Page:The American Cyclopædia (1879) Volume VIII.djvu/161

 GRANITE 153 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 rela- tions 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 pasty condition from below the latter, a the microscopic study of the minute cav- ities often found in the quartz of granites, rhich are filled with water or saline solutions, >rby has concluded that this rock must have jonsolidated at a temperature in some cases ipproaching a red heat, and under a degree )f pressure which implies that it was at that time buried beneath a very great weight of >ck. There is a popular notion that granite the oldest of all rocks, and is in fact the sub- ratum which underlies all others; but this lea rests upon certain misconceptions, and is jbably incorrect. It is true that it is found making up through the newer crystalline ratified rocks, the primitive slate formations ' some geologists ; but these are seen to rest i an older formation composed in great of highly crystalline gneiss, which, though ra granite-like in its aspect, is clearly strat- ied, and includes beds of quartzite, limestone, ad iron ores. This oldest known series, to rhich the name of Laurentian is given, was by >me of the earlier geologists mistaken for the anite which was supposed to underlie the )wer series, and it has been suggested witn mch probability that it is the gneisses of this Id series, which in a softened condition have een forced upward among the overlying for- lations, where they take the form of unstrati- ied granites. The primitive rock, which is ipposed to have been before all stratified de- sits, is everywhere concealed by these, and m chemical analogies may be supposed to lave been very unlike granite. The so-called lites of the Alps are now shown to be strat- [ rocks of eozoic age, which, by great and >rofound folds have been brought up and made n some cases to overlie the newer strata. (See ALPS.) The dikes of a fine-grained gran- ite, 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 ~ie rocks. Such veins, although often made in great part of feldspar and quartz with lica, are closely related to the veinstones of quartz and calc spar, which are so often the gangue of metallic ores. They frequently pre- sent 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 abun- dance 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 im- perfect skeleton crystals of quartz are so scat- tered through the masses of cleavable ortho- clase, 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 south- ern 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 consid- erable depths. This softening, which. Dolo- mieu called the maladie du granit, and as- cribed to the evolution of carbonic acid from the interior of the earth, depends upon a chem- ical 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 horn- blendes of the gneisses undergo a similar change. According to Sterry Hunt, this decomposition is not recent, and is not connected with an evo- lution of carbonic acid from below, but was effected in remote periods, when the whole at- mosphere 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 beau- ty 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 solid- ity, unbroken by seams and of remarkably uni- form structure. It is seen upon the sides of mountains covering acres, with hardly a crack or seam. But, however massive and unbro- ken it appears, it exhibits when quarried a ten- dency to divide more easily in some directions than in others, and is found to be realiy trav- ersed 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