The Fitld oj Geology— McGee. 203 



been made to explain antecedent deformation except by tbe unten- 

 able "contraction hypothesis;" »nd (2) that the antecedent move- 

 ments are now and ever have been important factors in developing 

 continents and mountains and initiating the various geologic proc- 

 esses whose products represent the material phenomena of the ter- 

 restrial crust. The source of these mysterious movements may be 

 sought; and to this end the quagmire of speculation must be skirted. 



Pratt and others have shown that a plumb-line suspended 

 over the sea-shore is generally defl^^cted seaward, despite the less 

 density of water than land, thus indicating much greater density 

 of the submarine portion of the earth than of its subaerial por- 

 tion; and Fisher and others have shown that when the plumb- 

 line is suspended at the base of a mountain range it may be de- 

 flected away from it despite the great density of the superficial 

 mountain rocks, proving the mean density of the mountain range 

 to be less than that of the adjacent plain. The discovery of this 

 relation and the American induction of consequent deformation 

 led to the development of Button's doctrine of isostacy, according 

 to which the entire terrestrial shell is in a state of hydrostatic 

 equilibrium — the continents floating higher than the sea bottoms 

 because lighter, and the mountain ranges overlooking the adjacent 

 plains, like icebergs the ocean, because their roots are less dense 

 than the medium they penetrate. 



A qualitatively adequate cause for the relation is not far to 

 seek, and has indeed been suggested by Faye: Deep sea soundings 

 have shown that the deeper waters of the ocean are cold, the mean 

 temperature of the ocean bottom being much lower than that of 

 the surface either of water or land. Now, water is a good con- 

 ductor and also a ready conveyor of heat; and it is evident that 

 the ocean bottoms are subjected to more rapid refrigeration than 

 the land, that the terrestrial shell is chilled to the greater depth 

 beneath the sea, and that the cooler suboceanic rocks are, ceteris 

 paribus^ denser than those forming the continents. And this 

 explanation of the observed inequality in density of the earth's 

 shell is at the same time an explanation of antecedent deforma- 

 tion ; for it is evident that with the progress of secular refrigera- 

 tion there is a constant tendency to depress and condense the 

 ocean floor and to relatively uplift and lighten the continents. 



It is true that the existing difference in temperature appears 

 inadequate alone to explain either the difference in density or the 



