1879-] Relations to that of the Glacial Epoch . 23 
were so gravity could have no tendency to produce motion, 
because the force which moves the ice must not only adt 
horizontally, but a 6 i more in one direction than in another ; 
and this it could not do were the ice of uniform thickness. 
Were the sheet of this uniform thickness the forces adting on 
it would balance each other, and no motion could result. 
If the sheet is to be forced out horizontally along the flat 
surface by its own weight, then there must be a piling up of 
the ice in the interior. If the ice comes from the centre, 
then the pressure must be greatest there ; but in order to 
this the sheet, of course, must be thickest at the centre. 
Supposing it should be asserted that it is not the pressure 
of the particle a that moves the particle b in front of it, and 
the pressure of the particle b that moves the particle c, and 
so on, but that each particle moves by its own weight, we 
are nevertheless led to the same conclusion. The weight of 
the particle (the force of gravity) will not move the particle 
unless the particle is allowed to descend. If a particle 
moves by its own weight from the centre of the sheet to the 
circumference, it must descend : it must pass from a higher 
to a lower level. It must move down an inclined plane from 
the centre to the circumference, but to allow it to do so the 
sheet must be thickest at the centre.* 
If, on the other hand, we adopt the “ Molecular ” theory, or 
the “ Dilatation ” theory of the motion of the ice, or any 
other theory whatever which attributes the motion of the 
ice not to gravity, but to some expansive force adting in the 
interior of the mass, we are equally led to the same conclu- 
sion as to the greater thickness of the sheet at the centre. 
Although such a force will of course tend to push the ice as 
powerfully inwards in the diredtion of the Pole, or centre, 
as outwards in the diredtion of the circumference, yet the 
motion of the ice will always take place in the latter direc- 
tion, and never in the former, for the latter will always be 
the diredtion of least resistance. The tendency of such a 
force is to produce an outward motion of the ice on the 
outer side, but to hold back or prevent such a motion taking 
* That the entire mass of the Antardlic ice down to the bottom is in a state 
of motion, and not simply the upper layers, as some suppose, is demonstrable 
from the fadt that icebergs are stratified down to their base. The iceberg is 
simply a piece broken off the edge of the sheet, and the stratified face of the 
berg is the counterpart of the edge from which it broke off; and as the ice- 
bergs are known to be stratified to ‘;heir base, it proves that the sheet from 
which they were derived is likewise stratified to the bottom. The fadt, there- 
fore, that stratified icebergs are continually breaking off the Antardtic sheet, 
and have been for ages, proves that the sheet down to its bottom must have 
been in a state of outward motion. 
