s brought to it by the
moving ice. These are not ordinary stream pebbles, for they have
strangely flattened sides which often show scratches, and look as
if they had been ground off against a grindstone. They are the
tools with which the ice does its work. The ice block takes up
the rock fragments which fall upon its surface or which it tears
from beneath, and carries them along, grinding every surface which
it touches. The fragments are dropped at the end of the glacier,
and the smaller pebbles are washed away down the stream that flows
from the melting ice.

[Illustration: FIG. 17.--GLACIER ON THE THREE SISTERS]

We follow up the little glacial creek, past icy snow-banks and
through groves of fir trees where the warm sunshine brings out
the resinous odors. Upon one side of the canon there lies a field
of black lava which not many hundreds of years ago forced this
glacial creek from an earlier channel into its present bed. Now
we come upon what appears at first to be a snow-bank lying across
the course of the stream, and from beneath which its waters issue.
Deep cracks in the outer mass of snow show the clear, pale-green
ice below. This is the lower end of the glacier which we have been
so long a time in reaching.

A short climb up a steep slope brings us to the top of the glacier.
It forms a perfectly even plain, extending back with a gentle slope
to the head of a deep notch between the two northern Sisters, while
above and beyond rise the steeper snow-fields, from which this
ice is continually renewed.

The glacier does not terminate in the usual manner, with a stream
flowing from its centre, for the outlet is at one side, while the
middle abuts against a low mound of rock. This mound we find most
interesting, for upon reaching its top we look down into a volcanic
crater. From this crater flowed the great stream of lava to which
we have already referred. The lava ran downward, bending this way
and that among the hollows, until it spread nearly to the McKenzie
River.

During the Glacial period, before the eruption took place, this
glacier was much larger. The summit of the Cascade Range was then
covered by glaciers. This fact we know from the presence of grooved
and polished rocks wherever the surface has not been worn away
or covered with newer lava. The Glacial period had passed away
and the climate had become much the same as it now is when the
volcanic forces broke out at the spot where the crater is situa