21S Fisher — Cause of Changes of Level in the Earth" 1 8 Crust. 



that is one including a coast line, the whole change of level 

 would be indicated on the coast line, although there might be 

 no change in the relative levels of the land and the sea bottom. 



By a somewhat complicated mathematical investigation,* I 

 believe that I have proved that if the continents contain 

 extensive areas nearly at the sea level, as is the case at the 

 present day, the substratum beneath these areas must be more 

 dense than the average beneath the oceans ; and that conse- 

 quently descending currents may be expected to be now in 

 progress beneath those low-land areas ; and on the whole, 

 ascending currents beneath the oceans, the suboceanic crust of 

 which may be partly supported by the upward currents press- 

 ing against it ; and in that case it would rise or fall in accord- 

 ance with the play of the currents. The present general low 

 level of the land areas may probably be exceptional, being the 

 final and temporarily stationary stage of a period of depres- 

 sion. Unfortunately my mathematical method does not lend 

 itself to the comparison of densities in the case of a land area 

 raised above the sea level ; but it seems reasonable to expect 

 that in such a case there would be a reversal of the direction 

 of the convection currents. Thus where land areas can be 

 proved to have existed formerly, where the ocean now rolls, it 

 seems probable that a change in the direction of the currents 

 in the substratum from up to down may be the cause of their 

 subsidence. 



I would therefore hazard the suggestion that the apparent 

 elevation or depression of the continents, which I gather from 

 the recent papers mentioned probably takes place simultan- 

 eously over the whole globe, are caused by the fall or rise 

 of areas of the sea bottom through the play of convection 

 currents. I conclude that during the periods of land elevation 

 there have been no extensive tracks of low land near sea level, 

 but that entire continents have stood higher ; as favored by 

 Hansen and lately considered by Spencer; — unless this is the 

 case, I do not think my suggestion will hold, because the 

 assumption on which I prove the substratum in the ocean to 

 be less dense than that of the land is, that the continental area 

 is at present on the average not much raised above the sea 

 level. If it were much raised I think the lesser density would 

 not appear as the result of the calculation. 



If instead of beneath the ocean an upward convection cur- 

 rent were to occur beneath the continent, the change of level 

 would be local and would affect the superincumbent area only. 

 This would cause the kind of uplift which is found in plateau 

 regions without much crumpling of the strata. 



* Proceedings Cambridge Phil. Soc. , xiii, Part II, p. 108, 1905. 



