Contrary to general opinion, most of the coal appears to have been formed in mild temperate climates such as prevailed in middle and even fairly high latitudes throughout most of geological times. Extreme cold or aridity prevents vegetation from growing with sufficient rapidity, while steady high temperature causes the dead plants to decay so rapidly that coal probably cannot be formed even where vegetation is luxuriant as in the Amazon Valley. This illustrates the importance of climatic optima as discussed in the last chapter. The earth's climate today is generally supposed by geologists to be somewhat more severe than when the coal was formed. Hence, most coal is found in regions that now are either fairly cool, like the United States, England and northern China, or else cold like Spitzbergen, Siberia, Alaska, and the islands north of Canada. Coal is correspondingly scarce in warm regions and deserts. Even where it occurs in such regions, as in the thick beds of Sumatra it is usually brown coal of poor quality.
The occurrence of petroleum may perhaps be influenced by climate, but relief is far more important. Just how this liquid oil and its accom panying gas are formed is not yet certain, but apparently both are derived from plants or animals which flourished in the ocean. Suppose that a region where oceanic deposits are full of the remains of such plants and animals is elevated, broken, crumpled and dissected as in a typical mountain region of high relief. Since both oil and gas move through the pores of the many kinds of rock with comparative ease, the tilting of the strata and the deep dissection will allow the supplies of both to seep rapidly away and be lost. Hence, these products are characteristic of relatively gentle topography and of places where the folding or bending of the crust has been only enough to form slight but unbroken arches of rock which may be many miles in diameter and only a few dozen feet in height. Because oil and gas are light they are gradually buoyed
up by the water in the rocks and finally accumulate in the top of the arches, provided a porous oil-bearing rock lies under one that is less porous. The rocks above such gently arched strata may be eroded into fairly rugged relief as in the Pennsylvania oil fields, but the valleys must not be deep enough or the rocks so crumpled and broken as to drain away the oil. Thus new supplies of the fuels are most likely to be found in regions of low relief, just as metallic ores can be most hope fully sought where the relief is rugged and the climate dry. Certain kinds of shale may indeed contain organic matter which can be con verted into oil by heating, and it matters little whether such rocks are only a little disturbed as in Colorado or much folded as in Scotland.
Summary of Effects of Relief.—The importance of relief, as of almost every other factor that influences business, appears greater as we study it more carefully. A rugged relief, as we have seen, diminishes the production of most of the great products that depend on plants and animals. Sometimes it acts through the thinness of the soil, sometimes through transportation, and again through climate. Certain products, however, such as sheep and trees or bushes, are produced in greatest abundance in rugged regions, for they are crowded out of the level regions but can thrive among rough hills and mountains. Upon minerals the effect of relief is even greater than upon plants and animals. Not only are most minerals more easily discovered in rugged regions, especially if the ruggedness also tends to cause dryness, but the metals, in distinction from the fuels, are segregated more abun dantly in such regions. Since relief also has a dominating influence upon transportation, it seems to stand second only to climate in its influence upon man's activities.