MOUNTAINS AND PLATEAUS 367 



having an equal area of surface and with its apex at the center, 

 contains the same amount of material, which it is impossible ma- 

 terially to increase or decrease. When a large quantity of material 

 is removed from the land by erosion and deposited in the ocean by 

 streams, the increased weight under the ocean and the decrease 

 under the mountains will cause the rock at a great depth to flow from 

 the area which is more heavily weighted, to that from which the 

 weight has been removed, and the approximate equality of material 

 in the segments will thus be restored. 



As the oceanic and continental segments are drawn toward the 

 center of the earth, the surface portions are subjected to great lateral 

 pressure produced by the crowding of the segments against one 

 another, and since the pressure cannot be relieved by the transfer 

 of material by rock flowage such as is possible at great depths, it 

 is relieved by folding and thrust faulting. Since, as has already been 

 shown, the materials of the great mountain ranges were formed from 

 the thick sediments of geosynclines whose basal portions were prob- 

 ably weakened to some extent by the invasion of heat from the 

 interior of the earth, it is clear that, if such thick but weak strata 

 are subjected to great horizontal compression, they will be likely 

 to be folded and faulted. According to the theory of isostasy, how- 

 ever, the folding of strata by lateral pressure could not cause the 

 elevation of a mountain range without the aid of the expansion of the 

 material of which it is composed, since otherwise the quantity of 

 material in the segment would be increased by folding and this 

 added weight would cause a slow sinking, and material would flow 

 from below the heavier segment to the lighter one, until the two again 

 balanced. 



This theory does not tell us definitely the cause of the elevation 

 of mountains and plateaus, but it positively states that the eleva- 

 tion of mountains or the depression of oceanic segments must be 

 due to an increase or decrease of density. The mountains are high 

 because their material is light, and their elevation is due to an ex- 

 pansion of the material in and under them ; the ocean deeps are 

 depressed because the material under them is dense and may be 

 sinking because this material is becoming denser. 



A number of examples of mountain ranges which owe their height 

 to vertical elevation can be cited. The present altitude of the 

 Appalachians, as has been stated, is the result of vertical movement 

 without the aid of lateral pressure, the folding of the strata long 



CLELAND GEOL. — 24 



