340 G. W. Bulman — Glacial Geology. 



great thickness of the ice itself prevents the intense external cold 

 reaching such springs, and they may continue to flow during the 

 cold period. We may even speculate on the probability that in 

 those ancient days, so much nearer the period of Tertiary volcanic 

 activity, warm springs were more numerous and powerful than they 

 are to-day. At first sight the suggestion doubtless arises, that the 

 springs would lack their usual source of supply, and early in the 

 glacial period become dried up. But, in the first place, the surface 

 ice is constantly melting, and the water at times sinking down 

 through cracks to the bottom ; and, in the second place, ice melts 

 through pressure and friction such as occur beneath an ice-sheet or 

 glacier. These two sources of supply might still feed the springs, 

 and serv'e to increase the streams formed by them. 



The possibility of stream action beneath a great continental ice- 

 slieet is indicated by a study of that of Greenland. Explorers 

 describe surface rivers cutting for themselves channels between 

 banks of ice, and all finally disappearing by plunging down through 

 openings in the ice to the bottom,^ These rivers then probably 

 flow along sub-glacial channels to the extremity of the ice-sheet, 

 eroding the rocks, and forming fluviatile deposits as they go. 



In connection with this, it may be noted, that it is held by many 

 that more heat was received from the sun during the summer of the 

 glacial epoch than at present, and that consequently this surface- 

 melting, and formation of rivers would be on a larger scale. 



And in " The Ice Age in North America," Dr. Wright notes the 

 occurrence of water-worn debris on the ice of the existing Muir 

 glacier : " Here a vast amount of water- worn debris covers the ice, 

 extending up the glacier in the line of motion for a long distance." ^ 



The streams issuing from the extremity of the ice, indeed, whether 

 glaciers or ice-sheets, are frequently not formed merely by thq 

 melting of the ice at the extremity, but are actually rivers which 

 have run for considerable distances beneath the ice. 



Thus, writing of the glaciers of the Alps, M. Eendu describes 

 streams running beneath the whole of that portion of the ice-river 

 which descends below the line of perpetual snow : 



" When we traverse the Glacier des Bois, at the bottom of each 

 crevasse we admire a little stream of fresh and limpid water, which 

 appears to flow over a surface of emerald. These streams reach 

 the edges of the glacier, lose themselves among the stones of the 

 moraine, and unite again beneath the ice at the bottom of the hollow 

 which contains it. Other streams reach the same destination by the 

 openings which penetrate to the bottom of the glacier ; and, lastly,, 

 springs and waterfalls fall also upon the banks of the icy river, and 

 take the same direction ; so that we are sure that there is a 

 snhterraneous river which flows beneath the whole extent of the 

 glacier, and which comes to light only at its lower extremity." * 



And what we must, I think, regai'd as an example of the mingling 

 of aqueous deposits with glacial occurs in the drift, near London. 



1 . Nordenskiijld, Geol. Mag. Vol. IX. p. 393. « p. 62. 



^ "Glaciers of Savoy," Translation. Edited by Prof. George Forbes, pp. 156-7. 



