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.
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]).
III. ALLUVIAL DEPOSITIONS.
269. These are described to comprehend all such substances as have been formed from previously existing rocks, of which the materials have been worn down, by long exposure to the agency of water and air, and afterwards deposited in nearly horizontal beds on the surface of the land. Alluvial deposits have been formed, and are still forming in every quarter of the globe. They occur both in mountainous regions and in flat countries, filling up the valleys or hollows in the one; and often forming vast and extended plains in the other.
They consist of peat, sand, gravel, loam, clay, and other substances.
IV. VOLCANIC ROCKS.
270. Volcanic rocks are composed of such mineral substances as have been ejected from volcanoes, or have been formed by the agency of subterraneous fires, and have undergone certain changes in such fires.
They are of two kinds; the one called pseudo volcanic, such as burnt clay, porcelain jasper, and earth-slag, which have been altered in consequence of the burning of beds of coal in their neighbourhood; and the other, called true volcanic minerals, such as stones, ashes, and lava, which have been thrown out of real volcanoes.
271. It will somewhat tend to illustrate the history of the mineral kingdom, to state, in conclusion, under a tabular form, the relative heights of the principal mountains, or masses of rocks, which occur in the different countries of the world; previously remarking, that the most lofty and magnificent of these, respecting which any account sufficiently authentic has hitherto been obtained, are the mountains of Nepaul and Thibet, in Asia, one of the former being 27,667, and the highest of the latter measuring at least 23,262 feet, or from 4½ miles to 5¼ miles in perpendicular height above the level of the sea. Previously to the knowledge that has lately been attained respecting the Asiatic mountains, those of the Andes, on the continent of South America, had been considered by far the highest in the world. One of them, Chimborazo, is 20,900 feet in height. Of the European mountains, the highest is Mont Blanc, in Switzerland, which measures 15,680 feet, or about 2¾ miles. The loftiest summit within the British islands is Ben Nevis, in Inverness-shire, Scotland, which does not exceed 4,380 feet, or somewhat more than three quarters of a mile; and the great pyramid of Egypt, the loftiest work of human art and industry with which we are acquainted, and which will serve as a point in the scale, measures only 477 feet.