Chapter II: Secondary Rocks

264. SECONDARY ROCKS _are composed of, or at least contain within them, the mineralized remains of organic substances_. These must necessarily have been formed at a period subsequent to the formation of those organized bodies the remains of which they enclose; and they have apparently been formed by the deposition of water. Hence it is that, to distinguish them from rocks of the preceding class, they have received the appellation of secondary. They always rest upon or cover primitive mountains, and sometimes lean upon their sides or invest them.

Werner, the celebrated German mineralogist, makes two divisions of secondary rocks. The first of these he denominates _transition rocks_, and states that they are less perfectly crystallized than the primitive rocks; and that they enclose the remains of marine animals, no species of which are at this time known to exist: the other division he terms _floetz_, or _flat rocks_, because they are generally disposed in horizontal or flat strata. Some of the latter contain the fossil remains of marine animals and shells, approaching in character and appearance to the kinds which are now found in the ocean; and others contain shells precisely similar to those now known to exist. These rocks usually occur at the foot of primitive mountains, or in deep valleys.

1. TRANSITION ROCKS.

265. _TRANSITION LIMESTONE is distinguished by containing marine petrifactions of corals, and other zoophytes which are supposed no longer to exist. It often contains veins of calcareous spar, and exhibits a variety of colours, which give to it a marbled appearance._

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._

The name of _freestone_ has been given to this kind of rock, from its capability of being broken or hewn, with nearly equal facility, in any direction. Hence, as well as from its great durability, it is peculiarly esteemed for buildings. It is also formed into cisterns and troughs of various kinds; into pillars for supporting corn ricks; into _rolling stones_; and into _grinding stones_ for cutlers, edge─tool makers, and workers in polished steel. _Paviors’ flags_, or the stones used for the paving of footpaths, yards, kitchens, and out─houses, are generally flat pieces of freestone.

_Scythestones_, or stones for the sharpening of scythes upon, are made of freestone. Considerable numbers of these are wrought in Derbyshire; and the dexterity that is displayed in cleaving and forming them is somewhat remarkable. The workmen use sharp─pointed picks, several very small wedges, and a hammer. A proper block of stone being selected, two or three of these small wedges are set in a row, by gentle blows of the hammer. These blows are successively repeated till the stone splits. The wedges are then set in a straight line into the face of the piece split off, and the stone is cleft again in that direction. In this manner the sub─divisions are continued, until a piece remains of size to make two scythestones, each an inch and a half square, and about twelve inches long. This the workman holds in his left hand, nearly upright; with the point of his pick he traces a deep nick down the middle of first one side and then the other; and then by a slight blow of his pick he separates it, into two, so dexterously, that not more than three or four in a hundred are broken in the cleaving. Such stones as are intended for round rubbers, are first reduced into an octagonal shape by the point of the pick, and then handed over to women and boys, who grind or rub them in a notch formed in a hard stone, until they are of the requisite shape. The square ones are finished by being ground on a flat stone.

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Other rocks, belonging to what is called the floetz, or flat formation, have been already mentioned, under the heads of _lime─stone_ (140), _gypsum_ (192), _rock salt_ (202), _chalk_ (141), and _coal_ (217).