SURFACE GEOLOGY. 69 



been questioned by high authority ; but it is also true that they have 

 been received with much favor by both geologists and physicists, and 

 have been generally accepted as affording a rational and plausible ex- 

 planation of phenomena which have hitherto been regarded as inexpli- 

 cable and mysterious. 



THE CAUSE AND MANNER OF MOTION IN GLACIERS. 



The theory that a great ice-sheet once covered much of North America, 

 and moved from the north southward, has been opposed by the argument 

 that there was no declivity down which it could flow ; that is, that the 

 surface over which it has been traced was too nearly level and too irregu- 

 lar to permit a glacier to pass over it moved by gravity ; and that no other 

 vis a tergo could have caused its motion. To which it may be replied, 

 that the record of the existence and reach of one or several great ice- 

 sheets stands graven in solid rock, and is indisputable. Also, that the 

 altitude of the northern highlands has, as we know, been greatly reduced, 

 largely by the action of the glaciers themselves ; and further, that the 

 relative levels of different portions of the glacial track may have been 

 changed by local subsidence or elevation. It should also be said that 

 ice is not an inflexible solid, like wood or stone, but that it is endowed 

 with, a plasticity that makes it comparable rather with resin or pitch. 

 This is shown by the manner in which it flows through valleys, expand- 

 ing and contracting according to the nature of the channel, flowing faster 

 at the surface than at the bottom and sides ; in short, behaving as water 

 does in similar circumstances. If piled high enough, even on a plain, ice^ 

 would unquestionably spread and sink by its own weight. If with a de- 

 pression of temperature snow were now to accumulate to the depth of 

 several thousand feet on the Canadian highlands, it would be compacted 

 below into ice, which would be pressed out on all sides, unless some im- 

 pediment restricted its flow. If impediments resisted its motion in cer- 

 tain directions, it would flow toward the point of least resistance. Dur- 

 ing the ice period the movement of the ice toward the north was pre- 

 vented by a continuous ice-sheet, held in adamantine solidity by perpet- 

 ual cold ; while toward the south it was softened by a mild temperature, 

 and in certain directions no impediment lay in its way, except irregu- 

 larities of the surface, which were relatively small. Hence it flowed out 

 in these directions to points where it was melted. 



The manner in which ice flows has been discussed with more prolixity 

 and bitterness than perhaps any other problem in physics. By Princi- 

 pal Forbes the practical plasticity of ice was called a viscosity ; in other 

 words, a freedom of motion of the particles on themselves, as in pitch ; 



