Rarely, indeed, does a skilled student of such phenomena have to
search in any region for more than a day before he obtains indubitable
evidence which will enable him to determine whether or not the field
has recently been occupied by an enduring ice sheet--one which
survives the summer season and therefore deserves the name of glacier.
The indications which he has to consider consist in the direction and
manner in which the surface materials have been carried, the physical
conditions of these materials, the shape of the surface of the
underlying rock as regards its general contour, and the presence or
absence of scratches and groovings on its surface. As these records of
ice action are of first importance in dealing with this problem, and
as they afford excellent subjects for the study of those who dwell in
glaciated regions, we shall note them in some detail.

The geologist recognises several ways in which materials may be
transported on the surface of the earth. They may be cast forth by
volcanoes, making their journey by being shot through the air, or by
flowing in lava streams; it is always easy at a glance, save in very
rare instances, to determine whether fragments have thus been
conveyed. Again, the detritus may be moved by the wind; this action is
limited; it only affects dust, sand, and very small pebbles, and is
easily discriminated. The carriage may be effected by river or marine
currents; here, again, the size of the fragments moved is small, and
the order of their arrangement distinctly traceable. The fragments may
be conveyed by ice rafts; here, too, the observer can usually limit
the probabilities he has to consider by ascertaining, as he can
generally do, whether the region which he is observing has been below
a sea or lake. In a word, the before-mentioned agents of
transportation are of somewhat exceptional influence, and in most
cases can, as explanations of rock transportation, be readily