I92I. No. II. THE STRAXDFLAT AND ISOSTASY. 229 



built up of waste. This accumulation of waste will have pressed the crust 

 down to some great extent. On the deeper sea-floor outside there may 

 probably also haye been formed thick layers of deposits, weighing down 

 the crust. But why is there such a comparatively steep and sudden 

 descent from the outer edge of the shelf on to the flatter sea-floor if the 

 shelf and the sea-floor are Ijuilt up of the same kind of material? 



There is also another difficulty with which we are faced. The level 

 surface of the continental shelf, whether cut in solid rock or built up of 

 loose material, indicates an earlier sea-level or rather sea-levels, when 

 either the land stood so much higher than now or the sea-level so much 

 lower. 



In Xorway there is furthermore this difficulty that the height of 

 tlie level of the shelf varies a great deal as was previously mentioned. 

 Outside Romsdal and Sondmor as well as outside Lofoten the level of the 

 continental shelf stands at a depth of lOO to 150 metres below sea-level, 

 and outside A'esterålen and Sen j en the depth below sea-level of the very 

 flat shelf is even less than 100 metres. Outside Helgeland the surface of 

 the continental shelf sinks down to 300 and 400 metres below sea-level. 



Considering the nearly perfect isostasy of the earth's crust, which 

 the crustal movements of Fenno-Scandia indicate, it seems extremely 

 difficult to understand that, rluring the long period when the shelf was 

 formed, the land as a whole can have stood so much higher as the depth 

 below sea-leve! of the continental shelf: and when we consider that the 

 other coasts of the globe must in that case also have been similarly 

 elevated, the idea becomes quite impossil)le. 



If the continental shelves of the world actually indicate one or more 

 earlier sea-levels we would therefore be bound to assume that there have 

 l:)een considerable fluctuations in the level of the Ocean, and that during 

 the periods when the continental shelves were developed it stood perhaps 

 100 to 150 metres lower than now [cf. Xansen, 1904, pp. 200, 211 f.]. 



Fluctuations in Sea-Level caused by the Formation of Ice-Caps. 



I'uring the Ice Ages great ((uantitics of water were accumulated in 

 the extensive ice-caps of the northern and southern hemispheres. Penck 

 [i88r. p. 76, 1894. II pp. 528. 660] estimated that the abstraction of water 

 from the Ocean caused by the glaciation of the northern hemisphere alone 

 sank the general sea surface 67 to 71 metres. Assuming that the glaciations 

 of the northern and southern hemispheres were simviltaneous, Drygalski 

 [1887, p. 274^ calculated that the lowering of the general sea-level thus 

 caused was about 150 metres. The present writer pointed out [1904 

 pp. 211 f.j that the ice-cajxs of the northern hemisphere may probably 

 during their widest extent have had a considerably greater area than 



