404 T. F. Jamieson — Oscillation of Land in Glacial Period. 



Now we have every reason to believe that from 1000 to 3000 feet 

 was quite a comnion thickness in many of the glaciated regions of 

 Northern Europe. Some authorities indeed place it at a higher 

 figure, while in America both Agassiz and Dana calculate that a far 

 greater depth of ice existed. The latter thinks that on the Canadian 

 watershed it must have been 11,000 feet thick (Manual of Geology, 

 2nd ed. p. 538), and on the northern border of New England he 

 supposes it had a mean thickness of 6500 feet. 



In some of the Fjords of Norway Mr. A. Helland puts it at 6000 

 feet. 



But without insisting on these high figures, it is evident that a 

 thickness of even 3000 feet of ice will give us a weight by no means 

 despicable, a weight which would require a marvellous rigidity 

 indeed in the earth beneath it to sustain such a load without yield- 

 ing in some degree. 



But no substance we are acquainted with is absolutely rigid ; it 

 always yields more or less to pressure, and when the pressure is 

 removed it tends to resume its former position, and will do so more 

 or less perfectly according to the elasticity of its nature. Some 

 bodies are so elastic as to regain their original form completely, 

 without suffering any permanent change ; and physical authorities 

 tell us that practically speaking almost every solid body may be 

 considered perfectly elastic up to a certain point. That is to say, 

 there is generally a limit of constraint from which it will recover 

 when the strain is removed. But if the strain is carried beyond 

 this limit, the body undergoes a permanent change in shape or size, 

 and acquires what is technically called a set from which it does not 

 recover. 



Now, that the crust of the earth is flexible and elastic the pheno- 

 mena of earthquakes sufficiently demonstrate. The surface heaves 

 like the billows of the sea, sometimes causing trees to bend so as to 

 touch the ground with their tops, or tossing up flagstones into the 

 air so as to make them come down bottom upwards. Waves of 

 elastic compression are transmitted many hundreds of miles from 

 the seat of disturbance, as in the case of the great earthquake at 

 Lisbon, when the water of lakes in Scotland rose and fell two feet 

 in vertical height as the shock passed along. All this implies some 

 amount of flexibility, which no doubt will vary a good deal accord- 

 ing to the nature of the strata and materials of which the earth is 

 composed. Geologists seem inclined to believe that whether the 

 great internal mass of the earth be fluid or not, there are, at all 

 events, great pools or lakes of melted matter here and there in the 

 interior, at various depths below the surface ; and if this be so, a 

 region lying above an elastic cushion of this sort would be more 

 likely to yield to pressure than where the substructure is more solid. 

 A verj' small amount of yielding is all that is required, spread over 

 so wide an extent as to be quite inappreciable to the eye when 

 viewed in a section drawn on a true scale ; and that the earth is so 

 absolutely rigid as to sustain with indifl'erence the imposition of a 

 weight of ice thousands of feet thick over areas of almost con- 



