24 The American (Geologist. January, i*** 
Flow of Glacial Molecules and Grains. 
Through all the patient observations and laborious theoriz- 
ing of physicists concerning, the methods of the flow of glacier 
ice, an ever increasing knowledge of the physical conditions 
and laws of. glacial flow is manifest. As Tyndall has gener- 
ously said, De Saussure, the pioneer of alpinists, doubtless 
knew the viscoid action of the glaciers in sweeping around the 
promontories and islands of their pathways. Scheuchzer and 
Charpentier seized something of the truth, or a close analogy 
of it, in attributing the ice flow to infiltering and freezing 
water. Rendu, without experimental proof, affirmed the river- 
like faster flow of the central ice; and Agassiz and Forbes, 
working independently, demonstrated this within the next 
year by measurements. Faraday and. Tyndall, in their studies 
of regelation, carried the physical theory a step beyond Forbes ; 
later. Mosele} r placed a useful emphasis on the influences of 
seasonal and diurnal changes of temperature; and lastly Croll, 
in his molecular theory, almost grasped the recently published 
doctrine which is based on the granulation. 
Figure S on plate II illustrates this view of Deele}' and 
Fletcher, concerning the changes of form which the glacier 
granules undergo, one being diminished and its fellow in- 
creased, in yielding to the pressure of gravitation and shear- 
ing motion, by which, in a way quite analogous to the flow of 
a viscous body, the glacier moves down its valley. They write 
as follows :* 
While a mass of Glacier-Ice is viscous in all directions, it has been 
found that a single crystal of ice is only viscous in a direction at right 
angles to the optic axis. A single crystal, or a portion of a crystal, will 
yield to continuous transverse stress applied in a direction parallel to 
the optic axis, but will not yield in a direction at right angles to the 
axis- in brief, viscous shear may take place in one plane only. Now, if 
all the crystalline grains constituting a glacier had their optic axes ar- 
ranged parallel with the direction of motion, or if there were a large 
majority of grains so arranged, it would not be difficult to account for 
the mode of motion of a glacier: but there is not any such relation be- 
tween the optical structure of the glacier grains and the direction of mo- 
tion. If we imagine shear to take place in any single grain, the motion 
will be stopped by adjacent crystals exhibiting rigidity in that plane. 
Indeed, it does not appear that a glacier in moving can make any use of 
the fact that ice-grains are viscous in one plane, for the direction of that 
plane varies in almost every grain. How, then, does the glacier move? 
Why does it, as a whole, exhibit viscosity? 
*Loc. cit., pp. 160, 161. 
