24*^ IKIOTJOF NANSKN. M.-N. Kl. 



Is it prohalilc tli.'it llic f^ciicral lc\cl of the Ocean lias been as much 

 lower as llu' (lc\flo])m(nl of these shelves near sea-level wouM necessitate, 

 and llial lliis low level has reniainerl fliirin^ such lon^ i^re^^lacial periorl.-, 

 as wonlil lie r((|uirefl for the planinj,'- of the hroad shelves, provifled that 

 tlicy are fonneil chiefly hy marine anr| suharrial fknudation an'l hv sedi- 

 mentation? 



The fol I o w i n,!^'' consideratifjns may perhaps help to find an explanation. 



The continual deposition of serliments on the sea bottom has a ten- 

 dency to raise the g^cneral sea-level. The rise of level thus caused is to 

 some extent compensated for b\ the i^radual subsidence of th? sea-floor 

 undt'r the weij^ht of the sediment. If the specific ^a'avity of the sediment 

 be 2.5, that of the ma_L;nia underlyinj^- the ri.^-id crust 3, the subsidence (u) 

 of the sea-floor caused by a la\er of sediment with the averajij^'e thickness 

 of 100 metres will be: 



ioo> 2.5 



u = ^ ^- o'i metres. 



3 



As the sediment came from the continents, these would be raised bv 

 an average height (//) of (cf. p. 235 ) 



// = 2.43 - 83 = 202 metres. 



Hence, although the general sea-level would be raised 17 metres, there 

 would be a negative shift of the shore-line. If we assume that the amount 

 of the upheaval of the coast is as much as (^o metres or mirlwa\- between 

 the upheaval of the continents inside and the subsidence of the sea-floo."" 

 outside, the negative shift of the shore-line would be 43 metres. 



These calculations are very rough. On the one hand, we do not kn.ow 

 the specific gravity of the plastic magma underhing the rigid crust, nor 

 do we know the exact specific gravity of the sediment on the sea-floor. 

 The oceanic water contained in this sediment has to be left out of our 

 calculation, as it was in the Ocean before the sediment was deposited. 

 What we wish to know is the specific gravitv and thickness of the sedi- 

 ment without the water. 



On the other hand the processes that actually take place are of course 

 much more complicated than here assumed. The deposit of scdiiuents is 

 distributed very unevenly over the Ocean floor. The deposition of terri- 

 genous matter is largely limited to the regions near the coasts and the 

 deposition of calcareous and silicious sediments (of organic origin) is 

 limited to the regions of the sea-floor which have moderate depths, while 

 there is extremely little deposition going on in the deepest areas of the 

 sea-floor. This means that the depression of the sea-floor caused by the 

 comparatively rapid rleposition of sediment in certain areas is largely 

 compensated for by the upheaval of the sea-floor in adjoining regions 

 wdiere the deposition is much less. In that case the rise of the general 

 sea-level caused l)y the deposition of sediment will naturally be consider 

 ably luore than found by our computation above. 



