DIASTROPHISM AND THE FORMATIVE PROCESSES 421 
it is the view of those inquirers who accept the modern evidences 
of increasing solidity with increasing depth that the idea of easy 
movement in the interior is to be scrupulously avoided, whatever 
may be the form or mode of movement. It is held by them that 
differential motion of all kinds in the solid interior takes place 
only in response to high differential stress. If the term flow is to 
continue in use, it should carry simply the idea of an intimate 
distributive method of deformation and be shorn entirely of all 
suggestion of easy movement, for that belongs to liquidity. Strong 
support for this view is found in the experimental work of Adams, 
which extends the zone of cavities to greater depths than formerly 
supposed possible, owing to the increasing strength of the rocks 
under cubical compression. 
There are increasing grounds for the view that the various 
special methods of deformation have a more intimate association 
with one another than has been generally recognized, and that 
processes heretofore confined to the upper zones may have applica- 
tion to greater depths. The true status of present knowledge of 
the movements in the deeper unseen zone has been most judi- 
ciously and trenchantly stated by Leith in his vice-presidential 
address before Section E of the American Association :! 
Notwithstanding these and other considerations, any conclusion as to the 
existence of a deep zone in which all rocks flow when deformed is hypothesis, 
not proved fact, and perhaps will always remain so. The environmental condi- 
tions are not accurately known, and even if each of the factors were measured, 
their conjoint effect is still speculative. Variations in the time factor alone 
may determine whether a rock flows or fractures. Rock flowage which has 
occurred in rocks now accessible to our observation fails to indicate increase 
_ with depth with sufficient clearness and definiteness to warrant confident 
downward projection. 
The general purport of the under-configurations developed by 
these shell studies carries at least an intimation that the principles 
of surficial diastrophism are to some extent applicable to the deeper 
problems of the continents as well. When the ocean basins sink 
and the continents are uplifted, some adjustment necessarily takes 
™C. K. Leith, “‘The Structural Failure of the Lithosphere,” Sczence, Vol. LIIL 
(1921), pp. 195-208. 
