YIELDING OF THE EARTH 's CRUST BOWIE. 239 



of material and the isostatic adjustment, if the material eroded and 

 deposited were of the same density as the material which moves 

 horizontally below the depth of compensation, but undoubtedly 

 there is a decided difference between the density of the material be- 

 low the depth of compensation and that which is eroded and de- 

 posited at the surface. It appears reasonable to suppose that the 

 deep material is at least ten per cent denser than the material at the 

 surface. If this is the case, a different volume of material is trans- 

 ferred from sedimentary to erosion blocks below the earth's crust 

 from that which is moved over the earth's surface in the opposite 



R05 \0N pirKfio nof increment of eroded mate rial 



l_ depih- of- -Qpnipsa s 



Probable direction of movement of material to 

 maintain equal weights of earth blocks. 



Fig. 4. — Movements in the earth's crust incident to isostatic adjustment. 



As material is eroded from a high area and carried over the surface of the earth to a 

 lower one and deposited as sediment, the earth block under the sediment sinks deeper into 

 the earth. This causes a deep-seated horizontal movement of material from the sedi- 

 mentary block towards the erosion block. Material enters the bottom of the erosion 

 block to maintain the isostatic equilibrium, thus forcing up the material of the block 

 under the area of erosion. The arrows show the direction of the movement. 



direction. The result of this difference is to lower gradually the sur- 

 face of the blocks that are undergoing erosion. If 1,000 feet of ma- 

 terial is eroded from an area, the thickness of the mass of material 

 that will come into the bottom of the block, as a result of the isostatic 

 adjustment, will be only 900 feet, thus lowering the height of the 

 block by 100 feet. In order to lower the surface of a mountain sys- 

 tem by 3,000 feet, on an average, it is necessary that approximately 

 30,000 feet be eroded from its area. 



It may be said that, while the theory of isostasy does not account 

 for the creation of mountain systems, it apparently does account for 

 the gradual lowering of a mountain system. The lowering of a 

 mountain system to the point at which erosion ceases is termed base- 

 leveling or peneplanation. It is evident that there must be a decided 

 slope to any ground to permit the streams and rivers to transport 

 material. As the lighter material of the mountain area is eroded 

 from the surface and an equal mass but smaller volume of material 



