This page of the book is from "The New Student's Reference Work: Volume 3" by Chandler B. Beach, Frank Morton McMurry and others.
MINING !233 MINING
deposits usually were horizontal to begin with, but the beds of ore, coal etc. may have been tilted the same as the beds of rock above and below. Veins likewise occur in any position, depending on the direction of the fractures in which the vein stuff was deposited.
(c) The third class of ore deposits is represented by gravel, sand or earthy material, in which there is some useful mineral which can be extracted with profit. Thus certain deposits of sand and gravel, like those of Cape Nome and various mountain valleys of the western part of North America, contain gold. Tin ores sometimes occur in the same way. In such deposits mines are opened. The specific gravity both of gold and tin is much higher than that of the associated sand and gravel, and this fact is taken advantage of in mining. Running water is caused to flow over the gravel and sand with a current which can be regulated. It washes away the lighter material, leaving the desired metals or ores behind. This is the principle of the method known as placer mining. Mines in these loose surface materials are placer mines. Certain other surface deposits are mined, but in a very different way. Thus in certain marshes and bogs bog-iron ore accumulates. This, in reality, is a bedded deposit, but is of recent origin, and not buried beneath later beds of rock or sediment. If worked at all, the ore is taken out bodily. Bog-iron ore is at present but little used.
The Problems of Mining. After the existence of valuable deposits of mineral matter has been determined, many conditions affect the method of mining it. These conditions are so complex that they must be considered individually in the case of each mine. In each case the methods must be adapted to the local situation. The aim always is to extract the maximum of ore with the minimum expenditure of time and money and with the least danger to life and property. The first things to be determined are (i) the shape and position of the ore body, whether it is in distinct beds or veins or disseminated through a large mass of rock; whether the veins or beds are horizontal, vertical or oblique etc.; whether it is near the surface, far from the surface or both; (2) the extent of the ore body; (3) the character of the ore itself as regards hardness, tenacity etc.; (4) the nature of the rock in which the ore occurs, so far as concerns its hardness, texture, structure etc.; (5) the topography of the immediate locality where the mine is located. Questions of another sort, as facilities for transportation to and from the mine, .water supply, fuel supply etc., have to be considered in connection with every mine.
Various means are employed for the determination of the exact position of the ore body. This is the work of exploration.
Exploration is carried on partly by sinking small test pits; partly by stripping the loose material from the surface, exposing the rock which contains the substance to be mined; partly by shafts sunk into the rock; partly by the help of the compass, as in the case of magnetic iron ores; and partly by means of drill holes. In each case the methods of exploration best fitted to the situation should be adopted. The test pits are very much like open, shallow wells. The stripping of the surface is the method often adopted where the mineral vein comes to the surface of the rock, but is covered by soil and other loose débris. Vertical shafts are often sunk alongside the vein, and tunnels or cross-cuts are then run from the shaft across the vein: Veins are often cross-cut in this way in many places by way of exploration. The drills which are used in preliminary tests are hollow iron tubes having an inner diameter of one or two inches. In the "bit" at the lower end of the tube diamonds are set. With the bit resting on the surface of the rock to be drilled, the tube is made to rotate at high speed and cuts its way down into the rock. A cylindrical core of rock appears in the tube or "core barrel" as the drill descends. Water is constantly pumped down the inside of the tube, and rises between the drill rods and the wall rock, washing up the fine grindings of the drill. With the drill it is possible to obtain specimens of the rock at various depths, and this is often of value in the further work of the mine. Drill holes have been made several thousand feet deep. This method of exploration is expensive, but effective.
Another problem with which the miner has to deal is the approach to the ore body. Roughly speaking, mining methods may be divided into two general classes: (1) surface mining or open work and (2) underground mining.
Open work is done where a small amount only of worthless material must be removed in order to uncover the deposit. This method is employed, for example, in the iron mines in the Mesabi range in Minnesota, where the ore is covered by glacial drift 10 to 100 feet thick. Such covering may often be stripped off with steam shovels. Where the ore is soft and friable, as in the case of some of the iron ore of the Mesabi range, it too can be removed by steam shovels; but if the material to be mined is hard, it is blasted and taken out in masses.
In underground work the cheapest and easiest method, if the topography and the position of the material to be mined permit, is to approach the ore by tunnels. This may be done where the ore body lies in the side of a hill or mountain, so that it may be approached from the side. If the ore can be taken out through a tunnel, it