VERTICAL UPLIFT OF MOUNTAIN RANGES 



323 



other ranges ; for wherever we find at high altitudes peneplains or marine 

 strata not folded, wherever we find mountain ranges with folded and 

 distorted strata extending to their lateral boundaries, or ranges limited 

 by normal faults, we may be sure that the elevation was due to vertical 

 and not to horizontal forces. 



The above facts militate against the idea that mountain ranges are due 

 to compression resulting from the cooling of the earth ; for the compres- 

 sion would be greatest at the surface, and many ranges have been raised 

 without compression of the strata. 



Relation of Isostasy to Folding and Uplift 



The objection to the principle of isostasy, due to the belief that moun- 

 tain ranges were elevated by horizontal compression, is seen to be invalid. 

 It must not be supposed that isostasy in any way contradicts the well 

 authenticated fact that folded mountain ranges have been greatly com- 

 pressed horizontally ; it merely denies that the great heights of mountains 

 are due to this compression. It is a necessary corollary of the principle 

 of isostasy, in conjunction with observations on the deflection of the 

 plumb-line and the intensity of gravity, that the great vertical movements 

 are due to changes in the density of the underlying mass, though we do 

 not know what causes these changes. Isostasy must not be blamed be- 

 cause it does not explain why the matter under mountain ranges changes 

 its density. The explanation must be sought in totally different lines of 

 investigation. Isostasy is no more capable of explaining it than the prin- 

 ciple of hydrostatic equilibrium is capable of explaining the difference in 

 the densities of oil and water. 



When combined with geological observations, isostasy presents the fol- 

 lowing picture of the course of events leading to a folded mountain range : 

 After the accumulation of sediments to a considerable thickness, forces 

 compress and fold the strata. This necessarily increases the amount of 

 matter in the compressed region and would naturally cause some eleva- 

 tion of the surface. On account of the disturbance of the isostatic 

 equilibrium, forces are brought into play which cause the region to sink 

 and drive out matter from below. When the equilibrium is reestablished 

 the region would be slightly higher than before, on account of the accu- 

 mulation of the lighter surface rock, which lowers the average density of 

 the mass. How great the actual elevation, due to compression, may be 

 depends on the amount of compression and on the relation between the 

 rates of compression and of readjustment of equilibrium. We do not 

 know this relation, and, in the absence of observations of deflection and 



