42 R. T. CHAMBERLIN AND W. Z. MILLER 
resulting from the compression under the load. Suppose there 
were 10,000 feet of rock piled up above the plane along which the 
fault is to occur. Assuming a specific gravity of 2.7 for the rock, 
the pressure resulting from this column will amount to about 
11,760 lbs. per square inch. If the stress along the axis of greatest 
stress, which is here horizontal, be taken to be three times this, it 
would need to be 35,280 lbs. per square inch. As the axis of least 
stress is vertical, the stress difference would amount to 23,520 lbs. 
per square inch. To cause faulting, this stress difference must 
equal the crushing strength of the rock under surface conditions 
augmented by the increased strength of the material induced by 
the hydrostatic pressure or cubical compression. This increased 
strength because of the depth is considerable, but pending more 
work of the type carried on by Adams and his colleagues this 
is not easily evaluated." | 
However, this stress difference clearly would not be sufficient at 
this depth to deform the stronger rocks, like granite, and probably — 
not rocks of average strength, though very likely it would be 
sufficient to deform the weaker rocks. Under these conditions, 
if the horizontal thrust were less than three times the vertically 
acting force, the stress difference would be proportionately still less 
effective in deformation. A ratio of more than three to one would, 
on the contrary, be more effective. A ratio of thrust to the weight 
of not less than three to one would seem to be required for extensive 
faulting through rock formations of average strength under a load 
ranging up to 10,000 feet of rock. This might lower the angle of 
faulting by 18° or less. But this reduction in angle from 45° 
falls far short of developing the approximately horizontal slippage 
planes of the great overthrusts. With loads greater than 10,000 
feet of rock, the resistance of the underlying rock is still further 
increased. While the increase in resistance probably does not 
mount up in direct proportion to the increase in balanced pressure, 
nevertheless for any thicknesses of rock likely to be piled up by 
diastrophic agencies there probably would not be a very radical 
change in the ratio of axes of stress necessary for faulting. At 
‘More data are now available. See Frank D. Adams and J. Austen Bancroft, 
Jour. Geol., SXV (1917), 597-637; also Louis Vessot King, ibid., XXV (1917), 638-58. 
