I92I. No. II. THE STRAXDFLAT AND ISOSTASY. 305 



isostatic uj^heaval oi the lanrl taking- place acconlin«^- as its surface is 

 fleiuuled by erosion. 



If the density of the rocks eroded from the surface of the crust be 2.6 

 and the density of tlie plastic substratum underlyint^- the rigid crust be 3.5, 

 it woulfi be necessary to erode a surface layer w ith an average thickness 

 of about 385 metres in order to reduce the average height of the land 

 above the sea by 100 metres. i)r()vifled that the isostatic readjustment 

 he complete. 



If there is less difference than assumed above between tlie densitv of 

 the surface recks and that of the plastic substratum, the thickness of the 

 surface layer which has to be er<;dcd in order to retluce the average height 

 of tlie land above tlie sea by 100 metres, will be proportionatelv greater. 

 If the density of the surface rocks be 2.6 and that of the sul)stratum be 3.0, 

 a surface layer with an a\erage thickness of about 770 metres would have 

 to be eroded in orrler to rerluce the average height of the land above the 

 sea by 100 metres. 



As the subaërial and f!u\ial erosion of a high land is most active 

 along the mountain slopes anfl in the river valleys as long as the floors 

 of the latter have sufficiently steep gradients and are sufficientlv elevated 

 above the sea — it is obvious that in many cases the ridges of the 

 mountains may be raised Ijy the isostatic upheaval to levels higher al)ove 

 the sea than those at which the summits of the mountains stoofl before 

 the erosion began. 



The result of the erosion during its first stage ma\- thus be to make 

 the mountain ridges higher above the sea than thev were before, while 

 the valleys are made lower. This may probably continue until the level 

 of the floors of the great river valleys approach the base level, after which 

 time the erosion will actually begin to reduce the height of the mountains 

 above sea-level, and will tend towards mnking the land less uneven. 



Tf, as above, we assume the density of the eroded rocks to be 2.6 

 and that of the i)lastic substratum to be 3.5, the erosion will not begin 

 to reduce the height above sea-level of the mountain ridges before the 

 thickness of the layer of rock removed from above them is as much as 

 74 per cent of the average thickness of rock removed bv the erosion from 

 the whole land surface. 



If the density of the plastic substratum be only 3, the thickness of 

 rock removed from above the mountain ridges would have to be as much as 

 87 per cent of the average thickness of the layer of rock removed by erosion 

 from the whole land surface, in order for the height of the mountain 

 ridges above sea-level to be reduced. 



The relation between the erosit)n, the isostatic movement ami the 

 reduction of the mountain heights ma>- be somewhat altered where there 

 is an active erosion by frost Hocal i,.a3ial erosion) in the higher jKirts of 



Vid.-SelsU. Skrifter. I. M.S. Kl. 1921. No. 11. 20 



