Hakdcastle. — On Crlacier-viotion. 333 



Professor Tyudall's experiments, to which I have alkided, 

 show that wet ice — that is, ice at and near its melting-point — 

 behaves, under sufficient pressure, as a plastic or quasi-fluid 

 substance. The ice at the bottom of a glacier is under both 

 conditions — it is wet, and under pressure. I do not know 

 what pressure is necessary to cause plastic flow in ice perfectly 

 free to move, but it is not verj' great. Suppose we say, for 

 the purpose of illustration, it is 1001b. per square inch, equal 

 to the weight of a column of ice, say, 300ft. high. If, then, 

 we have a portion of a glacier 400ft. deep, the base of which 

 is perfectly free to move, the lower 100ft. will be forced to 

 flow away by the weight of the 300ft. above. Supply a re- 

 sistance to movement everywhere equal to 1001b. per inch, 

 and the glacier must be 700ft. deep in order that the lower 

 100ft. may be squeezed away. 



It appears to me that, taking the glacier as a whole, or any 

 average cross-section of it, the ice at the bottom flows plasti- 

 cally under the weight of the ice resting upon it. In flowing 

 it will obey, however tardily, the laws of hydrostatics, flowing 

 from a region of gi-eater to one of lesser pressure, and, obeying 

 also the law of gravity, will flow preferably downhill. In 

 other words, the glacier and the neve, or icefield, each consists 

 of two mentall)^ sej)arable portions, moving in distinctly dif- 

 ferent ways. The lower portion is caused by the weight of 

 ice above it to move as a viscous fluid ; the upper portion re- 

 mains solid, and is borne along by the Hving stream beneath, 

 just as a mass of drift-ice or of logs is borne along by a river 

 of water. 



It is stated that " the surface-phenomena resemble those 

 of a solid in a state of flow." But every description I have 

 read (it has not yet been my good fortune to see a glacier) 

 shcv.o that this is true only of those portions of the surface 

 which are eiicountering obstructions. Elsewhere the surface- 

 phenomena, as described, are those of a solid in a state of 

 floating transport. Crevasses indicate a spreading of the 

 floatmg load, due to a spreading of the stream beneath : they 

 are closed by subsequent compressions of that stream. Where 

 the surface-ice encounters obstructions, such as a narrowing of 

 the channel, a sharp bend in it, or an island, the force of the 

 streaming ice beneath sets up horizontal pressures in the sur- 

 face sufiicient to produce plasticity in the ice immediately 

 obstructed. All the described peculiarities of surface-move- 

 ment are explicable as the consequences of the motion of a 

 stream-borne load of a substance hard and brittle when free, 

 but plastic under pressure. 



According to this view it cannot be generally true that the 

 surface of the glacier moves faster than the bottom. Never- 

 theless it must be true of a certain part or parts of each 



