l8o REPORTS ON INVESTIGATIONS AND PROJECTS. 



All the foregoing agencies involve thrust stresses in the main and 

 are only competent to explain thrust or compressive phenomena. It 

 remains to find an adequate agency for the production of tensional 

 faulting and other relaxative phenomena. These are much more 

 prevalent at the surface of the lithosphere than are compressional 

 phenomena, though their aggregate value is apparently less dynam- 

 ically. Tensional faulting is the " normal " form, and the sum total 

 of gaping fissures filled and unfilled is large. These tensional faults 

 are assigned in the main to secondary action growing out of the strains 

 developed by the protrusions incident to the primary actions already 

 described. The continental platforms rise from 2 to 8 miles above the 

 abysmal bottom of the ocean, with an average of perhaps 3 miles. 

 Three miles of rock exerts a pressure of 16,000 to 30,000 pounds to the 

 square inch, according to the class of rock. The continental platforms 

 may roundly be assigned a pressure of 20,000 pounds to the square 

 inch on their bases, and hence a tendency to creep laterally, much as 

 does a glacier. This is opposed by perhaps 5,000 pounds pressure per 

 square inch arising from oceanic waters. The difference between 

 the two pressures leaves a working margin that approximates to the 

 shearing strength of average rock, and hence it is conceived that the 

 continental platforms, yielding to this persistent pressure, creep 

 laterally in a slow, glacier-like way. The previous establishment of 

 a zone of shearing with the development of shear planes is presumed 

 to facilitate this lateral movement, and hence arises the conception 

 that a relaxative movement follows the primary movement, resulting 

 in the slow spreading of the continent with the development of fissur- 

 ing and normal faulting. The origin of earthquake tremors as an 

 incident of this process is an obvious suggestion. 



The conception of a comparatively thin superficial shell subjected 

 to compressive thrust during times of body shrinkage and general 

 deformation, followed by slow and persistent relaxative movement of 

 the opposite phase in the intervals between general deformations, 

 furnishes appropriate grounds for elucidating many of those very 

 gentle changes in the relations of land and water which characterize 

 the great periods of quiescence of the earth's history, without destroy- 

 ing the validity of the general conception of quiescence. It also 

 furnishes an alternative explanation of the remarkable behavior of 

 the continental borders, by virtue of which the exogenous growth of 

 the continents has been persistently mutilated and river valleys and 

 other channels submerged to extraordinary^ depths. These latter 

 have usually been assigned to vertical oscillations of the continental 

 masses ; but, besides making grave demands on dynamic possibilities 

 and introducing difficulties in disposing of the displaced oceanic 



