LOW-ANGLE FAULTING Dit 
vertical dimension of the vigorously deformed Appalachian block is 
probably less. Very likely the column need not have yielded to the 
deformation throughout its entire length, and so the belt actually 
deformed may be less than the true length of the column, but one 
could not safely assign to such a hypothetical column a greater 
length than the width of the continent. The very long Cordilleran 
chain would make even greater demands in this direction. Moun- 
tain ranges with their long dimension paralleling coasts from which 
the thrusting is assumed to have come, and with a lesser transverse 
dimension in the line of the thrusting, are thus to be considered 
as short blocks. With still less of vertical thickness involved, they 
are perhaps more closely analogous to the deformation of a thin 
prism or wall. 
It is possible, however, that very locally, in a portion of a moun- 
tain range, the conditions of the long column might be operative 
and low-angle faulting might result, but it hardly seems likely that 
this principle can, in any large measure, be responsible for the great 
overthrusts. The most that can well be claimed for it is that it 
may be a contributing factor. 
Shape.—The shape of cross-section of deformed masses is a 
factor in determining the character of the deformation which 
results under stress. The strength of a column depends on whether 
the ends are free to turn, or are fixed and thus incapable of turning. 
Columns with round ends bend quite differently from those with 
square ends.‘ The shape of a lenslike mass of sediment, weaker or 
stronger than the surrounding rock, may well be important in 
determining the nature of the deformation which takes place when 
the mass is stressed.’ 
D. EFFECT OF ROTATIONAL STRAIN 
As designated by Hoskins? and applied to structural geology by 
Leith,‘ strains due to compression are divided into two classes— 
rotational and non-rotational. Non-rotational strains are defined 
tT. P. Church, Mechanics of Engineering, pp. 300-61. 
2 Suggested by T. T. Quirke, personal communication. 
3L. M. Hoskins, ‘‘Flow and Fracture of Rocks as Related to Structure,’ 16th 
Ann. Rept. U.S. Geol. Surv., Part I (1896), p. 860. 
4C. K. Leith, Structural Geology, pp. 16-21. 
