RELATIONS TO OTHER SCIENCES 617 



the earth is found to be in an equilibrium similar to that which would 

 characterize the surface if it consisted of a number of floating blocks 

 of different densities, the less dense not sinking as deeply as the more 

 dense. Button has characterized this condition by the name isostasy. 

 It reveals an intimate relation between the greater forms of the earth's 

 surface and the density of the subjacent masses. We know nothing of 

 the more immediate cause of this relation, but we have no reason to 

 doubt that it has always obtained. At first glance the theory of iso- 

 stasy appears as a powerful support for the frequently voiced doc- 

 trine of the permanence of the continents, for it suggests the idea that 

 those masses which to-day are light and, therefore, stand high, have 

 always been so. But it is difficult to conceive how masses that once lay 

 deeply buried, and were, therefore, heavy, now stand high and appear 

 light. The Colorado Plateau of Arizona gives us an example of this. 

 It once formed the sea-floor, and to-day is a highland of horizontal 

 strata. We can only explain its elevated position by assuming that 

 the masses lying under the crust have there suffered a shifting, that 

 the foundation upon which the rock crust floats has changed. The 

 regular change in the character of the products of successive volcanic 

 eruptions which has been proved to occur in many places on the 

 earth's surface seems to argue in favor of the existence of such mag- 

 matic movements. 



Since shiftings of the magma lead to changes in the relative eleva- 

 tions of different portions of the earth's surface, they may also lead 

 indirectly to independent movements of the upper portions of the 

 earth's crust. Such movements must take place if high-lying crustal 

 blocks come to rest alongside of low-lying blocks. If one portion of 

 the crust is brought into such a position that it slopes steeply down 

 to a neighboring, lower-lying one, as the result of vertical dislocation, 

 then horizontal movements will be set up in the crust just as they 

 are in a mass of soil which has been heaped up steeper than its normal 

 angle of repose. E. Reyer many years ago compared the foldings of 

 the earth's strata with the phenomena which appear where great slid- 

 ings have taken place. In fact, there is no lack of evidence that 

 vertical movements have preceded the horizontal ones. Many folded 

 districts of the earth's crust, where the strata exhibit the multiple 

 phenomena of compression rather than the rarer regular folding, 

 originated in districts of subsidence in which enormous masses of 

 sediments were deposited contemporaneously with the shrinking. In 

 such cases it appears that the mountain-making elevation did not 

 immediately succeed the compression of the strata. The Appalachians 

 were folded long before they appeared as mountains. In other cases, 

 also, as, for example, the Alps, the elevatory processes have followed 

 the compression. It is conceivable that a recent change in the sub- 

 crustal masses has caused the formerly sunken land to rise again. 



