34 THE PARALLEL ROADS OF 
tered over Scotland and the other British 
Isles, let us see what general evidence we have 
that glaciers ever existed at all in that realm. 
The reader will pardon me, if, at the risk of 
repetition, I sum up here the facts which, 
from their identity with those produced by 
present glaciers, must be admitted, wherever 
they are found, as proof of their former exist¬ 
ence. Such a summary may serve also as a 
guide to those who would look for glacial 
traces where they have not hitherto been 
sought. 
In the first place, we have to consider the 
singular abrasion of the surfaces over which 
the glacier has moved, quite unlike that 
produced by the action of water. We have 
seen that such surfaces, wherever the glacier- 
marks have not been erased by some subse¬ 
quent action, have several unfailing character¬ 
istics : they are highly polished, and they are 
also marked with scratches or fine strice , with 
grooves and deeper furrows. Where best pre¬ 
served, the smooth surfaces are shining; they 
have a lustre like stone or marble artificially 
polished by the combined friction and pressure 
of some harder material than itself until all its 
