Earthy Impurities.—Independently, however, of those chemical com binations from which the metals can only be freed by the smelter when treated in the furnace, there are other mechanical impurities scarcely less important, which require to bo partially separated on the mine, and which therefore fall entirely within the province of the miner. Although large masses of the metallic ores, as before noticed, are con centrated in bunches or deposits, confined to particular portions of the vein, them is also a great proportion of the ore in all mines, which is more or less intermixed with the veinstone, or often indeed finely disseminated through it ; and as the expense of fusing this large mass of earthy matter would greatly exceed lthe value of the metal which it contains, while on the other hand it forms too large a propor tion of the produce of most mines to be thrown aside and rejected, great skill has been shown in all mining countries in contriving mechanical processes for effecting its separation as soon aa extracted from the mine. When this separation has been properly accomplished, the metallic residue, before worthless, can be profitably smelted, to the great benefit of all, more especially the poorer class of mines, while those in which the precious metals are worked are entirely dependent for their existence upon the skill and care with which it is performed. The mechanical impurities here spoken of are often sufficiently obvious even in band specimens of the metallic ores, which, unless purposely selected from the richest parts of the vein, will often exhibit thin alter nate layers of the ore and veinstone, or sometimes irregular masses of ore of different sizes which are completely intermixed with and imbedded in sparry and stony matter, and this must be considered the character of a very large proportion of ores in their natural state. It frequently happens too that ores of a worthless character are mixed up with the more valuable ones ; thus, copper and lead are very generally accom panied by iron pyrites and blende, both of which must be regarded as impurities, and therefore separated as far as possible previous to any process in the furnace. The great bulk of the metallic ores, when in their natural situation, constitute in fact a most heterogeneous mix ture, in which the really valuable metal exists only in a small propor tion, chemically combined with one or more mineralising substances, and completely intermixed with sparry and earthy matter and ores of inferior metals. A proper perception of this fact, which is scarcely noticed in works on mineralogy, is most essential to a right under standing of the art of mining, and of the various ingenious pro cesses which in this and other countries have arisen out of it, hav ing for their object the separation and concentration of the metallic matter drawn from the mine, previously to its being submitted to the action of fire.
Preliminary Mining Operations.—As the construction of a mine, or the arrangement of the underground works, must depend in a great measure upon the nature of the mineral deposit to be wrought, if we refer to the great division of mineral deposits into veins and beds, before noticed, it is evident that this construction must be principally of two kinds, adapted to each of the above cases, independently of the less definite processes adapted to the working of irregular mineral masses and fragmentary deposits. Thus, in working a mineral vein, as in a copper or tin mine, the excavations will be formed either verti cally or in a highly inclined position, and pursued laterally, or, as the miner terms it, " upon the course of the vein," while the advanced points tend progressively downwards, or "in depth." In working a mine
ral bed, on the contrary, taking a coal-mirie for example, the principal excavations will be formed horizontally around the pit or shaft by which access is first obtained to the deposit.
As a very large portion of the metallic produce extracted by the miner from the earth, and more especially of the soft metals, copper, tin, and lead, is derived from veins, it is to the former of these arrange ments that attention will be first directed. The working of coal and iron is considered hereafter, together with that of those minerals which are found in more irregular deposits. The general view of mining which falls within the limits of this article may conveniently be divided into the following considerations : the discovery of mineral veins or other deposits; the first opening of a mine and subsequent extension of the workings ; the machinery and other appendages required by these operations, both underground and at the surface, with a brief notice of the surface works, and of mining economy and statistics.
Works of Discovery.—Mineral veins or beds are seldom visible at the surface of the ground, being generally concealed by the thick covering of diluvial matter which is spread over almost every portion of the globe, and hides from our view the solid rocky strata in which they are enclosed. In some cases however where this covering is partially wanting, they may be distinctly traced at the surface, and still more frequently they are rendered visible by the indentation of excavated valleys, and the channels worn by mountain torrents. The same effect is often produced by cliffs on the sea-shore, where veins occur iu that situation, of which there are many examples on the coast of Cornwall.
As mineral deposits however present in most cases no trace of their existence at the surface, certain general indications must be had recourse to for their discovery. The most general of these indications (which can be but briefly noticed here) are furnished by geology, which teaches us that certain metals are most abundantly found in certain rocks, and further points out that they do not occupy any position indiscriminately in those rocks, but are almost exclusively found near their junction with other rocks of a different character, more especially near the contact of igneous masses with sedimentary strata, and that they commonly occur where rocks alternate together, and are broken and dislocated. Thus the tin and copper veins of Cornwall are situated chiefly in a species of clay-slate provincially termed " killas," and either near its junction with protruded masses of granite, or where it is inter sected by channels of a porphyritic rock termed " elvau." In Wales and the north of England the lead veins are chiefly situated in the carboniferous limestone and rocks associated with it, especially in places where they are intersected and broken up by enormous faults and dislocations. Similar circumstances to these are very generally observed in all parts of the world, and it is also well ascertained that the local enrichment of veins is greatly influenced by their intersection with one another; and often indeed closely corresponds with the points of junction.