The intersections of veins are very generally accompanied by a shifting and derangement of the metalliferous vein, the two portions of which on the opposite sides of the cross-vein, are often separated to a very considerable distance. As veins are most productive at or near the points where such intersections occur, the metalliferous mass on which the miner had previously been working is completely lost on coming to a cross-vein ; and it is frequently a work of considerable difficulty to recover it ogain, as the productive vein may be thrown or " heaved " completely out of its former course. The first object to be ascertained, in the search for the dislocated vein, is in which direction the " heave " has taken place, whether to the right hand or the left ; and from ruialogy the miner is generally, though not always, enabled to form a correct judgment on this head. Ile then continues the level upon the cross vein in this direction, till the metalliferous vein is again met with on the opposite side, when the level is continued upon it as before. If the search should be continued for a long distance without success, ho will drive in the other direction, in expectation of meeting with it there.
Ad/M.—Where a vein has been worked by driving a level towards it from a valley or other convenient point on the surface, the drainage to the point of intersection is, of course, complete; and hence in mount einem§ countries, where deep ravines occur, levels may be brought in ono below another, so as to prove the veins; and unwater the mines to a considerable depth, almost superseding the use of machinery for this purpose. Levels thus opening to the surface, and serving for drainage, are termed "thy-levels" or "rulits," and few mines are Nrithout one. in cases where mines have been opened by sinking down from the surface, which is the most common method, an relit is generally commenced from the bottom of some neighbouring valley (see Fig. 2), which is driven towards tho vein with a slight inclination, so that the water limy readily flow through it ; and in large mining districts edits have been formed of enormous length, traversing a considerable number of mines, and carrying off the water to the lowest practicable point of drainage. In driving an adit, if the length be considerable before it reaches the mine which it is intended to drain, the distance is often divided into two or more portions by sinking shafts upon its course, and driving from the bottom of each on reaching the requisite depth. ' The work is thus expedited In proportion to the number of limb; from which operations may be commenced. Adits may often be made valuable as works of discovery, by making them cross the direction of the veins occuning in the district, so as to intersect them in their course, when they afford the means of trial at a cheap rate, by driving upon them at the points of intersection. In some cases adits may be carried almost entirely upon veins, to which they thus afford an important trial.
It is evident that the shafts and levels may be indefinitely extended in the manner which has now been pointed out, and should the produce of the mine be considerable, the portions of the vein successively laid open continue productive, and other veins be discovered by cross cutting In its vicinity, this extension of the works may be continued for many years. Thus, in the course of time, from a few simple
excavations, we obtain that almost infinite complication of shafts, levels, croea-cuts, and other workings, which characterise the workings of an extensive mine, with all its numerous appendages of pumps, machinery, and buildings.
The operations which have now been described are applicable to all large and regular metalliferous veins situated in countries where no distinct stratification exists, and where, therefore, from the homo geneous nature of the rock, the metallic produce may extend to very considerable depths without any great fluctuation. The 'dis tricts of Cornwall, and those of Germany and Mexico, are chiefly of this class.
Mining in Stratified Districts.—In working mineral veins in dis tinctly stratified countries, this geological feature has a great influence on the arrangement of the subterranean works, as it impresses a peculiar character upon the contents of the veins, the metalliferous portions of which are in great measure confined to certain strata, while the intervening parts are poor and unproductive. Thus, instead of exploring the whole mass of the vein, as in the former ease, it is only necessary to lay open those limited zones which are embosomed in the strata most favourable to enrichment, and to which therefore the workings are chiefly confined. The lead-mines of North Wales, of Derbyshire, and of the North of England, are worked in the car boniferous limestone, and the grits and shales resting upon it, the two former being the productive rocks. In these mines therefore the mode of working above noticed is adopted, and, from the abrupt and mountainous nature of the country, great facility is afforded in carrying on the subterranean works, which are generally executed in the following manner. A point is selected in some valley or ravine where the edges of the strata are exposed to view, and from thence a level is commenced, if practicable, upon the vein itself, and in one of the beds known to be favourable to its enrichment, the progress of which effectually explores its produce, and admits of a convenient extraction of the ore. Should the vein itself not appear in any spot from which it can be directly driven upon, the level is driven as a cross-cut till it is reached, being either carried on one of the productive strata, or in some other bed adjoining them which may afford greater facilities for driving. Whenever bunches of ore are found in the progress of this level upon the vein, excavations are carried upwards and downwards into them, as far as the ore extends, thus laying it open in a convenient manner for extraction. Should the ore extend far enough towards the surface to render it necessary, levels may be driven from the " rises " to render it more accessible, and should another productive stratum be situated at no great height above the first, similar operatibns will be extended into that also. The vein having thus been laid open, the masses of ore are placed in a proper state for working ; they are broken from the vein either by the pick or by blasting, and the works so arranged that the stuff falls at once into the level below, whence it is transported in tram-waggons to the entrance, near which the dress ing-floors are usually placed. Fig. 4 represents the section of a mine worked in the above manner.