582 T. C. CHAMBERLIN 
fall below the creep pressure the residue of the strength of the 
buttresses opposes continental creep. The sub-oceanic buttressing 
is subject to further qualifications whose natures are more or less 
uncertain. The substructure of the border slopes of the conti- 
nents is unknown. To some geologists, the preferred picture of the 
continental borders is that of fault scarps with slopes of incoherent 
sediment banked against these. To others, it is that of warps 
or folds of indurated rock below with sheets of recent sediments 
above and banks of recent sediments on their outer borders. To 
still others, the picture is that of a graduated sedimentary series, 
soft and incoherent at the surface and on the abysmal face, grading 
downward and backward into more and more indurated rock. 
This may or may not be more or less warped and compressed by 
thrusts from the ocean bed, according to situation. There are no 
doubt other conceptions. The continental borders present a 
suggestive field for study which has not yet been adequately culti- 
vated.t. It would be a diversion from our main purpose to enter 
into a discussion of the details of the continental borders here, 
but, though opinions are diverse in other respects, they are at one 
on two features that bear on this discussion: (1) some notable por- 
tion of the border material is soft and feebly coherent, and (2) the 
borders of the continents have been specially subject to deformative 
processes throughout geological history and were hence probably 
weakened in their resisting powers thereby through the devel- 
opment of shear planes. There is good reason to believe also that 
creep takes place in the more recent soft material, whatever may 
be true of the indurated parts of the continental protuberance as a 
whole.?, These considerations make it difficult to judge how far the 
buttressing of the continents by the border slopes is effective in 
resisting the lateral pressure that tends-to spread the protruding 
masses. 
There is another consideration that affects the degree of resist- 
ance to lateral spreading in a possibly important way. It seems 
clear from an inspection of the folds of mountains that they are 
relatively superficial. It is probable that a shear zone has been 
t For some suggestions see Chamberlin and Salisbury, Geology, III (1906), 523-29, 
2 Ibid., pp. 527-28. 3 [bid., U1, 128. 
