DIASTROPHISM AND THE FORMATIVE PROCESSES 677 
this—but in the main only beyond this—shear movement or 
fracture movement may take place. Stresses must thus accumulate 
to a certain value before appreciable movement will set in. In this 
lies a basis for periodic movement in distinction from continuous 
movement, for under growing stress there is practically no move- 
ment until the yield-point is reached, when distinct and relatively 
free movement follows until the stress is eased. 
Notwithstanding the radical difference between the behavior of 
a plastic and of an elastic body respectively under constantly grow- 
ing stress, there are two ways in which deformative movements in 
bodies of the elastic type may simulate in outward aspect those of 
the plastic type. In an elastic crystalline body, a more or less 
gradual movement may take place when the interlocking crystals 
are so constituted that the stresses brought to bear on the whole 
mass are concentrated upon certain points at which the crystals 
press with so much special intensity and inequality upon one another 
that the point of solution, fusion or fracture is reached for these 
minute parts under these exceptional and unequal stresses. These 
minute parts then yield, and the dissolved, fused or granulated 
material readjusts itself at neighboring points of less stress. This 
material may or may not resolidify. By this yielding, similar 
intensified stresses are thrown upon other points which yield in turn. 
Thus a succession of yieldings of minute points is brought into play 
while the main mass remains more or less massively solid. It isa 
combination of two modes of movement—liquid, plastic, or granular 
movement at minute points, and massive movement for the rest. 
There is crystalline loss and usually a crystalline gain by transfer. 
The action is thus usually motion and metamorphism combined. 
The action is partial and distributive, in detail, and the tout ensemble 
very closely simulates a plastic movement—which indeed it is in 
part, but not as a whole. It is commonly interpreted as plastic, 
though it is not simply such. The best type is glacial motion.’ 
When the temperature of a glacier is o° C. and it is thus on the very 
border of liquidity, and when it is bathed in water on the very border 
t “A Contribution to the Theory of Glacial Motion,” Decennial Publications of the 
University of Chicago, 1904, pp- 193-204. See also Chamberlin and Salisbury, Ge ology 
I (1904), 294-306. 
