830 THE JOURNAL OF GEOLOGY. 
neighboring glaciers may also be explained in this way. For 
example, two glaciers subjected to the same climatic influences, 
but of unequal length, or if of the same length but of different 
mean velocity, would advance at different times in response to 
the same impulse, for the reason that the time required fora 
“‘wave’’ to reach their extremities would be different. 
The effects of variations in névé regions on the length of 
the glaciers flowing from them, have recently been discussed 
by Professor Reid in this JouRNAL™ and need not be considered 
further at present. 
2. Variation in the rate at which glaciers melt, might be con- 
sidered as a factor in studying the halts, advances and retreats 
observed at their extremities; but meteorological observation 
in Alpine valleys and the behavior of the ice streams entering 
the same valleys, do not show an intimate connection. 
3. The rate of flow of glacier ice is influenced, as already 
stated, by the percentage of débris mingled with it. The increase 
in the percentage of débris near the end of a glacier, may as we 
have seen, cause it to become stagnant and form a dam of débris- 
charged ice. When this occurs the terminus of the glacier will 
become stationary. If the current from above is sufficient 
to cause the ice to rise, and flow over the obstruction, an 
advance of the terminus will result. When the energy of the 
glacier is feeble, it may be held in check for a while, perhaps 
adding to the height of the débris-charged ice that retains it, 
and then retreat. The withdrawal of a glacier from its stagnated 
extremity is perhaps a more varied process than an advance 
beyond it. The extremity of a glacier that has been checked in 
the manner here considered, will be covered with superglacial 
débris. The effect of a surface covering on the wasting ice is 
varied. As is well known, a small amount of débris, especially 
if dark colored, will promote melting; while a larger amount 
will shield the ice beneath and assist in its preservation. For 
this reason, the abundantly débris-covered extremity of a glacier 
will waste more slowly than the less thoroughly covered portion 
farther up stream. In the case of a slowly retreating glacier 
tVol. III., 1895, pp. 278-288. 
