. 1872.) 89 [Perry. 
just the rapidity, of a glacier of to-day; still it moved. And while its 
presumed motion suggests many difficult points—some which cannot be 
readily explained—it involves others which are clear beyond a doubt. 
It should be distinctly remembered that the prevalent conditions of 
the glacial period were on a far grander scale than those connected 
with modern ice-streams, and in some respects very different from 
them. For such differences due allowances ought always to be made. 
Now glaciers have been compared with rivers; and, though there are 
important differences, there are also many points of similarity between 
an ice-stream and flowing water, so many that the comparison is to a 
limited extent just. A large amount of moisture condensing and 
falling on the table lands of a continent at once seeks the lower 
levels. The fact is essentially the same, whether it be in a liquid or 
in a congealed state. In the latter case, of course, the operation is 
slower, but the tendency is in no wise different as respects the point 
in question. This being so, we see that if an immense body of ice 
were lying to the north of New England it would naturally seek, 
though the slope were slight, all the lowlands and depressions. 
Were it lying on a horizontal plain there must be from simple weight 
a tendency to motion, from its centre outward, in all directions over 
the level surface. In case the plain were gently inclined toward a 
given point or points of the compass, we should expect the predomi- 
nence of the tendency to be determined, other things being 
equal, by the inclination. If heat were brought to. bear on 
the edge of the side most inclined, or occupying the lowest 
position, so as readily to waste this part of the mass, the ten- 
dency to motion in this direction would be increased. Should 
this exposed portion of the ice be largely removed by thawing the ten- 
dency must be greatly augmented; for room would be made for, and 
movement imparted to, the remainder of the mass lying in the rear. 
Were there also alternate freezings and thawings, alternate instances 
of contraction and expansion, there must likewise be successive in- 
stances of retardation and acceleration. Meanwhile there should be 
in addition, first, the motion secured by contraction of the ice-wave 
on an inclined plane, and next, the motion generated by the expan- 
sion from freezing, especially if moisture derived from the melting 
of the snows on the surface percolated all the crevices of the ice- 
‘sheet. The simple application of the principles suggested by these 
statements, and of others of a-cognate character which it is not nec- 
