290 EXTERNAL APPEARANCE OF GLACIERS. 
verse, narrow fissure lias been formed across tlio 
glacier, and that one of the many rivulets flow- 
ing longitudinally along its surface empties into 
it. As the surface-water of the glacier produc- 
ing these rivulets arises not only from the melt- 
ing of the ice but also from the condensation of 
vapor, or even from rain-falls, and flows over the 
scattered dust-particles and fragments of rock, it 
has always a temperature slightly above 32°, so 
that such a rivulet is necessarily warmer than 
the icy edge of the fissure over which it precip- 
itates itself. In consequence of its higher tem- 
perature it melts the edge, gradually wearing it 
backward, till the straight margin of the fissure 
at the spot over which the water falls is changed 
to a semicircle ; and, as much of the water dashes 
in spray and foam against the other side, the same 
effect takes place there, by which a corresponding 
semicircle is formed exactly opposite the first. 
This goes on not only at the upper margin, but 
through the whole depth of the opening as far 
down as the water carries its higher tempera- 
ture. In short, a semicircular groove is exca- 
vated on either side of the fissure for its whole 
depth along the line on which the rivulet holds 
its downward course. After a time, in conse- 
quence of the motion of the glacier, such a fis- 
sure may close again, and then the two semicir- 
cles thus brought together form at once one 
