212 FRANCIS PARKER SHEPARD 



THE ISOSTATIC DEVELOPMENT OP PLATEAUS 



As to the development of plateaus there has been no attempt 

 on the part of the isostasists to offer an explanation. Plateaus seem 

 to be connected with the formation of great magmatic bodies 

 beneath the surface. If these bodies are formed in place, the 

 expansion could be great enough to cause the uplift without increas- 

 ing the gravity. A broad arching of the crust such as occurs 

 under plateaus might relieve the pressure suiTiciently to cause a 

 zone that was already heated to become molten. As the magma 

 body cooled the plateau might be expected to sink, unless the erosion 

 of the surface was equal in rate to the contraction produced by the 

 cooling of the magmas. Plateaus are not known to have existed 

 through long ages and are probably only temporary features. 



EXPLANATION OF PENEPLAINS AND THEIR REJUVENATION 



Peneplanation of mountain ranges and the subsequent uplift 

 of these peneplains has seemed to many to be an insurmountable 

 argument against isostasy. Even BarreU considered that pene- 

 planation must indicate a successful resistance of the earth's crust 

 to isostatic forces.^ If the base of a range remains at a more or 

 less constant elevation throughout the process of peneplanation 

 and the range is losing weight continually by erosion, it might 

 seem certain that it would be getting out of isostatic adjustment 

 during the process. Bowie's answer to this objection is that a 

 range is being constantly elevated by isostatic adjustment but not 

 as fast as it is being eroded, because as the isostatic flow at depth 

 continues to add material to the base of the range, it adds less 

 material than is being substracted by erosion, because the material 

 at the base of the column is more dense than that at the surface, 

 and thus less has to be added to keep the column in equilibrium 

 (Fig. 3). It seems doubtful whether this factor would be sufficient 

 to produce peneplanation without the transference of tremendous 

 amounts of material, probably tens of miles, because while the 

 mountain column might be getting lower due to the adding of less 

 material at depth, it would be getting higher due to the expansion 



' Joseph Barrel!, Jour, of Geol., Vol. XXII, p. 30. 



