ISOSTASY AS A RESULT OF EARTH SHRINKAGE 



213 



of the material produced by the relief of pressure in the erosion, 

 and this expansion would oiiset much of the lowering. Neglecting 

 the expansion factor and considering the depth of compensation as 

 67 miles, a difference of density of 6 per cent would necessitate 

 erosion of some 50 miles of material to reduce to sea-level a mountain 

 range 3 miles high. 



Let us see what else could produce peneplanation. If a moun- 

 tain range is being maintained in spite of erosion because of having 

 the curvature of an arch, once this arch is broken or truncated by 

 erosion the lateral forces will no longer be competent to support the 



Fig. 3. — Diagrams illustrating Bowie's idea of the cause of peneplains. A, 

 Mountain column; B, Piedmont plain column. In each case A and B are balanced in 

 weight. 



range and a peneplain may result by cessation of uplift. If the 

 magmatic body beneath a mountain range cools or works its way 

 to the surface, the density conditions will change and the region can 

 no longer remain in equilibrium as a mountain range. If the lateral 

 support which is maintaining a mountain range can be transferred 

 to other zones of weakness, the mountain range will no longer be a 

 rising column and erosion can soon reduce it. Thus there are several 

 ways in which peneplains can be produced without loss of isostatic 

 adjustment. It seems only natural that a region which is held 

 above its surroundings by some abnormal means should be reduced 

 to its previous level by changing conditions. 



There remains to be explained the renewed upHfts of peneplains. 

 Supposing that the peneplain was reduced by the removal of the 



