FORMATION OP MOUNTAIN RANGES. 119 



this is not a partial equilibrium resisted by rigidity but enforced by 

 pressure ; it is original and without stress. 



2. Mountain-making Movements. 



I have so recently discussed this subject* that I shall have little more 

 to say now. Mountain ranges are of two types, namely, the anticlinal 

 or typical and the monoclinal or exceptional. The one are mountains 

 of folded structure, determined by lateral thrust, the other of simpler 

 structure and determined by unequal settling of great crust blocks. It 

 is only of the former that I shall speak now. The other or monoclinal 

 type will come up under another head. 



It will not be questioned that mountain ranges of the first type are 

 formed by lateral thrust, however much we may differ as to the cause 

 of such thrust; nor will it be questioned that they are permanent 

 features determined by continuous movement, however much they may 

 be modified by other kinds of movement or reduced or even destroyed 

 by subsequent erosion. I have placed them, therefore, among the effects 

 of primary movements — that is, movements determined by causes affect- 

 ing the whole earth. I have done so because until some more rational 

 view shall be proposed I shall continue to hold that they are the effects 

 of interior contraction concentrated upon certain lines of weakness of 

 the crust and therefore of yielding to the lateral thrust thus generated. 

 The reasons for, as well as the objections to, this view I have already on 

 a previous occasion fully discussed. I wish now only to supplement 

 what I have before said by some further criticisms of the most recent 

 and, some think, the most potent objection to this contractional theory, 

 namely, that derived from the supposed position of the "level of no 

 strain." 



It is admitted that the whole force of this objection is based on the 

 extreme superficiality of this level, and that this in its turn depends on 

 the initial temperature of the incandescent earth and the time elapsed 

 since it began to cool. Both these are admitted to be very uncertain. 

 I have already discussed these in my previous paper and shall not re- 

 peat here; but, as recently shown by Davison,f there are still other ele- 

 ments, entirely left out of account in previous calculations, which must 

 greatly affect the result, and these new elements all concur to place the 

 level of no strain much deeper than previous calculations would make it. 



These neglected elements are the following: (1) The earth increases 

 in temperature as we go down. Now the coefficient of contraction 



* President's address, Am. Asso. Adv. Sci., Madison meeting, 1893. 

 •f Am. Jour. Sci., vol. 47, 1894, p. 480. Phil. Mag., vol. 41, 1896, p. 133. 



