the most inflammable substances, carbon and sulphur, when subjected to intense heat in vacuo, undergo no change in their weight or properties. It is only when air or water obtains access to volcanic fire that it can produce effects analogous to those of combustion on the surface. Indeed, it appears probable that volcanic explosions and eruptions are occasioned by the access of water to subterranean fire. A sudden evolution of steam and vapour thus produced, will force a passage to the surface, in those parts where the incumbent rocks offer the least resistance, and the lava and fragments of rock will be ejected with a force, proportionate to the quantity of steam or air suddenly evolved.

CHAPTER XIX.

ON THE REPOSITORIES OF METALLIC ORES.

Metallic Matter disseminated through Rocks.-Masses of Metallic Ore.-Metallic Beds.-Metallic Veins.-Rake Veins.-Flat Veins.-Accumulated Veins.Cross Courses.-The remarkable Structure of the Botallack Mine worked under the Sea. On the Formation of Metallic Ores.-Remarkable Phenomena in Mines. Stream Works.-Gold disseminated in the Sands of Rivers in Africa, and North and South America.-Rocks in which certain Metallic Ores are found.

THE rocks and strata, described in the preceding chapters, are composed of earthy minerals, sometimes combined with a portion of metallic matter, which in such instances may be regarded as a constituent part of rocks. The mineral substances to be described in the present chapter, as forming beds or veins, or irregular masses, or grains imbedded in other rocks, consist of metallic matter either pure, or in combination with sulphur, oxygen, or acids.

The difference of external character between a pure metal and an earth is so great, that we find some difficulty, at first, in conceiving how metallic matter can form beds interstratified with earthy rocks; but the discoveries of modern chemistry have shown, that metallic and earthy minerals are closely allied. Nothing can appear more essentially different than a piece of polished iron and a piece of marble or slate; yet if iron be exposed to the action of air and water it is converted into rust, and in this state is known as ochre; and between ochre and powdered stone there is little difference of external character; nor would any one, unacquainted with chemistry, suspect that ochre was a metallic mineral. The ochre can, however, be easily reconverted into metallic iron: but, to convert the earths into a metallic substance is a difficult process,-yet, it has been effected; and it is further proved, that both earths and alkalies are metallic substances combined with oxygen. The metallic nature of the earths being ascertained, we can no longer be surprised that metallic minerals should be found, intermixed with earthy minerals, in rocks. Iron is found combined with earths in almost all rocks that are not white; and to the presence of iron they generally owe their colour, whether red, brown, or black.

The other metals rarely occur, chemically combined with rocks or strata, but are found either disseminated in grains or irregular pieces, or forming beds between earthy strata, or filling veins that intersect rocks in different directions, as represented Plate IV. fig. 4. a and b.

The metals, except gold and platina, are rarely found pure, but are generally combined, either with sulphur, oxygen, or acids; in this state, they are called ores. When the metals occur pure, they are called native metals: thus we have native gold, native iron, &c.

Metallic ores and native metals are sometimes disseminated in grains through rocks; and when they are abundant, the whole mass of the rock is worked as a mine; but this is seldom the case. Tinstone, or the oxide of tin, is sometimes disseminated, in grains, in granitic rocks in Cornwall, but it is generally in the vicinity of a vein of tin ore, that disseminated grains of tinstone are found in the rock. At Weal Duchy mine, near Callington, silver ore is obtained, both from a vein which intersects the hill, and from the rock itself, at a considerable distance from the vein. From a section of the mine shown me by the proprietor, it appears that in the rock, which is white killas (a silvery clay slate), the ore is disseminated in various parts, or is collected in bunches. The silver is found native in filaments, or in the state of vitreous silver ore, black silver, and ruby silver. Gold frequently occurs in grains, disseminated through solid rocks, or in the sands of rivers. Considerable masses of metallic ore are sometimes found in rocks, particularly of iron ore; but these masses are generally formed by the meeting of numerous veins, or are parts of metallic beds that are greatly enlarged :—they will be described with beds and veins.

Metallic Beds. Some metallic ores occur, taking the form of regular strata in the secondary rocks, or of beds in transition and primary rocks. Ironstone in thin strata, alternates with coal, coalshale, and sandstone, and has been described with the coal strata, in Chap. VIII.

Iron ore often forms beds of considerable thickness, interposed between rocks of gneiss, mica-slate, and slate. Metallic ores, in beds or strata, may be regarded as constituent parts of the rocks in which they occur, and must be cotemporaneous with them; the metallic and the earthy minerals have been deposited at the same time, and have probably, been separated by chemical affinity, during the process of consolidation. Sometimes, the metallic matter is intermixed with a bed of slate, or of other rocks, in such abundance, that the whole bed is worked as a metallic ore. When a bed of metallic matter swells out, irregularly, to a considerable thickness, it forms masses of ore, which, in some instances, attain the magnitude of small mountains;-such are the mountains of iron ore in Sweden and Norway. Metallic beds are, however, of limited extent; they seldom traverse a whole mountain or mountain range, but they gradually or suddenly become narrow and terminate, or in the miners' language wedge out. There are few known beds of metallic ores in England; the principal repositories of metallic matter are in veins. I have however ascertained, that the copper mines formerly wrought in the transition rocks of Cumberland, were beds of copper pyrites, interposed between the beds of the mountains in which they were found, and not intersecting them like veins. The beds of rock being highly inclined, the thin metallic beds between them have been mistaken for veins. I believe that several metallic repositories in

other counties, which have been described as veins, are in reality beds; the distinction between beds and veins not being well understood, they are both called veins by working miners. The manganese mines at Doddiscombe Leigh, in Devonshire, are irregular beds of oxide of manganese, in red sandstone. The iron mine at Dannemora in Sweden is an enormous bed, which has swelled out to the thickness of one hundred and eighty feet, of nearly compact ore. Copper pyrites sometimes occurs in beds; mercury has also been found, disseminated in beds of clay and sandstone. Black oxide of cobalt is found in beds, at Alderly Edge in Cheshire.

Metallic Veins.-Perhaps the reader may obtain a clearer notion of a metallic vein, by first imagining a crack or fissure in the earth, a foot or more in width, and extending east and west on the surface, many hundred yards. Suppose the crack or fissure to descend to an unknown depth, not in a perpendicular direction, but sloping a little to the north or south. Now, let us again suppose each side of the fissure to become coated with mineral matter, of a different kind from the rocks in which the fissure is made, and then the whole fissure to be filled by successive layers of various metallic and mineral substances; we shall thus have a type of a metallic vein. Its course from east to west is called its direction, and the dip from the perpendicular line of descent is called in miners' language the hading of the vein. Thus, it is said to hade or dip to the south or north, &c. Now it is obvious, that if the direction of the vein were changed, or its width increased or diminished, and the hade or dip were increased or diminished also, we should still have all the essential conditions of a metallic vein remaining. Let us now proceed to describe existing metallic veins. They appear to have been, originally, fissures cutting through different beds of rock, that have been subsequently filled with metallic ores, intermixed with other mineral matter, of a different nature from that of the rock which is intersected. Metallic veins are, therefore, considered to be of posterior formation to the rocks in which they are found: and where a vein cuts through different rocks, it is evident that its formation must have been more recent than that of the rocks which it intersects; but, where a vein is found only in one bed of rock, the fissure may have been formed and filled at the period when the rock was consolidated. Metallic veins are found principally in primary and transition rocks, or in the very lowest of the secondary strata: they are often separated from the rocks they intersect, by a thin wall or lining of mineral substances distinct from the rock, and sometimes also by a layer of clay on each side of the vein. The same substance which forms the outer coat of the vein, is also frequently, intermixed with the ore, or forms layers alternating with it: this is called the matrix, gangue, or veinstone. It appears as if the ore and the veinstone had at different times, been formed over each other, on the sides of the vein, till they met and filled up the fissure.

Sometimes, the ore extends in a compact mass, from one side of the vein to the other; but, not unfrequently, there are hollow spaces in veins, called druses, which are lined with crystals; in these cavities, the most beautiful and regular crystalline forms are obtained. Metallic veins, often, divide and unite again, and, sometimes, they separate into a number of smaller branches, called strings. A general idea of the different modes in which metallic veins intersect rocks, and are sometimes intersected by each other, is represented in Plate IV. fig. 4. ‘

To what depth metallic veins descend, is not known, nor is it ascertained, whether they generally grow wider or narrower in their descent. The opinions of miners on this subject are so various, that it may fairly be inferred, that veins differ, in this respect, in different situations. No instances, I believe, have occurred of a vein being absolutely worked-out in depth, though it often grows too poor to repay the labour of working deeper: more frequently, the further descent of the miner is stopped, by the difficulty or expense of removing the water. Veins are seldom rich in ore near the surface, but increase in richness as they descend, and at greater depths become poorer again. When Pryce wrote the "Mineralogy of Cornwall," it was believed that the richest state of a mine for copper in that county, was from eighty to one hundred yards deep; and for tin, from forty to one hundred and twenty yards. This account by no means agrees with the present state of the Cornish mines. Copper and tin are procured in considerable quantities at the depth of four hundred and fifty-six yards, in the Dolcoath mine. The Ecton copper mine, in Staffordshire, is now worked at the depth of four hundred and seventy-two yards: it is the deepest mine in England. The deepest mine that has been worked in Europe, or in any part the world, is one at Truttenberg, in Bohemia, which is one thousand yards below the surface.

of

Metallic veins frequently contain different ores at various depths. Iron ore, copper ore, cobalt ore, and silver ore, succeed each other in some of the mines in Saxony.

In France, there are mines which contain copper ore in the lowest part, silver ore above, and over that iron ore.

In Cornwall, blende, a sulphuret of zinc, frequently abounds in the upper part of veins that become rich in copper as they descend; the blende rarely continuing to any considerable depth. In the same district, tin is also commonly found at a small depth, in veins which afterwards prove rich in copper. "Among other instances that might be quoted, are the two deep extensive copper mines called Huel Unity, and Cook's Kitchen, both of which were at first worked for tin. In both, the tin was soon extracted; but it should be noted as an uncommon circumstance, that in the latter mine, after working to the depth of one hundred and eighty fathoms, first through tin, and afterwards through copper, tin was found again, and has continued