T. F. Jamieson — Oscillation of Land in Glacial Period. 403 



make this assumption, for I do not know of any trustworthy evidence 

 to show whether the glacial conditions were or were not simul- 

 taneous in both hemispheres. At present we cannot tell. 



Supposing, however, we grant an ice-cap at one pole of the size 

 Dr. Croll requires, viz. six miles thick at the centre, or about 32,000 

 feet. This would give a weight of 12,000 lbs. on the square inch, 

 which is equal to 800 atmospheres, and would amount to more than 

 21,000 million tons on the square mile. Could the bottom layers of 

 ice sustain such a weight without liquefying ? Supposing even this 

 to be possible, would not such a weight depress the ground beneath 

 it? Would not the centre of gravity of the earth be more likely to 

 pull the cap down than the cap to shift the centre of gravity ? The 

 earth would require to be a very rigid body indeed to sustain such a 

 weight for thousands of years without yielding. 



I, however, prefer resting my objections on the geological evidence, 

 which I think conclusively shows, from the unequal amount of sub- 

 mergence in adjoining areas, that the circumstance was not occasioned 

 by a rise of the sea, but by a fall of the land. I shall afterwards 

 point out that we have also a good deal of evidence of depression in 

 regions where there was no submergence, and therefore where move- 

 ments of the sea do not enter into the question at all. 



Hypothesis Proposed. 



It seems to me that the facts agree better with the notion that 

 the submergence was due to a sinking of the land, and my idea is 

 that the ice-covered regions were depressed by reason of the great 

 weight of ice placed upon them, and that when the ice disappeared 

 they rose again with extreme slowness, and may have eventually 

 attained nearly their former level ; but in most cases, I believe, 

 some amount of permanent depression probably occurred. 



The amount of depression would depend on two things, viz. : — 



1. The weight of ice. 



2. The elasticity or yielding nature of the area on which it lay. 

 The amount of ice that accumulated upon the land during the 



period of maximum giaciation is now admitted on all hands to have 

 been very great. Whether we ask the geologists of Scandinavia, or 

 of Switzerland, or of Great Britain, or of Canada, or of the United 

 States, all of them will tell ns that the thickness of the ice in these 

 regions must be reckoned by thousands of feet. 



Assuming the specific gravity of the ice to have been 875 com- 

 pared with water as 1000, or in other words to have been seven- 

 eighths of the weight of water,^ then the weight of a mass of ice 

 1000 feet thick, would be 378 pounds to the square inch, or equal 

 to fully 25 atmospheres, and would amount to 678,675,690 tons on 

 every square mile. If the ice was 3000 feet thick, it would at this 

 rate amount to over 2000 million tons on the square mile. If 4000 

 feet thick, it would give a pressure of a hundred atmospheres, or 

 1500 lbs. on the square inch. 



1 A. Helland found the sp. grav. of the Greenland icebergs to be 886. See Quart. 

 Journ. of the Geol. Soc. vol. xxxiii. p. 155, Feb. 1877. 



