ICSI. No. I]. THE STRANDFLAT AND ISOSTASY. 223 



, 15.156 



depth of the Ocean by an amount ot — ■ ooo >< 0.9 = 22.7 metres, 



361.1 



the area of the Ocean being about 361,100,000 sq. km. and the specific 



gravity of the glacial ice about 0.9. 



If the average thickness of the ice-caps be 1,000 metres, the increase 

 of the Ocean depth caused by their melting would be 37.8 metres. 



The final rise of the general sea-level caused by this increase of the 

 average depth of the Ocean, would be somewhat reduced by the gradual 

 crustal sinking of the sea-floor under the increased weight of the oceanic 

 water masses. If the specific gravity of the plastic rock or magma 

 underlying the crust be 3, the depression (it) of the sea-floor will be 



d 



It ^ — 



3 



where d is the increase of the average depth of the Ocean. Hence, if the 

 increase of the Ocean depth be 22.7 metres, the rise of sea-level will be 

 1 5. 1 metres. It will be 25.2 metres if the increase of the Ocean depth be 

 37.8 metres. This is provided that the crustal sinking of the sea-floor is 

 entirely compensated for by the upheaval of the areas which were covered 

 by the ice-caps. If not, as will generally be the case, the sinking of the sea- 

 floor will be somewhat reduced, as it will then have to be compensated for 

 by the upheaval of the continents surrounding the Ocean (cf. pp. 235 ff.). 



We see that the probable changes in the general sea-level caused by 

 the reduction of the existing ice-caps would be quite sufficient to explain 

 the negative shift of the shore-line after the first formation of the strand- 

 flat, if we add the probable isostatic upheaval of the land caused by the 

 removal from its surface of glacial and interglacial waste. 



The question is, however, whether we can assume that all ice-caps 

 had disappeared when the strandfiat was formed. The probability is that 

 during long preglacial periods, there have been no ice-caps of significance, 

 and that during those periods the Ocean had what we might call its normal 

 average depth, which was somewhat greater than the present one. 



During the warm interglacial periods it is also probable that the ice- 

 caps of the earth may have been essentially smaller than they are now. 

 The difficulty is. however, that the strandfiat has obviously not been 

 developed during these warm interglacial periods, but during periods with 

 a cold climate favouring a vigorous shore erosion by frost. And these 

 periods may have preceded the glacial periods, or have formed their be- 

 ginning, before the crust was pressed down by the weight of the accu- 

 mulating ice. 



The development may also have continued some time after this sub- 

 sidence of the land began, as the sea-level sank simultaneously because of 

 the subtraction of water from the Ocean by the increasing accumulation 

 of water in the ice-caps on land. But it should be noticed that this sub- 



