Glacier Work. — Scott. 233 
Viscosity is defined as tangential force per unit area 
divided by shear per unit time, and so the conditions for 
viscosity are here of the best. The force, due to pressure is 
enormous, and while the timie is indeterminate, the shear re- 
sistance is certainly small for ice masses as shown by experi- 
ment. The modulus of elasticity is low * as would be expect- 
ed, so that any small deformation produces permanent strain, 
resulting 'in a rearrangement of crystalline aggregates, 
if not indeed of the crystal molecules themselves. Thus it is 
obvious that in the zone of plasticity, continuous deformation 
of the ice mass results from the pressure exerted upon it, 
which pressure consists of both a vertical and a horizontal 
component. "Minerals deformed in the zones of flowage," 
says Van Hise, "show no interspaces under the microscope, 
but their remarkable shapes, undulatory extinction, and 
granulation in polarized light give evidence of their change 
of form." t And again "deformation may be partly ac- 
complished by the rearrangement of the mineral particles 
with respect to one another."! 
Such changes in ice crystals are apparently well exempli- 
fied by the studies of Deeley and Fletcher, mentioned previous- 
ly. Furthermore it seems entirely probable that the ice in this 
zone is assisted in its movement as a viscous mass through the 
prime agencies of continuous solution and recrystallization. 
The great pressure exerted in this zone, which is at the same 
time widely variable in magnitude and direction because of 
the opening and closing of crevasses and irregular movements 
in the zone of fracture, together with temperature effects, is 
obviously a cause for a state of interior strain in the ice mass. 
It has been shown by Barus § that the work done in strain- 
ing certain materials is largely jxDtentialized, and, further, that 
in the case of glass || the release from strain occurred 
through the process of crystallization. Van Hise H also savn 
concerning rock materials, that "as soon as a state of strain is 
produced, the processes of solution and recrystallization set to 
*See Smithsonian Tables. 
tPrinciples cit. p. GOG. 
tPiinciple.s, cit. p. G94. 
SThe mechanism of solid viscosltv, bv Carl Barus. Bull. T^. S. Geol. 
Sun-ey, No. 94, 1892. 
II The Compre.ssibility of Liquids, by Carl Barus. Bull. U. S. Geol. 
No. 92. 1892. 
HMetamorphism of Rocks and Rock Flowage. Bull, of the Geol. Soc. 
of America, Vol. 9, p. oOO. 
