GREATER PROBLEMS OF PHYSICAL GEOLOGY. 
59 
posits may vary much, but may be proportional to the time 
of accumulation, and here the time is measured by the geo¬ 
logic standard. The gross weight of such masses of sediment 
must be vast indeed. If there is any viscous yielding at all 
the problem becomes essentially that of the flowing solid,* 
which is in a large measure governed by hydrostatic laws. 
The intensity of the force must have a maximum value pro¬ 
portional to the thickness which lies above the isostatic level 
and also proportional to its specific gravity. The area cov¬ 
ered by the deposit enters as a quantity factor, but not as an 
intensity factor. The greater the area, the greater is the total 
potential energy of movement without any necessary increase 
of the intensity of the force. This intensity, being propor¬ 
tional to the thickness of the sediments, may become almost 
indefinitely great or it may be small. Indeed, it may, and 
in fact does, become negative when we apply the same statical 
theory to the movement or stress of the denuded land areas. 
But whether these forces are sufficient to produce actual 
flow is equally dependent upon the rigidity, or, as we may 
here term it, the viscosity of the masses involved. We have 
already seen reason to infer that the mean viscosity is not 
great, being far less than that of the surface rocks alone. 
Beyond this rather vague statement I perceive no way of as¬ 
signing a value to the resistance to be overcome. 
It remains to inquire what is the resulting direction of 
motion. The general answer is, towards the direction of 
least resistance. The specific answer, which must express the 
direction of least resistance, will, of course, turn upon the 
configuration of the deposition on the one hand, and of de¬ 
nudation on the other, and also upon the manner in which 
the rigidity or viscosity varies from place to place. Taking, 
then, the case of a land area undergoing denudation, its de¬ 
tritus carried to the sea and deposited in a heavy littoral belt, 
we may regard the weight of each elementary part of the 
deposited mass as a statical force acting upon a viscous sup¬ 
port below. Assuming that we could find a differential ex¬ 
pression applicable to each and every element of the mass 
