826 LLL JOCTLINAL OT MG TOO Gwe 
motion becomes less and less, and finally cease when stagnation 
results. The conditions most favorable for abrasion seem to be 
when the bottom layer of a glacier is lightly charged with small, 
hard and angular rock fragments. Other factors than those just 
mentioned, however, influence the abrasive power of glaciers ; as, 
for example, the pressure with which the débris is held against 
the rocks over which it is moved. In the middle course of a 
glacier, pressure is normally greater than near its extremity, 
where active waste is in progress; greater abrasion might, there- 
fore, be expected to occur in its middle course than near its 
extremity.. The firmness with which débris is held in its icy 
matrix, also influences it action as an abrading tool. It is reason- 
able to suppose that in a névé region the stones in contact with 
the rocks beneath, would be held less securely than in the com- 
pact ice of a glacier proper. This may be one reason why the 
upper portions of formerly névé-filled amphitheaters are fre- 
quently without smoothed and striated surfaces. Weathering 
in such situations, however, is more active than in lower regions; 
which may, perhaps, account sufficiently in many instances for 
the absence of ice abrasion referred to. 
Unconsolidated deposits beneath glaciers—tIn the well-known 
instance of Muir glacier, the ice, at its extremity, rests on uncon- 
solidated gravel. That the gravel well beneath the ice, however, 
in this and other similar instances, is really unconsolidated may be 
questioned. It is more reasonable, perhaps, to assume that such 
subglacial gravel is bound together by ice, and really forms a 
part of the glacier that rests upon it, but owing to excess of 
rocky material remains stagnant and allows the less highly débris- 
charged ice above to flow over it. Although this may be the 
explanation of the conditions now presented, in the example 
referred to, it does not explain how the ice first advanced upon 
the gravel. 
The gravel beneath Muir glacier was deposited by streams, in 
an unconsolidated condition, previous to the advance of the ice 
upon it, and differs both in character and in the manner of its 
accumulation from a ground moraine. 
A glacier advances, as has been shown by Professor Reid, 
