March 4, 1921] 



SCIENCE 



199 



the cleavage is merely the expression of tlie 

 yielding of a weaker formation or weaker 

 part of a formation by a slipping or differ- 

 ential movement between harder members. 

 Even in areas with regional cleavage, the 

 same interpretation may be applied on a large 

 scale when the harder units in adjacent ter- 

 ranes are taken into account. My own ob- 

 servations in old pre-Cambrian terranes tend 

 to the conclusion that cleavage, indicating 

 rock flowage, has been confined to compara- 

 tively narrow mesh-like zones between large 

 massifs. The evidence leading to this conclu- 

 sion that cleavage is the result of slipping be- 

 tween rock masses may usually be checked by 

 drag folds which develop simidtaneously in 

 the softer rocks, and by fissures and faults 

 which develop simultaneously in the harder 

 rocks. 



The zones of movement marked by cleavage 

 may have almost any inclination or direction, 

 but the plane of the cleavage itself has a 

 strong tendency toward steep inclination or 

 verticality. Both in strike and dip the 

 cleavage is more uniform than the movement 

 zone of which it is a part. The relation is 

 not unlike that between folds and cleavage, to 

 be presently discussed. This steep inclination 

 of cleavage does not necessarily indicate pre- 

 vailing horizontality of stresses on the as- 

 sumption that cleavage must develop normal 

 to stress. In part it may have this relation- 

 ship, but when considered in relation to folds 

 and relative movement of adjacent massifs it 

 more often indicates shearing stresses in- 

 clined to the cleavage. So far as any general 

 inference is possible, the tendency of cleavage 

 to show imifonn strike and steep inclination 

 over great areas suggests differential move- 

 ment in vertical or steeply inclined planes, 

 the movements in these planes ranging from 

 vertical to horizontal. It can not be ex- 

 plained by movement along planes tangential 

 to the earth, which would require prevalence 

 of flat or gently inclined cleavage. In short 

 the attitude of cleavage, so far as it may be 

 generalized, does not correspond to the con- 

 ception of the tangential shearing of a com- 

 petent surface zone over a mobile zone below. 



Cleavage has a definite relationship to folds 

 which is of great usefulness in interpretation 

 of rock structures, and which affords valuable 

 suggestions as to the general relations of 

 cleavage to the great zones of flowage of 

 which it is often an expression. Cleavage is 

 approximately parallel to the axial planes of 

 the folds. It therefore usually stands more 

 steeply than bedding and is more uniform in 

 dip and strike than bedding. Where cleavage 

 is noted in a rock outcrop, the direction and 

 inclination of the axial planes of the folds 

 are thereby indicated — not only for the folds 

 within the rock observed, but also, usually, 

 for the folds in the adjacent rocks as well. 



As a consequence of the fact that cleavage 

 is roughly parallel to the axial planes of folds, 

 it follows that the trace of any bedding plane 

 on the cleavage surface indicates approxi- 

 mately the direction and degree of pitch of the 

 fold, that is, the inclination of the axial line 

 of the fold to the horizontal. A single frag- 

 ment of cleavable rock appearing in an out- 

 crop may be sufficient to establish the pitch 

 for a considerable area. 



The inclination of bedding to cleavage — al- 

 ways remembering that the latter indicates the 

 attitude of the axial plane of the fold — indi- 

 cates faithfully the position of the observed 

 bedding on the fold, whether the fold be up- 

 right inclined or overturned. This principle 

 is useful in determining whether a bed is right 

 side up or overturned. Inferences of the same 

 sort may be drawn from strike observations on 

 bedding and cleavage in deformed areas. 



Still further, cleavage is a phenomenon of 

 rock flowage. The very existence of cleavage, 

 therefore, means the rock has been deformed 

 under the conditions of rock flowage, where 

 the folds are likely to be of a rather intricate 

 type, with much interior thinning and thicken- 

 ing of the beds. Even though evidences of this 

 folding are not immediately at hand, the very 

 existence of cleavage on a considerable scale 

 indicates with reasonable certainty the exist- 

 ence not only of folds, but folds of the rock 

 flowage type. 



All of these inferences may be made indue- 



