SCALING OFF BY PKESSUKE.

419

Glace. The protuberances at the base of the cascade aredue not only to the toning down of the ridges producedby the transverse fracture of the glacier at the summit of thefall, but they undergo modifications by the pressure locallyexerted at its base. The state of things represented inFig. 57 is plainly due to the partial pushing of one crumpleover that next in advance of it. There seems to be adifferential motion of the parts of the glacier in the samelongitudinal line; showing that upon the general motion ofthe glacier smaller local motions are superposed. The occur-rence of the seams upon the faces of the slopes seems alsoto prove that the pressure is competent, in some cases, tocause the bases of the protuberances to swell, so that whatwas once the base of a crumple may subsequently form aportion of its slope. Another interesting fact is also ob-served where the pressure is violent: the crumples scaleoff, bows of ice being thus formed which usually span thecrumples over their most violently compressed portions.I have found this scaling off at the bases of all the cascadeswhich I have visited, and it is plainly due to the pressureexerted at such places upon the ice.

Not only at the base of its great cascade, but thi’oughoutthe greater part of its length, the Glacier du Geant is in astate of longitudinal compression. The meaning of thisterm will be readily understood: Let two points, for ex-ample, be marked upon the axis of the glacier; if theseduring its descent were drawn wider apart, it would showthat the glacier was in a state of longitudinal strain ortension ; if they remained at the same distance apart, itwould indicate that neither strain nor pressure was exerted;whereas, if the two points approached each other, which