120 J. LE CONTE — EARTH-CRUST MOVEMENTS AND THKIR CAUSES. 



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 increases with 

 the temperature. This also would increase the rate of cooling and there- 

 fore of interior contraction. (3) The interior of the earth is more con- 

 ductive not only on account of its greater temperature, but also on account 

 of its greater density ; and this would be true whether the greater den- 

 sity be due to increased pressure or to difference of material, as, for ex- 

 ample, to greater abundance of unoxidized metals. (4) The materials of 

 the interior, aside from greater temperature and density, have a higher 

 coefficient of contraction. (5) The usual calculations go on the assump- 

 tion that the initial temperature was uniform for all depths. It prob- 

 ably 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 elements it 

 would come out many times greater still. The general conclusion ar- 

 rived 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 direction as long as they last. The resulting features are therefore 

 permanent, except in so far as they may be modified by other move- 

 ments or by degrading influences ; but nothing is more certain than that 

 besides these more steady movements there have been others of a more 

 oscillatory 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 uncon- 

 formities of the stratified series. Every line of unconformity marks an 

 old eroded land surface, and every conformable series of strata a sea 

 bottom receiving sediments. We give but two striking examples of such 

 oscillations. 



The Colorado plateau was a sea bottom, continuously or nearly so, 

 from the beginning of the Carboniferous to the end of the Cretaceous, 

 and during that time received about 12,000 or 15,000 feet in thickness 



