292 EXTERNAL APPEARANCE OF GLACIERS. 
One may often trace the gradual formation of 
these wells, because, as they require certain sim- 
ilar conditions, they are very apt to be found in 
various stages of completion along the same track 
where these conditions occur. Fissures, for in- 
stance, will often be produced along the same 
line, because, as the mass of the glacier moves 
on, its upper portions, as they advance, come 
successively in contact with inequalities of the 
bottom, in consequence of which the ice is 
strained beyond its power of resistance and 
cracks across. Rivulets are also likely to be 
renewed summer after summer over the same 
track, because certain conditions of the surface 
of the glacier, to which I have not yet alluded, 
and which favor the more rapid melting of 
the ice, remain unchanged year after year. Of 
course, the wells do not remain stationary any 
more than any other feature of the glacier. 
They move on with the advancing mass of ice, 
and we consequently find the older ones consid- 
erably lower down than the more recent ones. 
In ascending such a track as I have described, 
along which fissures and rivulets are likely to 
occur, we may meet first with a sand-pyramid ; 
at a certain distance above that there may be a 
circular opening filled to its brim with the sand 
which has just reached the surface of the ice ; a 
little above may be an open well with the rivulet 
