448 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 



rocks, — " That there has been a compression in the mass in a direction every- 

 where perpendicular to the planes of cleavage, and an expansion of this mass 

 along the planes of cleavage in the direction of a line at right angles to the line 

 of incidence of the planes of bedding and cleavage," or, in other words, to the di- 

 rection of the dip of the cleavage. From this view of the mechanical nature and 

 direction of the force engendering cleavage, I beg leave respectfully but explicitly 

 to dissent. 



Fan-like Arrangement of the Cleavage at the Anticlinal and Synclinal Axis Planes. 



A second general fact or law of direction of the cleavage planes in folded strata 

 must be here enunciated. At first view it is in seeming contradiction with the 

 universality of the primary rule above stated, of the invariable approximate pa- 

 rallelism of the cleavage planes to the axis planes of the flexures ; but closely 

 examined, it will be seen, I think, to be in beautiful accordance with that law 

 and with my hypothesis of the origin of the cleavage structure. The rule is this, 

 that where the cleavage is fully developed, and the anticlinal and synclinal flexures 

 are also conspicuous and very sharp, the cleavage planes immediately adjoining 

 those bendings are not parallel to the axis planes, but partially radiate from them 

 in a fan-like arrangement upward in the anticlinals, and downward in the syn- 

 clinals. 



This aberration from the normal direction is furthermore different in degree 

 upon the two sides of the geometric axis plane, being usually greatest upon the 

 inverted or steep side of the wave. 



Fig. 4. 



Kb 



Fan-like Arrangement of Cleavage at an Anticlinal Axis. 

 a, Cleavage in the Shale. b, b, Axis Plane. 



Another aberration of the cleavage planes from their normal direction of 

 parallelism to the axis planes, is their tendency to conform partially to the dip of 

 the strata, when the two are nearly coincident. This operates to flatten the 

 inclination of the cleavage upon the gentler slope of each wave, and steepen it 

 upon the more inclined one ; and as in every belt of uniform flexures closely 

 plicated with inversions, the uninverted or normal dips greatly exceed the inverted 

 ones, it produces in such cases a prevailingly lower inclination in the planes of 

 cleavage than in the planes bisecting the flexures. 



