The fact that the formation of ores is not only almost limited to a zone within a few thousand feet of the earth's surface but is highly spasmodic even there, is one reason why mining is so precarious an occupation. Ore bodies are prone not only to give out suddenly as soon as a mine penetrates below the level of enrichment, but to die out laterally. This is because underground water, whether from above or below, can penetrate the rocks only in fissures or in places where the rock has been shattered and crushed so that it is full of cracks, as happens when mountains are upheaved. That is why so many metallic ores are limited to narrow veins or to small zones on either side of places where the crust has been broken by faulting or shearing.
Why Ores are Abundant in Dry saw above that the dryness of such regions as Arizona and Mexico and the consequent scarcity of vegetation and the exposure of bare rock make it easy to discover whatever ores may exist. This dryness is partly due to mountains which shut out winds and rains from the ocean, thus adding another to the reasons why rugged regions are so important as sources of ore. But the world's largest dry areas owe their dryness to their latitude and continental position. The dryness allows the enrichment of ores by water to take place more effectively than in moist regions. When the deep-seated magmatic waters slowly well upward they finally reach a level where part of the dissolved metals is deposited. This level is much lower in dry regions than in moist, for it bears a definite relation to the level of permanent ground water. Because of the low level of ground water in dry regions the zone in which the enrichment of ores may take place is much thicker than in moist regions.
The Importance of explanation of the reasons why metallic ore are more abundant in rugged and dry regions than elsewhere emphasizes the need of the greatest care in conserving the metals. The happy-go-lucky person who says, " Don't worry! Our descendants will find a way to get all the metals they need," simply dis plays his ignorance. Some valuable minerals may indeed be found by exploring deep down beneath the plains or mountains, but only where the geological structure is favorable. New bodies of ore will also some day be segregated by nature, but that will take millions of years. The main fact that we must face is this: No matter how skillful man may become in exploring the interior of the earth, he is not likely to find many ores at great depths. Beneath the plains fewer ores are likely to be found than in the mountains, although ores may abound where old mountains have been worn to gentle relief, as is the copper and iron region of Lake Superior. Even among the mountains and in the outer ten
thousand feet of the earth's crust where ore deposits are formed, the chances of a constant succession of new discoveries of ore for generation after generation are not good. In scarcely more than half a century the work of mining has gone on so fast that in countries like the United States the best deposits are already known and are often well on the way toward exhaustion. Moreover, a large part of the ore formed in the past has long since been washed away by erosion and forever lost. Thus the prospect of finding new metals to last through the thousands or millions of years which may still be before mankind is slight. Knowledge of how slowly metals are segregated and formed into ores and of how limited are the regions where this is taking place ought to make the world extremely careful in its use of every kind of metal. It also adds to our realization of the value of rugged and dry regions to the future business of the world.
Why Fuels are Rare in Rugged effect of relief on fuels is quite different from its effect on ores. But with fuels, perhaps more than with ores, a wrong impression is likely to be created unless climate is considered h connection with relief. The original formation of coal long ago in geological times seems always to have taken place in regions of very gentle relief. Coal is formed in great swamps which must be surrounded by extensive plains, for otherwise the streams would bring in silt and destroy the purity of the coal. Moreover, if coal is uplifted into mountains, it is rapidly worn away. Of course, this is also true of metallic ores, but coal is relatively soft and easily eroded and it occurs chiefly in fairly moist climates where erosion is active, while ores, being formed at deeper levels and among harder rocks, are not eroded so rapidly. For these reasons most of the world's coal occurs in places like Illinois, England, eastern France, southern Russia, central Siberia, and northern China, where the relief is gentle. Some, to be sure, is found in rugged regions, and the coal which has been subjected to pressure by crustal folding and mountain building forms the best sorts, such as that of Wales, Sakhalin, and especially the anthracite of eastern Pennsylvania. There is not much of this, however, and vast quantities have been lost by erosion.