CONDITIONS OF CONTINENTAL ICE. 231 



could, in such a case, under amy circumstances have 



melted. 



But more than this, it must be borne in mind that 



these 9 feet represent the total quantity which could 



be melted during 1 the whole time the sheet was being 

 © © 



formed ; that is, from the time the bottom layer fell 

 in the form of snow on the surface down to the 

 present day. We have no means of ascertaining the 

 length of this period. If we assume it to be 10,000 

 years, and this is probably an under-estimate, then 

 9 feet of ice melted during that period would amount 



to only 1 inch in 92 years, or g^ of an inch annually. 



But whether the period be 10,000 or 5000 years, the 

 quantity is so trifling that it may be practically dis- 

 regarded in the present inquiry. 



Xor is this all ; for if the great mass of the ice be 

 as much as 2° below the freezing-point, which it 

 undoubtedly is, the total amount of heat generated 

 by compression and friction during the 10,000 years 

 would not suffice to raise the temperature of the ice 

 even to the melting-point. 



The Great Diminution in the Tliickness of the 

 Ice- strata, from the top downwards not due, as 

 supposed, either to Compression or to Melting. — 

 The thinness of the lower as compared with the more 

 superficial strata of the ice-sheet is considered by Sir 

 Wyville Thomson to be mainly, if not altogether, due 

 to two causes, — compression and melting of the ice, 

 particularly the latter. " The regularity of this 

 diminution," he says, "leaves it almost without a 

 doubt that the layers observed are in the same 

 category, and that therefore the diminution is due to 

 subsequent pressure or other action upon a series of 

 beds which were at the time of their deposition pretty 



