EXPLANATION OF THE MOVEMENT 295 



much rise of the column before the increasing density due to cooling 

 halts the downward movement. 



In order to visualize the results of the processes discussed, we may 

 compare the column of rock beneath the mountains to a cake of ice dc 

 floating in water, the surface of which is represented by aa^ of figure 4. 

 As the ice melts at the top under heat applied at c, the column floats 

 upward, maintaining always the appropriate proportion of the mass abo^'e 

 water. Thus the bottom of the column h must rise to h' in order that 

 the top c may be maintained at c'. As the melting of the ice from the 

 top proceeds, the column of ice continues to rise and the top can be 

 reduced to water level d only when the ice is all melted. 



Consider further the material which replaces the rising column of the 

 ice. As the bottom of the column b rises to h' the space &5' is filled with 

 material of the same density as that surrounding the ice. When removal 

 of material has proceeded to such an extent that the top c is reduced to 

 water level at d, the inflow of the denser material has replaced the lighter 

 and the column de will have the same mass as the ice ch. 



If the column he illustrates rock of low density sustained in rock of 

 high density, ac may represent mountains rising above sealevel. As the 

 mountains are eroded away, the column rises so that much more material 

 is removed from them than is represented by their reduction in altitude. 

 The mountains may be baseleveled only when the supporting column has 

 reached the density of the material surrounding it. 



In applying this illustration several principles must be considered, 

 some of which will strengthen and others modify this simple conception. 



As erosion of mountains proceeds, isostatic equilibrium is maintained. 

 Therefore, before baselevel is reached, a much greater amount of material 

 is removed than that originally above sealevel. Field observation con- 

 firms this principle. The amount of material removed in the baseleveling 

 process will. depend on difference in the density of the eroded material 

 and that of the material added to the lower part of the column by the 

 isostatic adjustment.^ 



The rise of the column of rock of low density and the entrance of heavy 



3 After this paper had left the writer's hands a volume by Nansen^ was received, which 

 contains much significant information concerning isostasy. In this volume the claim is 

 advanced that a balance, even more delicate than is advocated by the present writer, is 

 maintained. Nansen shows (page 305) that under this assumption of delicate balance 

 a mass of rock having a density of 2.6 at the surface, with a substratum of density 3.6, 

 a thickness of 385 meters must be eroded to lower the surface 100 meters with respect 

 to sealevel. If the density of the substratum is 3.0, 770 meters must be eroded to lower 

 the surface 100 meters. 



* Fridtjof Nansen: The strandflat and isostasy (Videnskapsselskapets Skrifter. I. Mat. 

 naturv. Klasse, 1921, No. 11), Kristiania, 1922. 

 XX — Bull. Geol. Soc. Am., Vol. 34, 1922 



