METALS AND CIVILIZATION Why Minerals Are Most Abundant among Mountains.—It is fortunate that the earth's intermittent contraction has bent and broken the crust and caused molten materials to move from lower to higher levels. Otherwise many minerals would be practically un known, for metals, which are the most valuable minerals, are heavy. For instance, iron weighs three and gold seven times as much as quartz. Consequently during the earth's cooling the metals seem largely to have sunk into the interior. That is probably one reason why the earth as a whole weighs twice as much per cubic foot as does the outer mile or two of the crust. If the crust had never been bent, broken, and uplifted, and if molten parts of the interior had never been forced upward, the heavier minerals would probably now be almost entirely buried far beyond our reach.
It is equally fortunate that erosion has taken place on a large scale. Otherwise most of the metallic deposits, even though uplifted, would be buried under an enormously thick layer of dense rock. During the lapse of millions of years, however, erosion—especially the work of rivers—has carried away thousands of feet of rock and ex posed many deep-seated deposits. Unfortunately, an immeasurable quantity of valuable minerals has thus been wasted by being carried to the sea as mud, but this is necessary if the underlying rocks are to be exposed.
The processes of bending, breaking, and uplifting the crust, of bringing melted materials from the interior, and of eroding the higher and more exposed rocks are all most active among mountains. There the ores and other deposits have been raised above the ordinary levels, so that they are easily exposed by erosion. Hence mining industries are largely concentrated in regions of rugged relief. Thus in the United States the chief mining regions are in the Sierra Nevadas, the Rocky Mountains, and the Appalachians. The mountainous relief of Arizona is one of the factors in its annual production of minerals worth more than $40,000,000, or a value of over $170 for each inhab itant. In the same way mountainous Montana produces over $175
per inhabitant. Texas, on the contrary, which consists largely of plains, produces minerals worth only $2.50 to $3 per inhabitant while the figure for the flat State of Mississippi is only 70 cents. None of the mineral wealth of Mississippi is metallic.
In some cases, such as the Lake Superior district, with its iron and copper, a mineral region presents almost the gentle relief of a peneplain, but the gently rounded hills still show that they were once mountains of rugged relief.
Mineral fuels, unlike most of the metals, are found in plains as well as in rugged regions. Thus coal is mined extensively in the plains of Illinois and Ohio as well as among the hills of western Penn sylvania. Among mountains, such as those of eastern Pennsylvania, the coal is likely to have been so folded, heated, and pressed that it is changed from the soft, bituminous form of the plains to the hard form known as anthracite.
Petroleum occurs almost entirely in regions where there has been little disturbance of the rocks. The world's great oil fields are often found in plains like those of California, Oklahoma, and Mexico. If the rocks are much bent, the petroleum with the accompanying natural gas is almost sure to escape.
Stages of Mining Industries.—(1) Prospecting.—The first stage of the mining industry is prospecting which includes any kind of search for valuable minerals. Among the mountains of Colorado or California, for example, one now and again meets an unkempt, unshaven prospector driving two or three donkeys laden with samples of rock to be "assayed" or tested. For months he has been camping alone and spending his days prowling among the mountains in search of rock that looks like good ore. Now and again he places a "loca tion" paper in an empty tin can on the end of an upright stick, and leaves it to declare that he claims this particular location. If the ore proves valuable, he will file the necessary documents with the govern ment.