244 
INTERNAL STRUCTURE AND 
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 arc frequent- 
ly 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 
grow. 
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 sum- 
mer 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 suc- 
cessively 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. Look- 
ing upon them as merely superficial phenomena, 
