THE MECHANICAL INTERPRETATION OF JOINTS 27 
question is, Can the essentially horizontal planes of fracture have 
been produced under similar conditions of strain or do they represent 
the result of rotational strain ? 
To prove the first assumption, we must find a cause for the 
great change in inclination of the shearing planes from rather high 
angles to a horizontal position. Cadell’s experiments and their 
own researches led Chamberlin and Miller to the suggestion that 
the additional vertical pressure resulting from the piling up of 
numerous thrust-blocks may tend to reduce the angle of shearing. 
In the light of the preceding discussion it appears that an 
additional load would rather have the opposite effect. It would 
be equivalent to increasing the hoop pressure in Karman’s experi- 
ments. It would render the rock less brittle and therewith the 
angle of shearing larger. Since the compressive stress, in this 
case, acts in a horizontal direction this would mean planes of 
shearing inclined at a steeper angle than before. 
Any given rock shows the smallest angle of shearing at the 
surface. To reduce this angle still more, an active tensile stress 
would have to be applied in a vertical direction, which is, of course, 
out of question. 
In a general way, then, we must picture to ourselves the planes 
of pure shear produced in the earth’s crust by a horizontal com- 
pressive stress as being least inclined near the surface and becoming 
progressively steeper at lower levels so as to approach finally the 
more or less vertical position of the planes of flowage existing at 
considerable depths. Conceptions such as are expressed, for 
instance, in Ulrich’s diagrams to illustrate the ‘‘inland migration 
of belts of folding in Southeastern North America” in his ‘‘ Revision 
of the Paleozoic Systems’’* require corresponding modification. 
For an understanding of the mechanics of the large horizontal 
overthrusts, however, we can but turn to the only alternative, 
which was clearly brought out by Chamberlin and Miiler, that, 
in contrast to the other type, they are essentially the result of 
rotational strains of the kind which Becker has termed “scission,” 
which occur “when a bar or plate is shorn by a pair of shears, or 
t Bull. Geol. Soc. Amer., Vol. XXII (1911), p. 441. 
