966 THE JOURNAL OF GEOLOGY. 
have simply slipped the plates over their neighbors, carrying the 
adjacent ones forward with them to some extent by dragging, 
but not visibly affecting the more remote ones. 
If a prism be made up of square cards and placed on its side 
and a transverse force be applied, the result will illustrate the 
apparent method of movement within the ice crystal in this last 
case. If such a prism be pressed at right angles to the cards, it 
will illustrate the bending of the first case, and if the cards be 
placed on edge, they will illustrate the effectual resistance to 
deformation of the second case. Variations of temperature 
through 10°, were not found to produce notable differences of 
result. | 
Not to mention other significant points, the investigation 
seemed to warrant the important conclusion that ice crystals 
yield to deforming forces by the sliding or shearing of the crys- 
talline layers at right angles to the principal axis. No analogy 
to the motion of a viscous fluid appeared. Dr Mugge had pre- 
viously found a similar method of deformation in other minerals, 
including gypsum, stilbite and vivianite. In respect to its mode 
of internal motion, ice is therefore to be classed with these miner- 
als rather than bodies properly called viscous. 
From these trenchant experiments it would appear that there 
is a suggestive analogy between the shearing movement of ‘the 
crystalline plates within the ice crystal and the shearing motion of 
the individualized layers of the Greenland (and presumably other) 
glaciers. The ulterior question of the source of motion in gla- 
ciers is not reached by these investigations, though they offer 
suggestions of the first importance. They seem, however, to cut 
at the roots of the viscosity hypothesis by showing that ice-defor- 
mation is not even analogous to the internal movement of a vis- 
cous body. The word Plasncity, as a very general term indicating 
the yielding of solid bodies, may still be applied to ice, but is 
not the term viscosity already sufficiently burdened with duplicity 
to entitle it to relief from further service in this much battled 
field? 
Ae. (C. 
