38 THE OPEN BOOK OF NATURE 
plain, may very probably be of different material] 
from the rocks of the ground where they are de- 
posited. For example, a glacier, say, passed down 
a steep slope formed of granite, and picked up some 
fragments of granite on its way. These fragments 
became attached to the under surface of the ice, and 
in their passage over the underlying rock scratched 
it, and became scratched themselves. The glacier 
farther down the slope passed over a different kind 
of rock, say, a slate; the slate was also scratched 
by the fragments of rock, and some loose pieces of 
slate were picked up by the glacier. These frag- 
ments of slate also scratched the underlying rocks, 
and became scratched themselves. By-and-by the 
glacier descended to a plain whose rocks were sand- 
stone ; there it melted, and dropped the pieces of 
granite and slate it had dragged along with it. Now, 
to-day you and I ramble over that sandstone plain, 
and find some ice-marked and scratched boulders of 
granite and slate. We want to know what they are 
doing out of their proper place ; they are foreigners, 
miles away from their homes. Geologists call them 
erratecs. Having found these erratic fellows, we 
should like to trace them to their homes. We 
ramble about with eyes very wide open ; we ascend 
the slope on to the region of slate; and, behold! on 
some exposed surfaces of the rock there are grooves 
and scratches—striations, say the learned scientists. 
We examine these striations carefully; they all 
point in the same direction! A few hundred yards 
farther up the slope there are more exposed surfaces 
and more scratchings, still pointing the same way. 
Still higher we toil, until we reach the granite. What 
