1921. Xo. II. THE STRAXDFLAT AND ISOSTASY. 233 



In Xurth America the lateglacial submergence was at least 500 

 metres, probably a great deal more. Computed in the same manner as 

 above (by the formula p. 231) this would correspond to a thickness of 

 the ice-cap of about 2085 metres. If, however, we do not take into 

 account the upheaval of the zone peripheral to the ice-cap, we find ac- 

 cording to the formula p. 230 that the thickness of the ice-cap mav have 

 been 1667 metres only. But this is a minimum value, and it was probablv 

 a great deal thicker. 



If we assume that the area of all Pleistocene ice-caps in the northern 

 and southern hemisphere, during their maximum development was at least 

 50 millions square kilometres, that their average thickness above sea-level 

 was 1000 metres, and that the average thickness of the extended Antarctic 

 and Greenland ice-caps were increased by a similar amount: if we further- 

 more assume that the area of the Ocean be reduced by 4 per cent bv the 

 ice-caps covering some parts of the continental shelves, and by the sinking 

 of sea-level, we find that, by the formation of the Pleistocene ice-caps, 

 the average depth of the Ocean was decreased by an amount of abcut 



50 



— -xiooo -c.g = 130 metres. 

 346 



I consider this to be a minimum value, for the average thickness of 



the greater part of the ice-caps during their maximum development was 



probably more than 1000 metres. It is also probable that the total area 



of all Pleistocene ice-caps during their maximum development was more 



than 50 millions sq. km. Drygalsky [1887^ and Penck '1894 II, pp.528 



and 660^ have estimated the reduction of the Ocean by the formation of 



the Pleistocene ice-caps to have been equal to a water layer 150 metres 



thick. 



Shifts of Sca-lez'cl and Shore-line caused by Changes in the Volume 



of the Ocean. 



\\ e thus see that there must have been an appreciable reduction of 

 the volume of the Ocean during the maximum development of the Pleisto- 

 cene ice-caps, which would cause an immediate sinking of the general 

 sea-level. 



By this shifting of water-masses, however, the equilibrium of the 

 earth's crust is disturbed. What will be the result? In the course of time 

 there will be a gradual and very slow readjustment. If perfect equilibrium 

 be restored, the crust under the ice-caps will be depressed to an extent 

 corresponding to the weight of the ice-caps, and the floor of the Ocean 

 will be upheaved to an extent corresponding to the weight of the re- 

 duction of the volume of the Ocean. 



If the specific gravity of the plastic rock or magma underlying 

 the rigid crust be 3, the upheaval (//) of the general sea-flcor, caused by 

 a reduction {d) of the average depth of the Ocean, would be: 



