THE WORK OF GLACIERS 191 



turing and later freezing together (regelation) of the surfaces of the 

 fractures when they again come into contact. Under the influence 

 of pressure a glacier is continually yielding to fractures of all sizes, 

 but after changing the position of its parts as a result of the down- 

 ward movement of the broken fragments, it is again united by regela- 

 tion. The effect of this constant rupture and regelation is thought 

 to cause the glacier to behave like a plastic or viscous body. That it 

 is not plastic is indicated by the failure of the Mer de Glace, moving 

 at a rate of only two feet a day, to withstand a change of slope in its 

 bed of even two degrees without fracturing. 



(4) Melting and Pressure. — The lowering by pressure of the 

 melting point of ice forms the basis of another theory. (Thompson.) 

 At the points of greatest pressure in a glacier melting occurs, and the 

 stress is relieved. The water thus formed moves to a point where 

 there is less pressure and immediately freezes. The forward motion 

 of the whole is, therefore, if the theory is correct, effected by a con- 

 tinual process of alternate melting and freezing. 



(5) Growth of Granules. — Since the crystals or granules of glacial 

 ice vary from one seventh of an inch or less to an inch or even four 

 inches in diameter, it has been held that the growth of the granules 

 of the ice is a leading factor in its movement. 



(6) Other theories of an importance perhaps equal to those pre- 

 sented have been suggested, but none seems to explain all of the ele- 

 ments of the problem. Some of the difficulties have doubtless arisen 

 from the desire to ascribe all of the phenomena of glacial motion to 

 a single cause. The movement of glaciers will undoubtedly be found 

 to be far from simple and to depend upon a number of factors, no 

 one of which is alone competent to produce the characteristic move- 

 ment of large bodies of ice. 



REFERENCES FOR GLACIERS 



Existing Mountain Glaciers 



Folios of the U. S. Geol. Surv., in Montana, California, Washington, Oregon, and 



Wyoming. 

 Gilbert, G. K., — Glaciers and Glaciation: Harriman Alaska Expedition, Vol. 3. 

 Martin, L., — The National Geographic Society Researches in Alaska : Nat. Geog. 



Mag., Vol. 22, 191 1, pp. 537-561. 

 Russell, I. C, — Glaciers of North America. 



Russell, I. C, — Malaspina Glacier: Jour. Geol., Vol. 1, 1893, pp. 219-245. 

 Salisbury and Atwood, — Topographic Maps: Professional Paper, U. S. Geol. Surv. 



No. 60. 



cleland geol. — 13 



