673 



MINING. 



MINING. 



674 



very much resemble. The layers of flint, which may be often seen 

 imbedded in chalk wherever a section of this rock is made, will convey 

 a good idea of a mineral-bed. Several of the metals, especially lead, 

 are occasionally found in beds ; coal, clay-ironstone, and rock-salt exclu- 

 sively so ; but the last-mentioned mineral is far less regular and 

 continuous than the former. 



Masses, or " pipe- veins," as they are often termed by the miner, are 

 not so easily defined : the best idea which can be given of them is that 

 of an irregular branching cavity descending either vertically or obliquely 

 into the rock, and filled up with metalliferous matter. Deposits of this 

 nature are less common than the two former classes ; they usually con- 

 tain either copper or lead, and some of the rich oxides of iron appear to 

 belong to a similar formation. 



Fragmentary Deposits occur associated with and indeed forming part 

 of many of the loose superficial beds of sand and gravel which occur in 

 the valleys of mineral districts, consisting of the detritus of the neigh- 

 bouring mountains, which has been washed down from thence at 

 remote geographical epochs. The mineral substances found in these 

 deposits, which may be considered as having originally been derived 

 from veins or beds in the vicinity, are not, in most cases, mixed up 

 indiscriminately with the alluvial matter, their greater specific 

 gravity having occasioned them to be deposited in distinct layers 

 by themselves, usually towards the bottom of the mass. Tin and 

 gold are the metals which most commonly occur in deposits of this 

 kind. 



Geological Position. The above-mentioned mineral and metalliferous 

 deposits are not found promiscuously distributed throughout all rocks 

 or soils ; on the contrary, there are certain rocks, or rather assem- 

 blages of rocks, to which they may be considered as in a great measure 

 peculiar. Granite, porphyry, and the older igneous rocks, generally 

 are metalliferous, and often eminently so ; but mineral deposits are on 

 the whole most abundant in rocks of sedimentary origin, and more 

 especially in and near situations where these two classes of rocks (the 

 igneous and sedimentary ) are in contact, or where a metamorphic struc- 

 ture has, from the action of internal causes, been superinduced upon 

 the latter. It does not appear that nature has confined particular 

 metals to any exclusive kind of rock, yet traces of a general association 

 may still be perceived. Thus, tin, copper, gold, silver, and certain 

 deposits of iron, are most abundant in the class of rocks usually termed 

 primary ; while lead, zinc, mercury, and the earthy ores of iron, are 

 most abundant in the older secondary rocks, which also contain our 

 principal deposits of rock-salt. Veins are of most common occurrence 

 in igneous and primary or metamorphic rocks, in the vicinity of which 

 the derivative fragmentary deposits are therefore most often found. 

 Mineral-beds, although sometimes occurring hi primary rocks, are most 

 common in secondary countries ; and irregular masses, or pipe- veins, 

 are most strikingly developed in limestone districts, where they probably 

 occupy original cavities in the rock. 



Metallic Ores and Mineralising Substances. The state in which the 

 metals are found, the full consideration of which belongs to chemistry 

 and mineralogy, will next require our attention, so far aa it is immedi- 

 ately connected with the business of the miner. It must be well known 

 that the metals are very rarely presented to us in a pure or metallic 

 state, although native masses of copper and iron are occasionally met 

 with ; but gold, from its small affinity for oxygen and other mineralis- 

 ing substances, although frequently alloyed, is never mineralised. The 

 great bulk of the metals then, with the exception of gold, are found in 

 the state of oret, that is, chemically combined with certain mineralising 

 substances, which completely disguise, and, in fact, till separated by 

 metallurgical processes, destroy their usually recognised and useful 

 properties. The most important of these mineralising bodies are oxy- 

 gen and sulphur ; the next in rank are chlorine, and the sulphuric, 

 carbonic, and phosphoric acids. The mode in which they combine 

 with the metals is in binary compounds. Of the former we have 

 examples in iron, lead, and mercury, which, when mineralised by sul- 

 phur, form respectively the following sulphurets, iron pyrites, galena, 

 and cinnabar : we observe the latter in all cases where the metals are 

 mineralised by acids, as in spathose iron-ore, or carbonate of iron, in 

 which one binary compound, the oxide of iron, is united to another 

 element, the carbonic acid. From this naturally compound state, in 

 which the metals almost invariably occur, arises the art of metallurgy, 

 which, although generally considered totally distinct from that of 

 mining, in nevertheless most intimately connected with it. 



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 be 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, there is also a great proportion of the ore in all mines, 

 which is more or leas intermixed with the veinstone, or often indeed 

 finely disseminated through it; and as the expense of fusing this large 

 man of earthy matter would greatly exceed |the 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 dull has been shown in all mining countries in contriving 

 mechanical processes for effecting its separation aa soon as extracted 



ABTB AND SCI. DIV. VOL. V. 



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 hand 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-mine 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. 



Worki 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 in 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 " elvan." 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. 



In addition to the general presumptions furnished by geological 



x x 



