424 ROLLIN T. CHAMBERLIN 
substance.t This variation applies especially to rupture, and it is 
uncertain how deep rupture may be safely projected. In rock 
flowage other conditions obtain. The inclination might be expected 
to become steeper. If it be true that in the lower reaches the border 
zones should become more steeply inclined than here drawn, that 
would amount to projecting the roots of the continental masses to 
greater depths. But the real significance of the location and — 
behavior of the bordering zones in the deeper portions of the 
globe can only be satisfactorily treated by tracing the diastrophic 
phenomena throughout the stages of the earth’s growth. Under 
the plantesimal view, each of the stages involved at first surficial 
diastrophism and later underwent the various deeper diastrophisms 
involved in the upgrowth of the continental and oceanic segments. 
This will be taken up in a later paper of this series. 
According to the general philosophy of which the wedge theory 
is a part, condensation of material in favor of greater density takes 
place throughout the deep interior under the influence of gravita- 
tive force. This causes shrinkage and the development of strong 
lateral thrusting in the outer portion of the globe. When the 
growing stresses reach and exceed the strength of materials under 
the conditions obtaining within the earth, a period of diastrophism 
sets in. The vaster and heavier oceanic segments take the lead 
in descending and as they do so, the continents, several or all, are 
wedged upward. Some may be wedged up more than others, or 
one side of a continent uplifted more than the other sides. A 
moderately thick shell forming only part of a continent may suffer 
notable shortening and be pushed up into a plateau. A thinner 
shell along the borders of a continent, yielding more readily and 
suffering much greater shortening, may be folded and faulted 
into a mountain system. The tangential compressive stresses 
necessarily extend throughout the outer portion of the globe and 
are not to be thought of merely as thrusts from an active oceanic _ 
mass against a passive continental mass, but the actual deformation 
into mountain systems is, for reasons to be brought out later, a 
t Walter H. Bucher, ‘‘The Mechanical Interpretation of Joints,” Jour. Geol.,. f 
Vol. XXVIII (1920), pp. 707-30; Vol. XXIX (1921), pp. 1-28. 
