26 WALTER H. BUCHER 
from such a complex arrangement, must be expected to vary con- 
siderably from place to place, while corresponding only in the aggre- 
gate to the simple fundamental stress relation. 
Whatever may be the true merits of this interpretation of the 
“‘double-sheet”’ structure, it serves well to bring out the fact that 
horizontal compressive stresses will produce planes of shearing at 
low angles only in highly brittle substances close to the surface, 
where the least principal stress is directed upward, in the direction 
of easiest relief. 
VII. LOW-ANGLE FAULTING 
This leads us finally to the important question of low-angle 
faulting which has recently been given a most valuable discussion 
by R. T. Chamberlin and W. Z. Miller. 
The heart of the problem is nowhere clearer eqnised than in 
the northwest Highlands of Scotland.3 A series of sediments of 
diverse nature, piled up, shingle-like, by thrust faults inclined 
on the average about 45°, has been cut across by several practically 
horizontal planes of fracture, along which the individual slices 
have been moved in the general direction of thrust for many miles. 
The strongly inclined faults clearly are planes of pure (irrota- 
tional) shear produced by the horizontal compressive stress. The 
* For a similar reason, the movement on individual shearing planes, into which a 
block has been broken under one-sided pressure, may be most diverse, one block being 
pushed out in one direction, the other in another. This explains the small success that 
has attended the attempts made by Dr. Salomon (1911) and some of his students 
(Lind, 1910, Dinu, 1912, Spitz, 1913, Engstler, 1913, Seitz, 1917) to determine the 
nature of the stress involved in the formation of joint systems from the direction of 
movement recorded on the slickensides. (For complete references to these most 
careful studies, see O. Seitz., Uber die Tectonik der Luganer Alpen, Inaug. Diss., Heidel- 
berg, 1917). 
The same criticism applies equally to the attempts made to infer from local 
observations of movements during an earthquake the nature of the major stresses - 
which have resulted in the formation of great fault lines. The cause of such movements 
along existing fault lines may be fundamentally different from that which caused the 
fracture itself. 
2 R. T. Chamberlin and W. Z. Miller, ‘‘Low-Angle Faulting,’ Jour. Geol., Vol. 
XXVI (1918), pp. 1-44. 
3B. N. Peach, John Horne, W. Gunn, C. T. Clough, and L. W. Hinxman, ‘The 
Geological Structure of the North-West Highlands of Scotland,’’ Mem. Geol. Survey 
of Great Britain, 1907. 
