of marine
formations, has never lain under the deep sea; but that its site must
always have been near land. Even its thick marine limestones are the
deposits of comparatively shallow water."

[33] _Antiquity of Man_, 4th Ed. pp. 340-348.

[34] _The Great Ice Age and its Relation to the Antiquity of Man._ By James
Geikie, F.R.S. (Isbister and Co., 1874.)

[35] This view of the formation of "till" is that adopted, by Dr. Geikie,
and upheld by almost all the Scotch, Swiss, and Scandinavian geologists.
The objection however is made by many eminent English geologists, including
the late Mr. Searles V. Wood, Jun., that mud ground off the rocks cannot
remain beneath the ice, forming sheets of great thickness, because the
glacier cannot at the same time grind down solid rock and yet pass over the
surface of soft mud and loose stones. But this difficulty will disappear if
we consider the numerous fluctuations in the glacier with increasing size,
and the additions it must have been constantly receiving as the ice from
one valley after another joined together, and at last produced an ice-sheet
covering the whole country. The grinding power is the motion and pressure
of the ice, and the pressure will depend on its thickness. Now the points
of maximum thickness must have often changed their positions, and the
result would be that the matter ground out in one place would be forced
into another place where the pressure was less. If there were no lateral
escape for the mud, it would necessarily support the ice over it just as a
water-bed supports the person lying on it; and when there was little
drainage water, and the ice extended, say, twenty miles in every direction
from a given part of a valley where the ice was of less than the average
thickness, the mud would necessarily accumulate at this part simply because
there was no escape for it. Whenever the pressure all round any area was
greater than the pressure on that area, the _debris_ of the surrounding
parts would be forced into it, and would even raise up the ice to give it
room. This is a necessary result of hydrostatic pressure. During this
process the superfluous water would no doubt escape through fissures or
pores of the ice, and would leave the mud and stones in that excessively
compressed and tenacious condition in which the "till" is found. The
unequal thickness and pressure of the ice above referred to would be a
necessary consequence of the inequalities in the valleys