life. Again, they have attributed to the glaciers the rocks, gravels,
and other material which they have found spread here and there long
distances from the mountains. The transportation of the so-called
erratic rocks has appeared inexplicable in any other way, and the
piles of rock and gravel have been considered so many _moraines_, that
is, deposits of diverse material transported by the glaciers. They do
not regard the probability of other agents taking the place of
glaciers, and undervalue the moving power of water. Water in liquid
state has often produced analogous effects, and it has often been the
error of the glacialists to confound the one with the other. The
erratic rocks and the moraines are undoubtedly the ordinary
indications of the ancient gravels, but, taken isolatedly, they are
not sufficient proof. In order to convince they should be accompanied
with a third indication, which is the presence of striated rocks which
we find in the neighborhood of our actual glaciers. When all these
signs are together then there is hardly a possibility of error, but
one alone is not sufficient, because it can be the effect of another
cause.

No doubt the temperature was really lower at the quaternary age and at
the epoch generally assigned to man's advent in European countries,
but the difference was not so great as some say. A lowering of four
degrees is sufficient to explain the ancient extension of the
glaciers. We can look on this figure as the maximum, for it is proved
to-day that humanity played the main _role_ in the glacial phenomena.
The beds of rivers and the alluvia are there to tell that all the
water was not in a solid state at that time, that the glaciers were
much more extended than in our days, and that the courses of the
rivers were infinitely more abundant. When this is understood we can
reasonably reduce the extension of the ancient glaciers, the lowering