A CENTURY OF GEOLOGY. 441 



longer cool or contract, but, tlie interior being still incandesccntly 

 liot, 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 irre- 

 sistible. 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. I^ow, 

 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 EartWs Crust over \Yide 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. Never- 

 theless, they often affect very wide areas — sometimes, indeed, of 

 more than continental extent — as, for example, in the crust move- 

 ments 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 



VOL. Lvr. — 34 



