EXTERNAL APPEARANCE OF GLACIERS. 291 
continuous circle, and we have one of the round 
deep openings on the glacier known as moulins , 
or wells, which may of course become perfectly 
dry if any accident turns the rivulet aside or 
dries up its source. The most common cause of 
the intermittence of such a waterfall is the for- 
mation of a crevasse higher up, across the water 
course which supplied it, and which now begins 
another excavation. 
These wells are often very profound. I have 
lowered a line for more than seven hundred feet 
in one of them before striking bottom ; and one 
is by no means sure even then of having sounded 
the whole depth, for it may often happen that 
the water meets with some obstacle which pre- 
vents its direct descent, and, turning aside, con- 
tinues its deeper course at a different angle. 
Such a well may be like a crooked shaft in a 
mine, changing its direction from time to time. 
I found this to be the case in one into which I 
caused myself to be lowered in order to examine 
the internal structure of the glacier. For some 
time my descent was straight and direct, but at 
a depth of about fifty feet there was a landing- 
place, as it were, from which the opening con- 
tinued its farther course at quite a different an- 
gle. It is within these cylindrical openings in 
the ice that those accumulations of sand collect 
which form the pyramids described above. 
