IQ2I. No. II. THE STR.WDFLAT AND ISOSTASV. 23I 



by the height of the wave. It is, however, extremely difficult to determine 

 what the height of this upheaval may have been at the time when the ice- 

 caps began to decrease, or, more correctly, when the sinking of the de- 

 pressed areas ceased. A great part of these surrounding areas are now 

 under water. The probability is, however, that owing to the upheaval of 

 these areas we have to add a considerable amount to the thickness of the 

 ice-cap computed from the late-glacial submergence of the land below its 

 present level. 



If the volume of the underlying magma be not altered by any chemical 

 changes, it is cbvicus that the total volume of upheaval must l)e equal to 

 the total volume of depression, without taking into consideration the 

 elastic compression which in this connection is insignificant. If we 

 assume that the area of the depressed land under an ice-cap is equal to 

 one fourth of the area of the upheaved region surrounding the ics-cap, 

 the average height (//) of upheaval must therefore be equal to one fourth 

 of the average height (/;) of depression. As, in the case of equilibrium 

 inside these regions, the pressure should be uniform at a certain level 

 below the earth's surface, this means that 



?/><3 = d: o.g — /1x3 



where d is the average thickness of the ice-cap, 3 the specific gravity of 

 the magma displaced by the depression, and 0.9 the specific gravity of 

 the ice. As 



u = 0.25x7? 

 we have 



0.9 



If for instance the average height (//» of the depression in the whole 

 depressed area were 200 metres, the average thickness of the ice-cap would 

 be 834 metres. If, however, the upheaved area were greater in proportion 

 to the depressed area the thickness of the ice-cap would be less. 



Thirdly, there is a probability that the specific gravity of the rock 

 or magma underlying the rigid crust, at depths of perhaps 120 kilometres 

 below the earth's surface, is somewhat higher than 3. In that case the 

 thickness of an ice-cap computed from its depression of the crust would 

 have to be increased accordingly. 



According to what has been just pointed out, the probability is there- 

 fore that the thickness of the ice-cap was a good deal greater than the 

 amount computed from the submergence of the land. 



In the Christiania region, with a lateglacial submergence of 218 

 metres, the corresponding minimum thickness of the ice-cap, computed 

 as on p. 230. would be 727 metres, and in the Trondhjem region, with a 

 submergence of about 200 metres it would be 667 metres. 



