This species of limestone occurs in immense beds, and forms a great portion of the mountainous parts of Derbyshire and Scotland; but it does not rise so high, on the sides of mountains, as primitive rocks (250). It often contains veins of valuable metallic ores. When cut and polished, many of the varieties of transition limestone are beautiful marbles; some of them have been already described. 266. _GREY WACKA is a transition rock, composed of pieces of quartz (76), flinty slate, felspar (110), and clay slate (120), cemented together by a basis of clay slate._ _It has various appearances, the pieces being sometimes as large as a hen’s egg, and sometimes so small that they can scarcely be perceived by the naked eye._ When the rocks of grey wacka are not covered by those of any other formation, they form round─backed hills, usually insulated towards the tops, and intersected by deep valleys. They are widely distributed: and are often extremely rich in ores, both in beds and veins. Almost all the mines of copper, lead, and zinc, in the Hartz, are in grey wacka; and, in Transylvania, this species of rock is traversed by numerous small veins of gold. 2. FLOETZ, OR FLAT ROCKS. 267. _OLD RED SANDSTONE, or MILLSTONE GRIT, is a floetz or flat rock, composed of large grains of sand or quartz (76), coloured by oxide (21) of iron, and usually cemented together by a kind of clay._ In several parts of Derbyshire this kind of rock forms the uppermost stratum; and in some places, is known to be 120 yards thick. What are known by the name of _peak millstones_ are formed of millstone grit. They are chiefly obtained from quarries near Nether Padley, in Hathersede, Derbyshire; a very inaccessible part of the country, but where the stone is of better quality than it can elsewhere be procured. These millstones are made of different dimensions, from two feet in diameter, and eight inches thick, to five feet and half in diameter, and seventeen inches thick. Some of the beds of millstone grit, which have spherical stains in them, of light red colour, are said to be infusible; and are consequently a valuable stone for lining the hearths of iron and other furnaces, where an intense heat is required. These are called _firestones_, and Roches quarry, near Upper Town, in Ashover, Derbyshire, is famous for them. The upper beds of this kind of rock are often thin, and capable of division, so as to make excellent _paving stones_, or _flags_. There is a particular bed of it at Stanton, in the Peak of Derbyshire, so porous that it is made into _filtering stones_ for the cleansing of turbid water. 268. _THIRD SANDSTONE, GRITSTONE, or FREESTONE, is another kind of floetz or flat rock, formed of very small agglutinated particles of sand. It is opaque, usually of whitish colour, and found in large masses, of various degrees of hardness._