Glacier Work. — Scoit. 22,1 
ice mass is also taking place, while during- periods of rising- 
temperature, the surface of the ice becomes more mobile, and 
the mass moves forward more easily under mechanical stress. 
The limit to which crevasses extend, as already pointed 
out, doubtless depends chiefly upon the w^eight of the super- 
incumbent ice, though it is also probable that different con- 
ditions of stress may exert some influence ; and, further, 
microscopic fracture's might exist where macroscopic ones 
are not discernible, and so under certain conditions througli- 
out a limited thickness below the zone of fracture, what miglit 
be called an intermediate belt of combined fracture and plastic- 
ity exists. 
Here under certain conditions of temperature, and acting 
pressures in the ice mass itself, the ice might either fracture, 
•or move as a plastic substance. Sudden pressures, or the low- 
ering of the temperature, or both, would produce fracture, 
when reverse conditions would cause the ice to behave like 
wax. It is probable that combined forces in this space may 
be largely responsible for the fact that the movement of any 
given point in a glacier may vary from day to day, or hour 
to hour, because folding and thrusting of the ice w^ould 
naturally occur, which, in turn, would make its influence felt 
on the observed surface ice above. And, further, a valley hav- 
ing a particularly irregular contour, would have such an effect 
upon the movement of ice in this space, as to auginent the 
formation of crevasses and irregularly directed chasins in the 
zone above. This accords with Russell's statement that * 
"The ice of glaciers is also broken along planes more or less 
inclined to their surfaces. Movement takes place along these 
cracks, and produces thrusts, analogous to the over-thrusts, 
or under-thrusts, sometimes seen in rocks that have been 
folded and broken. In fact, the counterpart of many of the 
structural features observed in rocks, such as faults, folds, 
joints, contortions, etc., may be observed in the ice of elaciers." 
Mention has been made by various writers of the movement 
upward in portions of some glaciers, as, for example, where 
Prof. Pfaff noted in one of the reservoirs of the Aletsch 
glacier, where the surface slope was 9°, that the direction of 
motion made an angle of about 40° with the horizontal. Sonu- 
*G)acki-s of North America, 1S97, p. 11. 
