60 
HAYFORD. 
The computation shows that continents of such dimen- 
sions and form as those which are now in existence would 
produce stress-differences as great as 4 tons per square inch 
at depths of from 600 to 1,000 miles. The stress-differences 
due to the weight of the continents would, according to the 
computation, decrease both downward and upward from a 
maximum value at a depth of from 600 to 1,000 miles, 
becoming zero at the center and approaching zero at the 
surface. The stress-difference at 70 miles below the surface, 
according to the computation, is about one ton per square 
inch.* The computation, based on the assumption stated, 
also shows that parallel mountain ranges of density 2.8, 
rising 13,000 feet above the intermediate valley-bottoms, 
would produce stress-differences of 2.6 tons per square inch. 
The statement that the stress-difference is 4 tons per square 
inch at a given point within the earth means that at that 
point the compressive stress is 4 tons per square inch greater 
in the direction of maximum compressive stress at that point 
than it. is at that point in the direction of minimum com- 
pressive stress. This form of statement eliminates any refer- 
ence to the actual compressive stress in either direction. It 
is stress-difference which tends to deform and rupture the 
material, not hydrostatic pressure. A stress-difference of 4 
tons per square inch corresponds in its effect to a compres- 
sion of 4 tons per square inch applied to two faces of a cube 
in a testing machine while the other four faces of the cube 
are left free from pressure. 
Darwin’s conclusion, as stated in his paper, is that since 
such continents exist, since they do not sink down or the sea 
bed rise up, the materials composing the earth are strong 
enough to bear the computed stresses. For example, he 
writes: “From this discussion it appears that if the earth be 
solid throughout, then at a thousand miles from the surface 
the material must be as strong as granite.” He states in a 
table the breaking stress-difference in granite to be from 
2.45 to 4.91 tons per square inch. 
* Scaled from figure 6, plate 20, of Darwin’s paper. 
