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Rh information required for the most extensive and important mining operations. Even when the outcrop is oxidized, and

the mineral character and richness of the deposit is altered thereby, it is possible to determine variations in thickness and the extent and distribution of the rich and barren areas by outcrop measurements. Information of this sort obtained by surface exploration is often as conclusive as similar information obtained from underground workings. If the deposit shows great variations in thickness in its outcrop along the surface it is probable that a drift or a slope would show the same thing in depth. If the workable areas are poor, and appear only at long intervals along the outcrop, the chances of discovering richer areas by a shaft are very small.

In many cases underground exploration is necessary. For example, the deposit does not outcrop as in the case of blind veins and flat deposits below the general level of the country; or the outcrop lies beyond the limits of the property or under water or water-bearing formations, or is covered by quicksand, or is deeply buried. For such

buried deposits boring is cheaper than sinking. In the case of coal, salt, iron ore, pyrite and other homogeneous minerals, boring may give all the information required. With a number of holes the average thickness and probable extent of the deposit may be determined, at least approximately. When the deposit is vertical or steeply inclined, horizontal or inclined bore-holes will be necessary. This will increase the cost of boring and will render the holes more likely to swerve from the true direction. In the case of metalliferous deposits of varying thickness or irregular distribution the information from bore-holes is less satisfactory. A large number of holes must be bored to obtain, even approximately, the average thickness and value of the ore and the shape and size of the ore bodies. In extreme cases the results from boring are likely to be untrustworthy and misleading unless the work is done on such a scale that the cost becomes prohibitory.

While the information obtained by surface explorations is always valuable, and sometimes conclusive, as to the value of the deposit, it is usually necessary to supplement and confirm it by underground work. The outcrop of a metalliferous vein is generally more or less altered by oxidation, and often a part of the valuable mineral

has been converted into a soluble form and leached out. These conditions sometimes extend to a considerable depth. Below the oxidized outcrop the vein is often increased in value by secondary enrichment, sometimes to a depth of several hundred feet. In the case of such altered deposits surface exploration alone is likely to be misleading, and it is important to push the underground exploration far enough to reach the unaltered part of the deposit, or at least deep enough to make it certain that there is a sufficient quantity of altered or enriched ore to form the basis of profitable mining operations. As the sinking of shafts or the driving of narrow entries or drifts is expensive, and as the mineral extracted rarely pays more than a small fraction of the cost, it is usual to plan this exploratory work so that the openings made shall serve some useful purpose later. The mistake is often made of sinking large and expensive shafts, or driving costly tunnels, before it is fully proved that the deposit can be worked on a scale to warrant such developments, and, indeed, too often before it is known that the deposit can be worked at all; and in too many cases large amounts of money are thus unnecessarily lost by over-sanguine mine managers. It is, however, often advisable that the money spent in surface or underground exploration should at the beginning be spent for information alone. The information so gained not only determines the value of the deposit, but also serves to indicate the best methods of development and of working. The money so spent, if judiciously used, insures the undertaking against loss by diminishing the mining risk, and is thus analogous to premiums paid to insure against fire or other sources of loss.

Development.—As soon as it appears reasonably certain that the property is workable the mine will be opened by one or more shafts, drifts or tunnels, and the underground passages

for active mining operations will be started. A drift or entry is a horizontal passageway starting from the outcrop and following the deposit. The former term is used in metal-mines and the latter in coal-mining. A tunnel differs from a drift in that it is driven across the strata to intersect the deposit; Either may be used for drainage of the mine workings, in which case it becomes an adit. A mine should always be opened by drift or entry if practicable, as thereby the expense of hoisting and pumping is avoided. Drifts, entries and tunnels find their chief application in mining regions cut by deep valleys. When, however, the deposit lies below the surface the mine must be opened by a shaft. If the outcrop of the vein or bed is accessible the shaft may be inclined and sunk to follow the deposit. This is in general a cheaper and quicker method of development for inclined deposits than by a vertical shaft, and it has the added advantage that much information as to the character of the deposit is obtained as the shaft is sunk. When the deposit lying below the surface is horizontal, or nearly so, or when the outcrop of an inclined deposit is not accessible, a vertical shaft will be necessary. Vertical shafts are better adapted to rapid hoisting, and have therefore somewhat greater capacity, than inclined shafts. They are to be preferred also for very deep shafts, or for sinking in difficult ground. Drifts and inclined shafts following the deposit may prove difficult of maintenance when the workings become large and settlement of the overlying strata begins. Large pillars of mineral should be left for the protection of the main openings, whether these be shafts or adits. In the case of very thick beds and mass deposits the main shaft or tunnel will Preferably be located in the foot-wall.

Figs. 1 and 2 illustrate the development of a metal-vein by two adits, two inclined shafts in the lode, and by a deep vertical shaft connected with the lode by horizontal cross cuts. The stippled areas represent the ore shoots and the white areas the barren portions of the lode. The levels are supposed to be 10 fathoms (60 ft.) apart. As the mine is opened the deposit is subdivided into blocks of convenient size by parallel passages, which form later the main haulage roads, and by transverse openings for ventilation. In metal-mines the main passages are known as levels, and these are connected at intervals by winzes or small shafts. In coal mines, entries and headings, bords and walls serve similar purposes. The size of the blocks or the distance between the

main passages is determined mainly by considerations of convenience and economy in excavating and handling the