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 
haye 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 ig 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; 
