292 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Correlation of Hirer Valleys and Glacier Valleys. — Thus far 
the consequences of glacial erosion have been described as if they 
were unlike those of river erosion, especially in respect to the pro¬ 
duction of hanging valleys. A just comparison of the two agen¬ 
cies will show that their resemblances are more marked than their 
differences, when due allowance is made for their individual 
peculiarities. 
The likeness of glaciers and rivers has been frequently con¬ 
sidered. The motion of water streams and ice streams is retarded 
by bottom and banks, and is fastest in mid-channel where farthest 
removed from all hindrances. The motion is faster on strong than 
on gentle slopes, and in large than in small streams : the line of 
fastest motion departs from the medial axis towards the concave 
bank. Forel (’ 97 , 204) and Gannett (’ 98 , 422) have justly com¬ 
pared ordinary valley glaciers, not to rivers that mouth in the sea, 
but to rivers that descend from mountains to wither away on pied¬ 
mont deserts. The terminal moraine of the glacier corresponds to 
the terminal delta-like fan of withering rivers. The fluctuation of a 
withering river following changes of weather or season corresponds 
to the secular fluctuations of glaciers, as during the period of about 
thirty-five years in the Alps. The advance and retreat at the end 
of large glaciers does not occur synchronously with the advance 
and retreat of small glaciers, although both large and small glaciers 
accomplish their periodic variations of length in the same interval; 
and it is probable that the same contrast obtains in withering 
rivers of different length, although I cannot find any direct state¬ 
ments to this effect. Meunier (’ 97 , 1043) has suggested that certain 
peculiar successions of drift deposits in Switzerland may be the 
result of the enlargement of the drift-bringing glacier by the 
capture of the head reservoirs of another glacier, after the analogy 
of rivers. • Gannett and Penck (see abstracts below) have gone 
further still and have shown that the hanging valleys, so char¬ 
acteristic of strongly glaciated drainage systems, have a perfect 
analogy in the valley systems of ordinary rivers in non-glaciated 
areas. This comparison is so instructive that it deserves full state¬ 
ment here. 
The “ nice adjustment of declivities ” that characterizes the main 
and the side valleys of a river system is found only in maturely 
developed valleys. The adjustment or accordance between main 
trunk and lateral branch obtains only with respect to the surface of 
the streams or to the floor of their valleys. The beds of the trunk 
