66 EEV. A. IKVING on the 



So far as we have now proceeded in the argument, this difference 

 of velocities would appear to represent the work done within the 

 glacier ; and we might reason in a similar way as to the difference 

 of velocities of the median and marginal portions. 



In the above reasoning I have assumed only the action of a part 

 of the shoving force due to gravity in making the glacier slide upon 

 its bed. As a matter of fact, other causes may promote sliding by 

 diminishing friction, such as (1) the non-contact of the ice with the 

 rocks in places where the glacier streams are flowing, (2) the ther- 

 mal effect of the heat flowing- from the earth's crust by conduction 

 from below, (3) comparatively warm water rising in places from 

 deep-seated springs, (4) the partial liquefaction of the ice by pres- 

 sure against the rocks. All these, by diminishing friction, dispense 

 with some portion of the shoving force due to gravity ; so that the 

 proportion of that force used up within the mass of the glacier is 

 even much greater than the estimate from the difference of velocities 

 alone would give. 



So far as any supposed excavating action is concerned which 

 could form rock-basins, the differential movement of the upper 

 portion of the glacier, as compared with its base, is the most im- 

 portant point. Measurements taken by Prof. Tyndall in the case 

 of the Glacier du Geant, at the foot of the Tacul, showed a move- 

 ment of the portions near the surface more than double that of the 

 base. A fortiori, this differential movement must be greater, owing 

 to the greater retardation of the base of the glacier, when on a 

 horizontal bed — so much so, that the greater pressure acting at right 

 angles to that bed (which varies cceteris paribus with the cosine of 

 the angle of inclination of the bed) would seem to avail nothing, 

 since the movement of the base of a glacier lying upon a horizontal 

 bed would be nil. The only propelling force to which it could be 

 subjected would be the shoving force acting against it from the 

 weight of the glacier lying upon an inclined slope immediately 

 above. Prof. Tyndall * has shown us how this would act. "When 

 the glacier passes from a steeper to a less steep gradient, the 

 crevasses close up, the yielding-property of the ice comes into play, 

 the ice at the surface is thrown into a series of transverse terraces 

 or huge wrinkles, the differential motion is increased so much that 

 stones of the medial moraine, which have fallen into the crevasses, 

 are brought again to the surface. From all which it would appear 

 that the movement of the base of the glacier upon a horizontal 

 bed is nil ; and therefore here, where a theory of excavation most 

 requires it, its erosive action is almost nil. This reasoning seems 

 further confirmed by observations made on the Morteratsch f . 

 Some distance up the glacier the movement, at its maximum, was 

 found to be 14 inches per diem ; yet at the snout, which lies on a 

 nearly horizontal bed, even without any ice in front to offer any 

 resistance to its motion, the movement forward was only 2 inches 

 in a day. It is no reply to this argument to say that, higher up, the 

 erosive power must be greater. The ordinary law of valley-contour, 

 * Forms of Water, p. 18U. t Ibid. pp. 96, 97. 



