longer cool or contract, but, the interior being still incandesccntly hot, would continue to cool and contract. The interior, therefore, cooling and contracting faster than the exterior crust, the latter following down the ever-shrinking nucleus, would be thrust upon itself by a lateral or tangential pressure which would be simply irresistible. If the earth crust were a hundred times more rigid than it is, it still must yield to the enormous pressure. It does yield along its weakest lines with crushing, folding, bulging, and the formation of mountain ranges.
This is the barest outline of the so-called "contractional theory of mountain formation." Very many objections have been brought against it, some of them answerable and completely answered, but the complete answer to others must be left to the next century. Perhaps the greatest objection of all is the apparent insufficiency of the cause to produce the enormous amount of folding found not only in existing mountains but in the folded structure of rocks where mountains no longer exist. But it will be observed that I have thus far spoken only of contraction by loss of heat. Now, not only has this cause been greatly underestimated by objectors, but, as shown by Davison and especially by Van Hise, there are many other and even greater causes of contraction. It would be out of place to follow the discussion here. The subject is very complex, and not yet completely settled.
We have given, the barest outline of the history of mountain ranges and of the theory of their formation as worked out in the last third of the present century, and, I might add, chiefly by American geologists. So true is this, that by some it has been called the "American theory."
Oscillatory Movements of the Earth's Crust over Wide Areas.—We have already spoken of these as modifying the effect of the ocean-basin-making movements, and therefore now touch them very lightly. These differ from the movements producing oceanic basins on the one hand and mountain ranges on the other, by the fact that they are not continuously progressive in one direction, but oscillatory—now up, now down, in the same place. Again, they do not involve contraction of the whole earth, but probably are always more or less local and compensatory—i.e., rising in one place is compensated by down-sinking in some other place. Nevertheless, they often affect very wide areas—sometimes, indeed, of more than continental extent—as, for example, in the crust movements of the Quaternary period or ice age.
These are by far the most frequent and most conspicuous of all crust movements—not only now, but also in all geological times. If ocean-basin-forming movements are the underlying cause and