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). 256. _MICA SLATE, or MICACEOUS SCHISTUS, is a primitive rock of slaty structure, consisting principally of quartz (76) and mica (123)._ Like gneiss, it is rich in ores. It often contains beds of magnetic ironstone (235), galena (239), copper, blende (241), cinnabar (228), and sometimes even gold. It frequently has garnets, and sometimes tourmalines (69), interspersed in different parts of it. Mica slate occurs in many parts of Scotland; the mountain of Schehallien, and the rocky adjacent country, are in a great degree composed of it. 257. _CLAY SLATE is a primitive rock generally of dull blue colour, more or less compact, always slaty, and always stratified._ Under the appellation of clay slate are included _roofing slate_ (120), _whet slate_ (122), _drawing slate_ (121), and some other kinds already described. Few rocks abound more in veins and beds of valuable metals than slate. In different countries it contains ores of tin, lead, cobalt (247), silver, and copper; and gold, and mercury (228) sometimes occur in it. The celebrated quicksilver mines of Idria (228), and the immense mass of copper at Parys mountain in the island of Anglesea (230), are in clay slate. Crystals of pyrites (236), and sometimes garnets (70), and thin layers of quartz (76), and felspar (110), are all occasionally found embedded in it.