EXTERNAL APPEARANCE OF GLACIERS. 293 
still pouring into it ; or, higher up, we may meet 
an open fissure with the two semicircles opposite 
each other on the margins, but not yet united, 
as they will be presently by the closing of the 
fissure ; or we may find near by another fissure, 
the edges of which are just beginning to wear in 
consequence of the action of the water. Thus, 
though we cannot trace the formation of such a 
cylindrical shaft in the glacier from the begin- 
ning to the end, we may, by combining the sep- 
arate facts observed in a number, decipher their 
whole history. 
In describing the surface of the glacier, I 
should not omit the shallow troughs, which I 
have called “ meridian holes,” from the accu- 
racy with which they register the position of 
the sun. Here and there on the glacier there 
are patches of loose materials, dust, sand, peb- 
bles, or gravel, accumulated by diminutive water- 
rills, and small enough to become heated during 
the day. They will, of course, be warmed first 
on their eastern side, then, still more powerfully, 
on their southern side, and in the afternoon with 
less force again on their western side, while the 
northern side will remain comparatively cool. 
Thus around more than half of their circumfer- 
ence they melt the ice in a semicircle, and the 
glacier is covered with little crescent-shaped 
troughs of this description, with a steep wall on 
