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. 
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. 
