DIASTROPHISM AND THE FORMATIVE PROCESSES 681 
or fusion limit of the stressed points is reached, deformation 
naturally becomes fundamentally periodic. As already remarked 
by way of qualification, a multitude of minute local periodic 
motions may so combine as to give the semblance of a continuous 
motion, but even in this case the precise mode of action and the 
nature of the effects differ radically from continuous fluidal or plastic 
flow. Normally, in an elastico-rigid earth, strains should accumu- 
late until they attain marked intensity and should then yield either 
to a slow massive movement of great magnitude—as is the common 
case in ordinary diastrophism—or else to a sudden swift movement 
of less magnitude—as is the case in earthquakes. The latter type 
is demonstrably non-continuous. There is certainly a period of 
accumulating strain, with inappreciable motion, followed by quick 
and often catastrophic movement. Between this class of demon- 
strative actions and the almost continuous phases of creep, there is 
believed to be a graded series of cases in each of which the movement 
is in essence periodic though the general expression varies from 
nearly continuous movement to sharply periodic movement. In 
the latter case there may be long intervening stages of relative 
quiescence. 
Periodic movements of all these varieties are consistent with the 
acceptance of a qualified doctrine of isostasy, i.e., approximate 
isostasy attained by periodic movement, not complete isostasy 
attained by continuous movement. ‘This qualified isostasy may be 
nearly complete, so far as vertical balancing is concerned, just after 
the periodic movement has ceased. Afterward there is a gradual 
departure from a strict balancing while stresses are reaccumulating. 
This leads on to a renewed movement of adjustment and so the 
periodic process continues. There is in this a rhythmical approach 
to isostasy. 
Now this seems to meet the conditions of the case. It has 
already been repeatedly urged that the base-leveling process cannot 
normally reach the stage of an advanced peneplain if continuous 
warping of the crust is in progress. On the contrary, the crust 
must hold an approximately static attitude long enough for the 
gradational process to accomplish the results observed. It has 
been further urged that there goes with the base-leveling process an 
