251. GRANITE or moonstone is a compound rock composed of felspar ([110]), quartz ([76]), and mica ([123]), each in crystalline grains of various size, and promiscuously arranged; sometimes one and sometimes the other of these ingredients predominates, but generally the felspar.
This is one of the most common and most widely extended rocks that are known; and is considered as the foundation on which the secondary rocks are deposited. In Cornwall it is very abundant, and veins both of copper and tin are found in it. Granite forms the summits of the highest mountains in Scotland, of the highest of the Grampian Hills, the Alps, and the Pyrenees; and indeed the loftiest parts of most of the countries of the world. The Logan or rocking stones, in Cornwall, are immense blocks of granite.
The uses of this stone are numerous and important. Millstones, steps, troughs for stamping mills, and innumerable other articles, are made of it. The streets of London are chiefly paved with granite, and its hardness and durability render it peculiarly eligible for this use. Weather has little effect upon it. Consequently, when applied to architectural purposes it is found infinitely preferable to Portland stone, of which nearly all the public buildings of modern date in London have been constructed, and many of which are fast going to decay. This circumstance induced the proprietors of the Waterloo Bridge to adopt granite in the construction of that edifice. Mr. Smeaton also chose it for the outer walls of the Eddystone Lighthouse.
252. Scottish Granite.—Scotland is remarkable for many kinds of granite, some of which are susceptible of an excellent polish. The greatest part of the mountain of Ben Nevis, near Fort William, is composed of a reddish granite, one of the best and most beautiful that is known. This mountain is nearly a mile in perpendicular height, and is said to contain granite enough for all the kingdoms of the earth, although they should be as partial to this stone as the ancient Egyptians were. Columns and obelisks of any size and height might be cut from it: for the rock is one uniform mass, without appearance of strata, division, or fissure of any kind. A convincing proof has been given of the strength and hardness of this granite, in a fragment of several tons’ weight, which fell from nearly the top of a precipice five hundred yards in height, upon a hard and solid rock below, and yet continued entire.
253. Granite of Ingria.—A beautiful red granite is found in some parts of Russia, remarkable on account of the felspar ([110]) that it contains, appearing in round or oval pieces, from half an inch to two inches in diameter. This granite, when polished, exhibits shining spots of round or oval shape, which give to it somewhat the appearance of being studded with precious stones.
The royal summer garden at Petersburg is decorated with a superb colonnade of Ingrian granite. The columns are sixty in number, and each of a single piece twenty feet high, and three feet in diameter. Many of the public buildings in Petersburg are of this granite. An immense block of it thirty-two feet long, twenty-one feet broad, and seventeen feet high, forms the pedestal of the celebrated equestrian statue of Peter the Great, in that city.
254. Graphic Granite.—A singular kind of granite has been discovered in the island of Corsica, and lately near Portsoy in the north of Scotland. The ground of this granite is a whitish or reddish yellow felspar, in which are embedded crystals of quartz each from an inch to an inch and half long, and several lines in diameter. The name of graphic granite was given to it in consequence of an imaginary resemblance which the sections of these crystals have to Hebrew, or Arabic, and sometimes to musical characters.
255. GNEISS is a primitive rock, consisting, like granite, of felspar ([110]), quartz ([76]), and mica ([123]), but differing from that rock in its structure, being slaty.
Mountains of gneiss are not so steep as those of granite, and their summits are usually rounded. Ben Lomond and others in Scotland, and mount Rosa in Italy, are almost wholly of gneiss, as well as the middle part of the Pyrenees. It is not an uncommon rock, but in Britain is of less frequent occurrence than granite.
Many valuable metallic ores are found in veins of gneiss. This rock also sometimes contains crystals of garnet ([70]), and tourmaline ([69]).