purest form constitutes kaolin or porcelain clay. The feldspars and hornblendes of the gneisses undergo a similar change. According to Sterry Hunt, this decomposition is not recent, and is not connected with an evolution of carbonic acid from below, but was effected in remote periods, when the whole atmosphere was highly charged with this gas, and has ceased in modern times; although it is not impossible that some such changes may now be going on in localities where an abundance of carbonic acid is given off from the earth. The red granites from Peterhead, near Aberdeen in Scotland, are especially esteemed for their beauty of color. Similar red granites are found on the coasts of Maine and New Brunswick; and the hornblendic granites of Rockport and Quin-cy in Massachusetts are quarried in very large quantities and shipped to distant points. - True granite is found in masses of great solidity, unbroken by seams and of remarkably uniform structure.
It is seen upon the sides of mountains covering acres, with hardly a crack or seam. But, however massive and unbroken it appears, it exhibits when quarried a tendency to divide more easily in some directions than in others, and is found to be really traversed by parallel seams, which separate it into blocks more or less symmetrical. Having great durability, and being so hard and compact that the finer varieties are susceptible of a good polish, and when carved retain better than any other rock used for architectural purposes the sharp edges of mouldings, granite has always occupied the first rank among building stones. Its great strength to resist pressure is exhibited in the trial of the Aberdeen granite used in the construction of the piers in the vaults of the London custom house. A half-inch cube of the best stone required to crush it the pressure of 24,556 lbs. It is easily split in large blocks by a very simple process. With a hand drill and hammer a workman bores a succession of holes from 3 1/2 to 0 in. in depth, and 2 to 6 in. apart, along the line where he wishes to • open the stone. The depth and number of the holes are proportioned to the size of the block.
Into each of the round holes thus made he introduces two slips of iron called half-rounds, buckings, or feathers, being of wedge form, but round on one side, and running to a point. He then inserts a small steel wedge between the flat faces of the two half-rounds and gently tightens it with the hammer. This being done, he moves along the line tapping each wedge in order, and repeating the process till the strain causes a crack, which gradually opens, separating the block. Good granite of close grain and uniform texture should in this way make a clean separation, the crack going straight through twice or three times the depth of the holes. It may even be quarried out of the solid ledge in the same manner, provided there are natural seams; and where practicable the process is much to be preferred to blasting, which wastes the stone, breaking it into irregular fragments. But the latter is necessary in quarrying, to expose suitable faces for splitting, and to open seams. Blocks of great size may be obtained from good quarries, much larger indeed than there is any demand for. They are often split out from 40 to 80 ft. in length, and are afterward reduced to smaller sizes.
They are sold in the rough blocks commonly by the ton of 14 cubic feet, or if dressed, by the superficial foot of hammered surface. In many parts of the country gneisses of great homogeneousness and with little evidence of stratification are quarried under the name of granite, for which they furnish an excellent substitute. Such is the case with some of the gneisses of the Lau-rentian in New York, and still more with the fine-grained gray gneisses of the Montalban series in New England and further southward in the Blue Ridge. The so-called granites of Hallowell and Augusta in Maine, and of Concord in New Hampshire, are examples of these granite-like gneisses. They are somewhat more tender than the true granites, but are more easily wrought, and from their beauty of color and texture are greatly esteemed for architectural purposes. A very fine variety of so-called granite is largely quarried on the James river near Richmond, Virginia, but it is not certain whether it is a true granite or one of the gneisses above described.