LOW-ANGLE FAULTING. IO 
But as the favorite angles of fracturing for these blocks of homo- 
geneous material appear to be 30°, or 35°, or 40, the effect of fric- 
tional resistance due to the component of stress normal to the 
fracture plane clearly is not an adequate explanation of the great, 
low-angled overthrusts whose planes of fracture are commonly 
inclined at only 5° or ro from the horizontal. There must be 
other factors. 
B. EFFECT OF HETEROGENEITY OF MATERIAL 
It is to be recognized that the angle of faulting is to some extent 
dependent upon the uniformity or heterogeneity of the material. 
Tests on the strength of materials seem to show that most blocks 
fail by a combination of shear and splitting. Only short blocks of 
very uniform texture will fail by shear entirely across the section 
in one plane. Heterogeneous material which introduces differences 
in composition introduces irregularities in fracturing, and these 
irregularities are prevailingly in the nature of a lowering of the 
angle of fracture. But this alone will hardly explain the great 
overthrusts. 
C. POSSIBLE INFLUENCE OF LENGTH AND SHAPE 
Length.—The short column, whose length is not more than five 
times its diameter, fails by direct crushing. A longer column fails 
partly by crushing and partly by bending.* Thus a distinction is 
made in engineering practice between the behavior of the short 
block and the long column. The long column first bends and then 
splits obliquely. That the long column should be weaker than 
short blocks of the same material and cross-section is evident, but 
the theoretical treatment of its behavior is much less satisfactory 
than in other cases of flexure. The breaking load for long columns, 
however, is represented by Euler’s formula: 
cad a 
P 
cast iron the usual value of @ is about 35°, corresponding to a value for ¢ of 20° (Arthur 
Morley, Strength of Materials [London, 1913], pp. 55-56). Hodgkinson’s experiments 
with cast iron have shown that 35° is the common angle of rupture for this material 
(cited by Church, Mechanics of Engineering, p. 220). 
P= 
t James E. Boyd, The Strength of Materials, 1911, p. 48. 
2R. J. Woods, op. cit., p. 205. 
3J. P. Church, Mechanics of Engineering, 1913, p. 360. 
