Geological facts; or, the crust of the earth, what it is, and what are its uses

by W. G. Barrett

SECTION OF A MINING DISTRICT.

Here the metalliferous vein, we may suppose, has cropped out on the surface of the ground, or, as the miners say, has “come above grass.” Let us now suppose that the position of this vein of ore, copper, lead, or tin, has been ascertained—that is, how it runs, whether from north to south, or from east to west—and also that the “captains” of the mining district around have given their opinion as to the extent and thickness of this underground vein. The next thing is to obtain this mineral wealth. For this purpose shafts (a a a a) must be sunk, which must reach the vein at a certain depth; then will probably follow cross-cuts (c c c), called adit levels (technically an “additt”), driven, as may be seen, at the lowest convenient point above the level of the highest water of the valley; and these, in connexion with the shaft, will serve the purpose of draining the mine and carrying the ore above ground. It will also be seen, by reference to the diagram, that the shafts of a mine do not always correspond; sometimes they are sunk vertically to meet the vein, sometimes they are commenced in the very outcrop itself. On this matter the best geological lesson is a visit to Cornwall, where the student will see that everything depends on the locality of a mine, the nature of the slope of the hill, or the character of the rock in which the vein appears, and so on. “The act of sinking a perpendicular shaft downwards to a depth where it is calculated the lode should be cut, may seem to require little further skill than is necessary to determine correctly the spot on the surface where the work is to commence. But the process in this way is exceedingly tedious; and in a mine at work, where many galleries already existing are to be traversed, much greater rapidity is desirable. In such a case the shaft is sunk in several pieces (see diagram below), or, in other words, the sinking is commenced at the same time in different levels; and no small skill is required to lay out the work, so that the different portions of the shaft thus formed may exactly fit when they are joined together. An exceedingly small error of measurement, in any one of these various and dark subterranean passages, would, in fact, be sufficient to throw the whole into confusion; but such an accident rarely happens, although works of the kind are common in the Cornish mines.”[^[21]] As an illustration of the immense quantity of water in the mines, we may add—and this is almost as startling as any romantic fiction—that the various branches of the principal level in Cornwall, called “the Great Adit, which receives the waters of the numerous mines in Gwennap, and near Redruth, measure on the whole about 26,000 fathoms, or nearly thirty miles in length; one branch only, at Cardrew mine, extends for nearly five miles and a half, and penetrates ground seventy fathoms beneath the surface. The water flows into a valley communicating with a small inlet of the sea, and is discharged about forty feet above high-water mark.”[^[22]] In this method about forty millions of tons of water are raised by steam-power out of the mines in Cornwall.

EAST WHEAL CROFLY COPPER MINE, CORNWALL.

Here, then, we have seen two of the economic uses of geology in connexion with granite alone; and as we think of these mineral treasures, requiring only the labour and skill of man to bring them out for his service and for the civilization of the world, our boast is in our native land, which, though insular and small, combines within itself everything needful to develop its three sources of national wealth—mining, manufactures, and agriculture—to their highest point. Our boast is not the warrior’s boast, which Shakspeare puts into the mouth of one of his heroes—that this our isle is

“That pale, that white-faced shore,

Whose foot spurns back the ocean’s roaring tides,

And coops from other lands her islanders”—

but rather that, without impropriety or irreverence, the words of Holy Writ may as legitimately be applied to Great Britain as to Palestine. It is a land wherein “thou shalt eat bread without scarceness, thou shalt not lack anything in it; a land whose stones are iron, and out of whose hills thou mayest dig brass.[^[23]] When thou hast eaten and art full, then thou shalt bless the Lord thy God for the good land which He hath given thee.” (Deut. viii. 9, 10.)