THE EARTH, A FAILING STRUCTURE. 
61 
Let us take Darwin’s computations and reason from them 
in another way. Instead of deducing from the computations 
the strength of the materials composing the earth, let us 
take some of the known facts as to the behavior of materials 
under stress at the surface of the earth and endeavor to esti- 
mate whether the material composing the earth probably 
would bear, without permanent deformation, the stresses 
brought to bear upon it by the continents and mountain 
ranges. 
When the subject is approached from this point of view, 
at least six reasons appear for believing that the material 
would undergo permanent deformation : 
1. It may be doubted whether even granite will stand for 
geologic ages, without permanent deformation, a stress-dif- 
ference as large as 4 tons per square inch. This is the 
breaking stress-difference of a good granite, according to the 
figures given by Darwin. All materials begin to suffer per- 
manent deformation at an elastic limit considerably less than 
the stress-difference necessary to break them. The longer 
the period of application of the stresses, the farther the 
effective elastic limit falls below the breaking stress-differ- 
ence. In the case in hand the stresses exist continuously 
for geologic ages. A standard authority on engineering 
practice gives the safe working load for granite masonry at 
30 tons per square foot, or one-fifth of a ton per square inch, 
and states that this is about the maximum in existing struc- 
tures. According to Darwin, the material within the earth 
must stand for geologic ages a stress-difference of 4 tons per 
square inch — twenty times as great as the safe load pre- 
scribed by engineers for granite — or it must fail. 
2. Granite is one of the strongest of the materials found 
abundantly in the accessible part of the earth’s crust. One 
may well doubt whether the heterogeneous mixture com- 
posing the crust is as strong on an average as continuous 
good granite. 
3. Failure — that is, permanent deformation — at any point 
tends to concentrate further failure at and near that point. 
The heterogeneous mixture of material composing the earth 
