ich we
shall see to be intimately connected with snow-strata, and which
eventually become so prominent as to be mistaken for the cause of the
lines of stratification, do nevertheless tend, when properly understood,
to make the evidence of stratification more permanent, and to point out
its primitive lines.

On the plains, in our latitude, we rarely have the accumulated layers of
several successive snow-storms preserved one above another. We can,
therefore, hardly imagine with what distinctness the sequence of such
beds is marked in the upper Alpine regions. The first cause of this
distinction between the layers is the quality of the snow when it falls,
then the immediate changes it undergoes after its deposit, then the
falling of mist or rain upon it, and lastly and most efficient of all,
the accumulation of dust upon its surface. One who has not felt the
violence of a storm in the high mountains, and seen the clouds of dust
and sand carried along with the gusts of wind passing over a
mountain-ridge and sweeping through the valley beyond, can hardly
conceive that not only the superficial aspect of a glacier, but its
internal structure also, can be materially affected by such a cause. Not
only are dust and sand thus transported in large quantities to the
higher mountain-regions, but leaves are frequently found strewn upon the
upper glacier, and even pine-cones, and maple-seeds flying upward on
their spread wings, are scattered thousands of feet above and many miles
beyond the forests where they grew.

This accumulation of sand and dust goes on all the year round, but the
amount accumulated over one and the same surface is greatest during the
summer, when the largest expanse of rocky wall is bare of snow and its
loose soil dried by the heat so as to be easily dislodged. This summer
deposit of loose inorganic materials, light enough to be transported by
the wind, forms the main line of division between the snow of one year
and the next, though only that of the last year is visible for its whole
extent. Those of the preceding years, as we shall see hereafter, exhibit
only their edges cropping out lower down one beyond another, being
brought successively to lower levels by the onward motion of the
glacier.

Other observers of the glacier, Professor Forbes and Dr. Tyndall, have
noticed only the edges of these seams, and called them dirt-bands.
Looking upon them as merely superficial phenomena, they have given
explanations