334 Transactions. — Geology. 



glacier. Glaciers either tliiu out near their terminations, or 

 are dammed by high moraines. In the former case certainly, 

 in the latter case usually, the lower part of the glacier will not 

 have weight enough to maintain its base in a hydrostatic con- 

 dition. The terminal end of a glacier is therefore a stranded 

 solid, which acts as a dam to the iiowingand floating ice above. 

 The pressures which these exert upon the dam may be consi- 

 derable, and, as the upper part of the dam will yield most 

 readily, it will be pressed forward over its base. In respect of 

 the stranded terminal portion of the glacier, then, it is true 

 that the top moves faster than the bottom. Similarly, and 

 for the same reasons, it is true of any other portion of a glacier 

 which is " stranded," as on a shoal, or where it approaches a 

 fall. But it is clear that such portions can form no large part 

 of the total length of a glacier. Is it too much to assume 

 that the observations from which it has been inferred that the 

 upper strata move faster than the lower were made on some 

 stranded portions of glaciers ? I think not ; for naturally more 

 importance would be attached to observations which gave a 

 positive than to those which gave a negative result. There is 

 one observation by Principal Forbes which must always have 

 been given great weight, in which he found a considerable 

 acceleration of the upper ice in a vertical side of a glacier 

 exposed as it flowed past a cliff. This, however, can be ex- 

 plained otherwise than by a general law that the surface 

 moves faster than the bottom; and must be otherwise ex- 

 plained, since it is nearly everywhere seen that the surface 

 does not flow, and that forces cannot be found to make it 

 do so. 



No theory of ice-motion which assumes greater mobility of 

 the surface as a normal condition will explain the scooping-out 

 of rock-basins or fiords. The theory here offered explains it 

 readily, as it transfers the scene of greatest activity to the 

 base of the glacier, and the deeper the ice the more energetic 

 will be its action on the rock beneath. 



So far as I can judge, this theory '' fits all the facts." It 

 may be sunnnarized thus : Glaciers and icefields flow through 

 the lower portions, being reduced to plasticity or quasi-fluidity 

 by the weight of the upper portions, and the former in flowing 

 a;Way bear the latter with them. The pressure necessary to 

 effect such reduction at any point, and therefore the critical 

 depth of the ice at that point, depends upon the sum of resist- 

 ances to hydrostatic movement at the base — chiefly upon 

 distance to a point of no resistance, gradient of bed, and 

 amount of obstruction presented by the form of the channel 

 or course of flow. 



