198 



SCIENCE 



[N. S. Vol. LIII. No. 1366 



higUy successful. The various structures re- 

 sulting from rock failure have usually been 

 explained on the simple conception of the 

 application of a non-rotational stress — either 

 tension, causing elongation in the direction of 

 pull, or simple compression, producing a short- 

 ening parallel to the principal stress and elon- 

 gation at right angles to it. A fold, for in- 

 stance, is assumed to indicate application of 

 stress normal to its axial plane; a set of com- 

 pressive joints is taken to indicate application 

 of stress at 45° to the fractures; cleavage is 

 taken to indicate application of pressure nor- 

 mal to its plane. Experimental work on rock 

 deformation has been conducted mainly with 

 the same limited assumptions, and the results 

 have been widely quoted and applied to the 

 interpretation of rock structures in the field. 

 These conceptions may be correct as far as 

 the immediate feature is concerned, but the 

 forces are only minor constituents of the 

 major causal movement and give no clue to 

 its direction. 



Much less attention has been paid to the 

 conception that the compressional forces may 

 be rotational, that is, that they may be ap- 

 plied in the form of a couple. Under this 

 conception, the net result is a shearing be- 

 tween the heterogeneous rock units along 

 planes ranging from parallel to 45° to the 

 principal axis of stress, the shearing usually 

 accompanied by local tension — in other words, 

 no matter what the origin of compressional 

 stresses and their angle of application, when 

 applied to the heterogeneous rock masses con- 

 stituting the earth they tend as a whole to 

 act in couples and are resolved into compo- 

 nents usually acting in directions inclined to 

 the resulting planes of movement. A moun- 

 tain making movement imder this conception 

 is a shear of certain rock masses over others, 

 resulting in faults, joints, folds, and cleavage. 

 Tensional stresses may be minor consequences 

 of such shear. Field observations within the 

 range of my own experience favor this view 

 of the dominance of shear. It is the view 

 also which geologists have commonly applied 

 to an assumed shear of a thin brittle crust 

 over a thin mobile zone below, though curi- 



ously enough not to the local structures that 

 can be observed. 



Illustration of Shear Structures. — To illus- 

 trate the prevalence of shear structures: Most 

 folds are not symmetrical and indicate by the 

 inclination of their axial planes a drag of one 

 structural unit past another. When this rela- 

 tion is conspicuous they may be called " drag 

 folds." A fold has usually been regarded as 

 indicating direct shortening normal to its 

 axial plane, and therefore application of stress 

 normal to this plane. The Appalachian folds, 

 for instance, have been ordinarily discussed as 

 indicating pressure from the northwest and 

 southeast. The same results, however, can 

 equally well be produced by a differential or 

 shearing movement acting in directions in- 

 clined to the trend of the fold axes or to the 

 moimtain range as a whole. Experimental 

 reproduction of Appalachian folds under 

 shearing stresses gives more satisfactory re- 

 sults than experiments with normal shorten- 

 ing.^ The folds indicate the direction of the 

 shortening or elongation, in other words of 

 the nature of the strain, but not the angle of 

 application of the stress. 



The interpretation of rock cleavage or 

 schistosity, a common though not the only 

 evidence of rock flowage, affords an especially 

 good illustration of the danger of using nar- 

 row assumptions as to its relations to causal 

 stresses. Cleavage is a capacity to part along 

 parallel surfaces determined by the parallel 

 dimensional arrangement of mineral particles. 

 There is abundant proof that the schistose 

 rock has been elongated parallel to the cleav- 

 age surface, and cleavage thus becomes evi- 

 dence of elongation. It does not follow, how- 

 ever, that the stress producing elongation was 

 applied normal to it. 



The elongation may well have occurred 

 under a shearing stress of the sort which 

 exists when a mass of dough is rolled out on 

 the table by the application of stress inclined 

 to the table surface. Field studies of cleavage 

 seem to indicate that in the majority of cases 



2 Mead, W. J., ' ' Notes on the Mechanics of Geo- 

 logic Structures," Jour. Geol., Vol. 28, 1920, pp. 

 521-523. 



