326 



SCIENCE. 



[N. S. Vol. V. No. 113. 



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 ejfec*s of 

 interior contraction concentrated upon cer- 

 tain 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 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 super- 

 ficiality 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 ad- 

 mitted to be very uncertain . I have already 

 discussed these in my previous paper and 

 shall not repeat here; but, as recently 

 shown by Davison,* there are still other 

 elements, entirely left out of account in pre- 

 vious 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 follow- 

 ing : (1) The earth increases in tempet'a- 

 ture as we go down. ]!^ow the coeflQcient 

 of contraction increases with temperature. 

 This would increase the depth of the level 

 of no strain, and also, of course, the amount 

 of interior contraction and, therefore, the 

 lateral thrust. (2) The conductivity in- 

 creases with the temperature. This also 

 would increase the rate of cooling and, 

 thei'efore, of interior contraction. (3) The 

 interior of the earth is more conductive not 



*Am. Jour. Sci., Vol. 47, 1894, p. 480. Phil. 

 Mag., Vol. 41, 1896, p. 133. 



only on account of its greater temperature, 

 but also on account of its greater density ; 

 and this would be true whether the greater 

 density be due to increased pressure or to 

 difference of material, as, for example, to 

 greater abundance of unoxidized metals. 

 (4) The materials of the interior, aside from 

 greater temperature and density, have a 

 higher coefiicient of contraction. (5) The 

 usual calculations go on the assumption 

 that the initial temperature was uniform 

 for all depths. It probably increased with 

 the depth then as now. This would again 

 increase in an important degree both the 

 depth of the level of no strain and the 

 amount of lateral thrust. 



The final result reached by Davison is, 

 that while according to the usual calculations 

 the level of no strain may be only a little 

 over two miles (2.17) below the surface, yet, 

 taking into account only the first element 

 mentioned above, the depth of that level 

 would be increased to nearly eight miles 

 (7.79), and taking into account all the ele- 

 ments it would come out many times greater 

 still. The general conclusion arrived at is 

 that the objections to the contractional 

 theory, based on the depth of the level of 

 no strain, must be regarded as invalid. 

 3. Oscillatory Movements. 



The movements thus far considered 

 are continuously progressive in one direc- 

 tion as long as they last. The resulting 

 features are therefore permanent, except 

 in so far as they may be modified by 

 other movements or by degrading influ- 

 ences ; but nothing is more certain than 

 that besides these more steady movements 

 there have been others of a more oscil- 

 latory character — that is, upward and 

 downward — in the same place, affecting 

 now smaller, now larger areas, and often 

 many times repeated. These are the most 

 common of all crust movements, and are 

 shown everywhere and in all periods of the 

 earth's history by unconformities of the 



