678 T. C. CHAMBERLIN 
of solidity, as it is when melting is in progress, the change from ice 
to water and water to ice takes place with the utmost ease and is 
greatly facilitated by slight differences of stress. Glacial motion 
may then be almost as continuous and free from appreciable strains, 
ruptures and starts as the motion of a viscous body. But when the 
temperature is much lower and pervades the whole mass, its texture, 
and behavior are more rocklike, and strains, ruptures, starts, and 
microseismic phenomena are more pronounced. The mass is of 
course a crystalline rock of singular purity and the phenomena of 
strain, rupture, and start imminent in all cases under sufficient 
differential stresses. Crystalline orientation prevails at all stages. 
The second case is found when the pressure on all sides is so 
intense that no separation of particles is possible and hence con- 
tinuity of contact is forced, whatever may be the nature of the 
deformation that arises from the differential portion of the pressure, 
which alone can give a typical deformative movement. Such a defor- 
mation may take on the aspect of plastic deformation even though 
the minute process be one of granulation, or of progressive recrystal- 
lization, or otherwise. 
These modes of pseudo-plastic movement, or, at most, partially 
plastic movement, qualify the distinctness between deformative 
movements normal to viscous or plastic bodies, on the one hand, 
and to rigid elastic bodies, on the other, without destroying the 
radical differences in their fundamental natures. The similarities 
in external aspect are of course a source of difficulty in interpreta- 
tion. It is none the less necessary to take account of the distinc- 
tiveness of the opposed qualities. The nature of viscous or plastic 
bodies is favorable to continuous deformative movements so long 
as unequal stresses continue to arise. The nature of elastico-rigid 
bodies requires the delay of deformative movements until the 
inequalities of stress have reached the elastic limit of the aggregate 
or of its most stressed points. This difference gives the key to the 
interpretation of terrestrial deformations. It is in this difference 
that the phenomena of base-leveling, shelf-building, and sea- 
transgression, of the types we have set forth, find their elucidation. 
They seem to imply a crystalline elastic-rigid constitution of the 
earth-body. 
