200 



SCIENCE 



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



tively from a surprisingly narrow range of ob- 

 servation. 



These remarks on cleavage apply to the 

 structure ordinarily associated with the de- 

 formation of rocks which is almost without 

 exception inclined to bedding or other pri- 

 mary structures. They do not apply to cleav- 

 age developed solely by load or gravity, which 

 might reasonably be expected to be horizontal. 

 The latter type of cleavage has been described 

 for certain terranes and districts, as for in- 

 stance in the Belt series of the Canadian 

 boundary; but within my own observation of 

 deformed areas it is a phenomenon of such 

 local and special character as not to invalidate 

 the generalizations above made. So far as 

 load cleavage is assumed to develop under 

 static conditions of load, without movement, 

 I doubt its existence. Cleavage iisually indi- 

 cates movement, not static pressures. 



The interpretation of jointing and faulting 

 has likewise suffered from far too narrow and 

 simple assumptions of the mechanical condi- 

 tions. Quoting from a recent paper by Mead,^ 

 such a simple structure as an open fissure or 

 joint " obviously due to tensional stresses (so 

 far as the fissure itself is concerned) may be 

 {in incident in simple elongation, shear, cross- 

 Jsending, compression or shortening, or tor- 

 sional warping. A reverse fault implies con- 

 ditions of shortening or compression but may 

 in addition to this possibly be an incident 

 in a general shearing movement, or a phenom- 

 enon of simple cross bending, or may be due 

 to torsional warping." In my own field of ex- 

 perience I have been impressed with the fre- 

 quency of joints and faults developed as inci- 

 dents in differential or shearing movements. 

 ^There is rapidly accumulating evidence of the 

 existence of great thrust faults with low dips 

 as prominent features of diastrophism. 



When the shearing movements have been de- 

 termined by the study of a single type of struc- 

 ture like folds, important corroborative evi- 

 dence may be obtained from other structures. 

 Instead of regarding structures as independent 

 units, each with its own set of mechanical con- 

 ditions, they may be viewed as a group expres- 



a Loc. cit., pp. 505-506. 



sion of some major movement. When so 

 viewed the shearing nature of the movement 

 pften becomes obvious. 



, Distribution of Movements. — ^Within our 

 iZone of observation, it is difficult to say induc- 

 tively whether or not there has been more 

 movement or less movement with depth, 

 ^either is it possible with any satisfactory de- 

 gree of definiteness to discern controlling atti- 

 tude or pattern in the complex of movement 

 zones. The zones range from vertical to hori- 

 zontal, are parallel or intersect. The original 

 Jiorizontal position of stratified rocks naturally 

 suggests dominance of the horizontal element 

 in movements affecting them, because of reso- 

 Jution of forces along bedding planes of weak- 

 ness, but the beds soon become inclined or ver- 

 tical when deformed and disturbed zones may 

 be anything but horizontal. The less deformed 

 piasses between may have almost any shape, 

 locally they may be discoidal, or sheet-like, or 

 oval or rod-shaped, or rhomboidal. Interesting 

 attempts have been made to discern some con- 

 trolling pattern, both in large and in small 

 ^ructural features, but subjective hypotheses 

 pnter to so large an extent that the reality of 

 the pattern presented is often not convincing 

 to others. 



Possible Increase of Roch Flowage with 

 Depth. — Within a few hundred or at most a 

 ;few thousand feet of the surface, fracturing, 

 pnuch of it open, is clearly the dominant proe- 

 less, though even here soft rocks may yield by 

 fiowage. In the lower part of the zone of ob- 

 servation combined fracture and flowage is the 

 jule. Fractures are more commonly of the 

 closed shearing type. It has been easy to as- 

 sume that this combination is merely transi- 

 tional to a zone of flowage below. The fact 

 that rocks which have been deeply buried are 

 often highly schistose as a result of rock flow- 

 age has been cited as indicating increased rock 

 flowage with depth. I have shared in this 

 view. From some familiarity with ancient and 

 formerly deeply buried terranes, I am not sure, 

 however, but that a careful inductive study of 

 £eld sections requires considerable qualifica- 

 tions of this generalization. Many instances 

 piay be cited of rock flowage occurring high 



