284 EXTERNAL APPEARANCE OF GLACIERS. 
at sunset ; it is no uncommon tiling to see a full 
cascade come rushing out from the lower end of 
a glacier during the heat of the day, and vanish 
again at its decline. Suppose one of these riv- 
ulets should fall into a deep, circular hole, such 
as often occur on the glacier, and the nature of 
which I shall presently explain, and that this 
cylindrical opening narrows to a mere crack at 
a greater or less depth within the ice, the water 
will find its way through the crack and filter 
down into the deeper mass ; but the dust and 
sand carried along with it will he caught there, 
and form a deposit at the bottom of the hole. 
As day after day, throughout the summer, the 
rivulet is renewed, it carries with it an additional 
supply of these light materials, until the open- 
ing is gradually filled and the sand is brought 
to a level with the surface of the ice. We have 
already seen, that, in consequence of evapora- 
tion, melting, and other disintegrating causes, 
the level of the glacier sinks annually at the 
rate of from five to ten feet, according to sta- 
tions. The natural consequence, of course, must 
be, that the sand is left standing above the sur- 
face of the ice, forming a mound which would 
constantly increase in height in proportion to the 
sinking of the surrounding ice, had it sufficient 
solidity to retain its original position. But a 
heap of sand, if unsupported, must very soon 
