INFLUENCE OF DEBRIS ON FLOW OF GLACIERS. 829 
of their accumulation, with the two varieties of terminals just 
cited, may also be recognized. 
The influence of débris on the behavior of glaciers that 
advance upon a plain and build morainal embankments, like 
those at the mouth of Bloody Cafion, California, might be traced, 
but space forbids such an extension of this paper. 
Variations of glaciers—Much attention is now being directed 
to fluctuations in the lengths of glaciers. As is well known, 
many Alpine glaciers alternately advance and retreat in the course 
of a few years, or remain stationary for a term of years and 
then undergo marked variations. These changes are usually 
considered to be due directly to variations in meteorological 
conditions. Glaciers in the same group, however, which, so 
far as one can judge, are exposed to the same climatic 
changes, frequently fluctuate differently. One glacier may be 
advancing, while its neighbor, perhaps draining the same névé 
field, is retreating. What has been stated above, however, in 
connection with the stagnation of the extremities of glaciers, 
when congested with débris, suggests that fluctuations in their 
lengths may be due to other causes than climatic changes. 
Advances and retreats of the end of a glacier may evidently 
result from (1) variations in the rate at which snow is accumu- 
lated on its névé, (2) to changes in its rate of melting, and (3) 
to fluctuations in its mean rate of flow. 
1. Variations in the accumulation of snow on the névé of a 
glacier may be considered as causing pulsation, or ‘‘ waves,” 
which would progress throughout its length and on reaching its 
extremity cause an advance or retreat. How an increase or 
decrease in the rate of accumulation on a névé would affect a 
glacier flowing from it, can, at present, only be conjectured. 
But it is reasonable to suppose that a moderate ‘“‘wave”’ pro- 
duced in this manner would become less and less well defined 
the greater the extent of the glacier it traversed, and its 
final effect on the length of the glacier be inappreciable. 
Marked changes in the volume of a névé would, however, 
unquestionably affect the glacier flowing from it and cause 
variations in its length. The opposite changes exhibited in 
