526 Correspondence— Rev. 0. Fisher. 



C0I^I^:Es:P0I^^D:EI^^c:E. 



ON THE DEPEESSION OF ICE-LOADED LANDS. 

 SiK, — To any one who belie"ves in a yielding condition of the 

 earth's crust, the depression of a tract of country upon its being- 

 loaded with any adventitious accumulation of matter appears an 

 inevitable consequence. But I prefer to argue that the depression, 

 which seems invariably to occur under such circumstances, shows 

 that the crust, irregular though its surface be, nevertheless has a 

 form appropriate to its equilibrium, and that there must be a plastic, 

 if not a liquid, substratum to allow of the observed changes of level, 

 when the load is freshly distributed. 



So obvious is the connexion of these conditions, that it must occur 

 to every one who has had his attention drawn that way, that any 

 region when loaded with ice must sink. But since the question of 

 priority in making this suggestion has been raised by Mr. Gardner, 

 I would point out that, as long ago as in 1865, Mr. Jamieson wrote 

 as follows : " It has occurred to me that the enormous weight of ice 

 thrown upon the land may have had something to do with this 

 depression. Agassiz considers the ice to have been a mile thick in 

 some parts of America ; and evei-ything points to a great thickness 

 in Scandinavia and North Britain. We don't know what is the 

 state of the matter on which the solid crust of the earth reposes. 

 If it is in a state of fusion, a depression might take place from a 

 cause of this kind, and then the melting of the ice would account 

 for the rising of the land, which seems to have followed upon the 

 decrease of the glaciers."' 



I have discussed this and kindred phenomena in my Physics of the 

 Earth's Crust ; and have there shown that, if the crust yielded freely 

 to the added weight, using certain probable assumed densities for the 

 crust and substratum, it would require a thickness of 2310 feet of 

 ice to depress the land, so that on its emergence the old shore-line 

 should be found at an altitude of 700 feet, at which height raised 

 shell-beds occur in Scandinavia. 



The subject of mountain attraction, referred to by Mr. Jamieson, 

 in the October number of this Magazine, is also discussed in chap, 

 xi. of my book. 



With respect to the yielding of the crust, I think we cannot but 

 lament, that mathematical physicists seem to ignore the phenomena 

 upon which our science founds its conclusions, and, instead of seek- 

 ing for an admissible hypothesis, the outcome of which, when sub- 

 mitted to calculation, might agree with the facts of geology, they 

 assume one which is suited to the exigencies of some powerful 

 method of analysis, and having obtained their result, on the strength 

 of it bid bewildered geologists to disbelieve the evidence of their 

 senses. Such appears to most of us the conclusion, that the earth is 

 excessively rigid from its centre to its surface. For we know that 

 down to quite a recent period it has yielded freely to pressure. 

 bth Oct. 1882. 0. FlSHEE. 



1 Journal of Geological Society, August, 1865, p. 178. 



