196 
THE SEINE, THE MEUSE, AND THE MOSELLE 
divide will move more or less to one side or the other as it 
weathers away, on account of the unequal rate of wasting of its 
two slopes. The possible causes of unequal wasting are various- 
The declivity of the two slopes may differ, .in which case the 
steep slope wastes faster than the other and the divide is veiy 
slowly pushed toward the flatter slope. The rocks underlying 
the two slo]3es ma}^ be of different resistance ; then the weaker 
one will, as a rule, waste away the faster, and the divide will 
gradually migrate toward the more resistant rocks. Again, the 
agencies of erosion may be of different activities on the two 
slopes; one slope may have a greater rainfall than the other, or 
may suffer a greater number of alterations from freezing to melt- 
ing. Although the last is generall}’’ a subordinate cause, it prob- 
ably contributes in a small way to the solution of the problem 
as a whole. 
The shifting of the divide as thus explained is generally accom- 
] dished by a slow migration. In some cases, however, when the 
divide is pushed to the very side of a stream whose basin it 
inclosed, then a little further change diverts all the upper drain- 
age of this stream into the encroaching basin, and with this 
change the divide makes a sudden leap around the upi)er waters 
of the diverted river, after which the slow migration may be 
resumed. The movement of a divide may therefore be described 
as alternately creeping and leaping. 
Whether this process is of very general importance or not can 
hardly be decided at the present time; but there are certain 
regions in which its application is most illuminating to the 
studies of the physical geographer. Philippson has brought the 
subject to general attention in his Studien ilber Wasserscheiden, 
where a full account of what others have done up to 1<S85 may 
be found. Oldham has told how certain headwaters of the In- 
dian rivers are pushing their divides through the innermost of 
the Himalayan ranges, and thus acquiring drainage area that 
formerly Ijelonged to the interior streams of the elevated Thi- 
betan plateau. This example is one of the best in which the 
process depends chiefly on the unequal declivity of the slopes 
on the two sides of the divide. Heim has described the depre- 
dations of the Maira in beheading the upper course of the Inn, 
thus accounting in a most beautiful manner for the little lakes 
at the head of the Engadine valley, wdiere this contest is going 
on. The special map of the Ober-Engadine, published in 1889, 
on a scale of 1 : 50,000, by the Swiss topographical bureau, gives 
