348 A. KEITH OUTLINES OF APPALACHIAX STRUCTURE 



the high and low sjoots of the crust would cease to exist. If, however, the 

 rocks had the elasticity of a steel spring, an enormous amount of pressure 

 might be stored up to ])e later released under favorable conditions. Such 

 movements, however, would be strictly limited by the capacity of rocks 

 to be condensed and later to expand when free. That rocks can be con- 

 densed and do expand when free from pressure is well known, and the 

 process has been observed many times in quarries and engineering works. 

 The amount of these motions, however, is very small and is doubtless 

 much less than one per cent of the linear dimensions of the body con- 

 cerned. The 200 miles of shortening in the Appalachians would thus 

 require more than 20,000 miles of background, if this alone were the 

 cause. It is therefore seen that this process of compression and release 

 does not even begin to account quantitatively for the great lateral move- 

 ments of the crust. 



Another cause appealed to as an agent in supi^ort of isostatic adjust- 

 ment is the increase of heat due to the blanketing of the ocean bottom 

 by the strata and the consequent rise of the isogeotherms and expansion 

 of the bottom. This cause, however, can hardly be considered to favor 

 the process because it diminishes the density of the rock where it oper- 

 ates. Similarly, erosion of the land lowers the surface and is considered 

 to bring the underlying heated beds nearer the surface, thus facilitating 

 expansion and uplift. In this case, too, the rise of isogeotherms wo.uld 

 be counteracted by increased loss through radiation. The lowering of 

 the surface might even be so slow that the radiation loss would be suffi- 

 cient to maintain the position of the isogeotherms. Here, too, the 

 amount of expansion due to increase of heat is a very small matter, even 

 at the surface where expansion is not checked. At depths . beneath the 

 surface, however, how can it be said that there will be any expansion 

 due merely to a few degrees of increased temperature ? It is more likely 

 that the expansion is potential and is absolutely stopped by the infinitely 

 greater force of gravity. Actual expansion is also definitely limited, for 

 no rock can expand if in so doing it must move other rocks and exert a 

 pressure greater than its own crushing strength. 



As was stated above, there can be no doubt of the tendency of the ocean 

 bed to sink under load and of the denuded land to rise. The theory 

 requires that the flow from sea to land be deep-seated, and it obviously 

 does not occur near the surface. Deep-seated flows must, however, en- 

 counter the obstacles of friction and rigidity, which inevitably increase 

 downward. The deeper the flow, therefore, the greater is the demand 

 made on the original slight differences of gravity in order that they may 

 really function as a cause. It would seem as though it would require a 



