account for Glacial Submergence. 343 



Consequently, if I be the volume of an ice-sheet and fi its 

 density, M the volume of the magma displaced and a its 

 deusity, we must have 



fiI = <rM. 



The amount of depression of the crust and the corresponding 

 submergence around the ice-covered area will consequently 

 depend upon the lateral dimensions of the displaced magma. 

 The wider these are the less will be the submergence at the 

 edge of the depressed area. 



As the most favourable case theoretically possible, let us 

 suppose the lateral dimensions of the ice-sheet and of the 

 displaced magma to be the same, and that their mean thick- 

 ness and depth are x and z respectively. Then the volumes 

 will be proportional to x and z, and we shall have 



flX = GZ. 



For ice, /jl = 0*9 17 6, and we may put the density of the magma 

 of the substratum at 2*96. Hence 



,-0-9176 3 



that is, in the most favourable case conceivable the submer- 

 gence would be 0*31 times the mean height of the ice-sheet. 

 If the submergence had been 1000 feet, the least possible 

 height of the ice-sheet would have been 3226 feet. On the 

 whole, what we must conclude from the above is that, on 

 this theory, for a submergence of 1000 feet the ice-sheet 

 must probably have considerably exceeded 3226 feet in mean 

 thickness. 



Such a thickness for the ice would not appear to be exces- 

 sive, and points to the theory of subsidence through the weight 

 of the ice as a probable explanation of the phenomena. At 

 the same time it must be remembered that this involves the 

 necessity of a highly plastic, if not liquid, substratum, and 

 affords one further argument towards rendering such a con- 

 dition of the interior probable*. 



We will now consider the second hypothesis, namely that 

 the submergence has been caused by the attraction of the ice- 

 sheet upon the ocean, thereby raising its surface in a slope 

 towards itself and causing submergence in its neighbourhood. 

 Upon the hypothesis of solidity of the interior, the ice-sheet 



* See a paper " On the hypothesis of a Liquid condition of the Earth's 

 Interior considered in connexion with Professor Darwin's theory of the 

 Genesis of the Moon," by the author (Proc. Camb. Phil. Soc. vol. vii. 

 p. o35, 1892), wherein this question is dealt with in relation to the tides. 



2 B2 



