246 DYNAMICAL GEOLOGY. 



glacier around the two rocky islets in the Brenva Glacier (south of Mont 

 Blanc), that the movement was by molecular displacement. 



(d) Slipjnng along planes of bedding or straticulation, or those of the blue 

 bands. — This slipping has been shown to be a fact in several glaciers, by 

 Porel (1889) ; among them, the Bossons Glacier at Chamouui. In the lower 

 part of a glacier these planes have a dip up stream, and as a consequence 

 the mass of the glacier above, as it flows along, rises by slipping along one 

 or more of the planes of lamellar structure. 



Mr. Forel observes that the fact explains the difference of velocity 

 between the upper and lower beds ; the little movement at the extremity of 

 a glacier ; the reappearance, at the surface, of bodies buried in the interior 

 of the glacier; and the preservation of the thickness of the ice at the lower 

 extremity, notwithstanding the annual loss from melting. The cause must 

 have great influence over the direction of crevasses, and in all adjustments 

 to resistances (1889). Guyot described (1832) the up-stream dip of the 

 stratification at the termination of a glacier, and attributed to it the origin 

 of the caverns. 



(e) Sliding along the bottom of the valley. — By the preceding methods, the 

 ice might move by yieldings and adaptations to surfaces, and not necessarily 

 move on the surface beneath so as to abrade it. But the amount of abrasion 

 in glaciated regions shows that these means of yielding and adaptation only 

 help toward an actual flow or sliding of the material along its valley in 

 river-like style. 



(/) Movement through the dilatation of freezing in the permeating tvater of a 

 glacier. — This cause of movement was appealed to by Agassiz, and has been 

 sustained by others. It has taken a new form through Forel, who has sug- 

 gested that movement may be produced by the freezing of water between the 

 large crystalline grains constituting the glacier. Freezing at night, accord- 

 ing to the view, by expanding the mass, would force the glacier forward. 

 The fact that the grains are so much larger in the lower than in the upper 

 part of the glacier is supposed to favor it ; but this is far from conclusive. 

 Forel proposes to give the subject further investigation. 



With regard to the motion of glaciers, Canon Moseley, after experimenting on the 

 shearing force of pure ice, and making it too great to be overcome by gravity alone, pre- 

 sented a view that glaciers descend as a plate of lead descends a sloping surface, through 

 alternate changes of temperature, the movement from expansion by heat being mainly 

 downward because of gravity, and contraction working the same way. To this theory 

 the obvious objection holds, as has been observed, that glaciers do not undergo the needed 

 change of temperature. 



Professor Croll, in his Climate and Time (and in earlier memoirs) accepts Moseley's 

 deduction as to the shearing force of ice, and brings forward a molecular theory to 

 account for the motion of glaciers. He says : " We find that the heat applied to one 

 side of a piece of ice will affect the thermal pile on the opposite side ' ' ; and explains this, 

 not by radiation through the ice, but on the view that the heat applied passes from mole- 

 cule to molecule through the mass ; the transmission of the heat-energy conveyed by A to 

 B melts B, but crystallizes A, and so it goes on through the ice. Gravitation is the source 



