spelled. This red snow is an organic
growth, a plant springing up in such abundance that it colors extensive
surfaces, just as the microscopic plants dye our pools with green in the
spring. It is an _Alga_ well known in the Arctics, where it forms wide
fields in the summer. With the above facts before us concerning the
materials of which glaciers are composed, we may now proceed to
consider their structure more fully in connection with their movements
and the effects they produce on the surfaces over which they extend. It
has already been stated that the ice of the glaciers has not the same
appearance everywhere, but differs according to the level at which it
stands. In consequence of this we distinguish three very distinct
regions in these frozen fields, the uppermost of which, upon the sides
of the steepest and highest slopes of the mountain-ridges, consists
chiefly of layers of snow piled one above another by the successive
snowfalls of the colder seasons, and which would remain in uniform
superposition but for the change to which they are subjected in
consequence of a gradual downward movement, causing the mass to descend
by slow degrees, while new accumulations in the higher regions annually
replace the snow which has been thus removed to an inferior level. We
shall consider hereafter the process by which this change of position is
brought about. For the present it is sufficient to state that such a
transfer, by which a balance is preserved in the distribution of the
snow, takes place in all glaciers, so that, instead of increasing
indefinitely in the upper regions, where on account of the extreme cold
there is little melting, they permanently preserve about the same
thickness, being yearly reduced by their downward motion in a proportion
equal to their annual increase by fresh additions of snow. Indeed, these
reservoirs of snow maintain themselves at the same level, much as a
stream, into which many rivulets empty, remains within its usual limits
in consequence of the drainage of the average supply. Of course, very
heavy rains or sudden thaws at certain seasons or in particular years
may cause an occasional overflow of such a stream; and irregularities of
the same kind are observed during certain years or at different periods
of the same year in the accumulations of snow, in consequence of which
the successive strata may vary in thickness. But in ordinary times
layers from six to eight feet deep are regularly added