EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 239 



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 

 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, 

 therefore, of interior contraction. (3) The interior of the earth is more 

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

 account of its greater density; and th is 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 coefficient 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 elements 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 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 

 unconformities 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 

 of sediments. During the whole of this time the area of the earth's 



