238 EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



It is evident that, according to either view, but especially according 

 to mine, the material of the ocean basin areas down to the center of the 

 earth must be as much denser than the material of the land areas down 

 to the center as the subocean radii are shorter than the subcontinental 

 radii, and therefore that the two areas must be in perfect static equilib- 

 rium with one another. Thus in the formation of continents the claims 

 of isostasy are completely satisfied. I say completely, because 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 1 that I shall have little 

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

 anticlinal or typical and the inonoclinal 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 

 affecting 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 reason 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 farther criticisms 

 of the most recent and, some think, the most potent objections 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 repeat here; but, as recently shown by Davison, 2 there are 

 still other elements, entirely left out of account in previous calculations, 

 which must greatly affect the result, and these new elements all concur 



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



2 Am. Jour. Sci., Vol. 47, 1894, page 480; Phil. Mag., Vol. 41, 1896, page 133. 



