DIASTROPHISM AND THE FORMATIVE PROCESSES 581 
the square inch. Over against this would be the pressure of 12,000 
feet of water or roundly 5,000 pounds to the square inch, leaving 
a differential pressure of 9,000 pounds to the square inch, which 
may be taken as representing the pressure at the base of the border 
prisms of the continent. The prisms that would form the summits 
of the continental swells, taken at 6,000 feet above the sea-level, 
would suffer a differential pressure of about 16,000 pounds per 
square inch at their bases. Now unbalanced pressures of 9,000 to 
16,000 pounds per square inch are equal to the crushing strength 
of weak rock and approach that of average rock. Oblique or trans- 
verse shearing would not unlikely take place in a prism of rock 
instead of crushing and this would require appreciably less stress, 
but just how much less has not been well determined as yet, so 
far as I know. There would be, at any rate, at the base of each 
such ideal prism of the continents, internal stresses that would 
approach the average strength of the rock of which they were com- 
posed and, in the weaker cases, would probably exceed it. 
If now, instead of our idealized continental swells, we take the 
actual case, the stresses will be found much more intense. For 
example, the present Tibetan plateau over a wide area has an 
elevation of 15,000 feet or more above the sea-level, and the ocean 
bed, not far away, is considerably more than 12,000 feet in depth, 
so that after allowing for the oceanic pressure, there would be at 
the base of a Tibetan prism an unbalanced lateral pressure of 25,000 
or 30,000 pounds to the square inch. In an isolated column this 
would be opposed only by the rigidity of the rock, and if this were 
of the average type, creep would certainly take place in the lower 
portion, if crushing did not anticipate it. The phenomenon of 
creep in mines and canyons under much less pressure leaves no 
room for doubt on this point. 
But massed as the ideal prisms actually are in the continents, 
and surrounded by low slopes running out under the edges of the 
oceans, the actual case presents a modified aspect. The prisms not 
only lend some support to one another—though they must then 
carry one another’s burden in some degree—but the sub-marine 
continental slopes buttress them. These buttresses are subject 
to lateral pressures of their own, but in so far as these pressures 
