82 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 17, 



in the first place, suppose former glaciers in western Europe to have 

 been due to this cause alone, and determine the general elevation of 

 that region which would be necessary to produce such effects, rejecting 

 the supposition, as altogether improbable, that each individual moun- 

 tain which exhibits glacial phsenomena was locally elevated indepen- 

 dently of any corresponding elevation of the surrounding region. I 

 shall then proceed to the hypothetical cases of the previous section. 



24. The present mean annual temperature of the Alps may be 

 taken at 55°*5 F. (art. 13, p. 68), or 23°*5 above the freezing tem- 

 perature. The height of the snow-line is about 9000 feet, and that 

 of the line of 32° F. about 7000 feet, which gives a decrease of 1° F. 

 for about 300 feet. If the surrounding region should be elevated 

 several thousand feet together with the Alps, we may take the de- 

 crease of temperature for 1° from the level of the sea for about 400 

 feet (art. 20, p. 11). This would give the height of the line of 32° 

 above the level of the sea, after the elevation, equal to about 9300 

 feet ; and assuming the height of the snow-line to exceed this, as at 

 present, by 2000 feet, the height of that line would be 11,300 feet. 

 It is probable that the glaciers would not in this case descend quite 

 so far below this line as at present. Suppose them to descend 4300 

 feet below it ; the height of their lower extremities would then be 

 about 7000 feet above the level of the sea. Consequently the gla- 

 ciers would descend into the level of the Lake of Geneva, provided the 

 elevation of the region placed that lake 7000 feet above the sea, or 

 about 6000 feet above its present level. 



Hence, if blocks on the Jura have been transported from the Alps 

 by the agency of ice, the Alps must, according to our present hypo- 

 thesis, have been at least 6000 feet higher than at present, supposing 

 the surrounding region to some extent to have been elevated at the 

 same time. 



The present mean annual temperature of Snowdon is about 49°* 5 F., 

 or 17°^ 5 above the freezing temperature. Assuming the general ele- 

 vation of western Europe, Snowdon would stand on a wide elevated 

 table-land, and we may take the decrease of temperature at 1° F. for 

 about 450 feet*. This would give the height of the line of 32° equal 

 to rather more than 7800 feet, and, supposing the snow-line 2200 feet 

 higher, we have 10,000 feet for the height of this latter line. Now 

 to produce glaciers of considerable magnitude on Snowdon, its summit 

 must probably rise some 1000 feet above the snow-line, or to the 

 height of 11,000 feet. Hence the whole region must be elevated 

 between 7000 and 8000 feet above its present level. The glaciers 

 might then be expected to descend to the table-land at the foot of the 

 Snowdonian mountains, i. e. about 2000 feet below the snow-line. 

 Further descent to any extent would be prevented by the compa- 

 ratively horizontal surface of the immediate foot of the mountains. 



In higher latitudes the required elevation would be smaller ; but, 

 speaking generally, in order that glaciers should exist on our present 



* I take this number greater than for the Alps, because the elevation of the 

 surrounding region would in this case form a higher table-land than that imme- 

 diately around the Alps. 



