422 ROLLIN T. CHAMBERLIN 
place between the segments. Near the surface the adjustment 
may well be accomplished by thrust-faulting; by movement along 
distributed shear planes where clear-cut faulting does not develop; 
by accommodative folding, and in part by the deformation involved 
in the formation of flow cleavage, and possibly in other ways. It 
is important to understand clearly that the border zones between the 
more deformed blocks and the less deformed blocks are not held to 
be simple fault planes, though thrust-faults do so commonly emerge 
at the surface in these situations, but instead are held to be zones 
of more composite nature in which adjustments between the larger 
units are accomplished in the various ways outlined above. 
In full agreement with the view that the results of deformation, 
in the zone of observation, afford perhaps the best criterion we 
have at present for judging of the behavior in the deeper zones, 
we may make the projecting planes, or zones, between the oceanic 
and continental segments the line of approach to the deeper 
problem, éspecially if the problems of the deeper horizons are as 
strictly the deformations of solids as are those of the surface, how- 
ever different their conditions of temperature and pressure. With 
depth, fracture should become an exceptional phenomenon, though 
certain types of shear should be more persistent. And it would 
seem likely that deformation by distributive movement involving 
crumpling, cleavage, and general rock flow, should predominate in 
the deeper horizons. 
Cleavage, by recrystalization, develops parallel to the elonga- 
tion of the mass, whatever be the nature of the compressive stress 
which produces it.1 From the experiments of Daubrée* upon the 
orientation of mica flakes under various conditions of compression, 
we learn that the direction of elongation is determined by the 
direction of least resistance. The deformation is controlled by the 
difference between the stresses along greatest and least axes of stress, 
and in this the least axis of stress is most important. By chan- 
ging the position of the orifice (direction of easiest relief) in a com- 
pression cylinder, such as that used by Daubrée, without changing 
™C.K. Leith, Structural Geology (1913), pp. 84-87. 
2 Op. cit., pp. 407-22. 
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