Van Hise and Hoskins — Pre- Cambrian Geology. 209 



than in a stronger bed, it follows that cleavage in a soft layer 

 more nearly accords with bedding than it does in a hard layer. 

 Further, in the more plastic beds cleavage may be developed, 

 and may be absent or imperfect in the more rigid beds. The 

 change in direction may be abrupt, but if there is a gradation 

 between the hard and soft layer the cross cleavage of the hard 

 stratum may vary by a curve into the nearly parallel cleavage 

 of the soft stratum. Since on opposite sides of a fold the 

 readjustment is in opposite directions, the cleavage dips in 

 opposite directions. On opposite sides of an anticline the 

 cleavage usually diverges downward ; on opposite sides of a 

 syncline it usually converges downward. In areas of much 

 overturned monoclinal folds the readjustment between the 

 layers is all in the same direction, and hence the cleavage is 

 rotated in the same direction, and is monoclinal throughout. 



Fissility in heterogeneous rocks may develop in two direc- 

 tions at the same time. In a common case one of these is in 

 shearing planes, and the other normal to tensile planes. The 

 fissility formed in shearing planes may have the same relations 

 to the bedding of hard and weak layers as does cleavage. As 

 rocks in the deep-seated zone in which cleavage develops must 

 pass through the zone of combined fracture and flowage and 

 into the zone of fracture, before they reach the surface, it fre- 

 quently happens that fissility is developed in rocks which had 

 a prior cleavage. As the cleavage planes under these condi- 

 tions are likely to be shearing planes, the fractures of fissility 

 are likely to be controlled in direction by the previous cleav- 

 age, whether it is the direction of maximum tangential stress 

 or not. It is, therefore, concluded that fissility developing in 

 the shearing planes is usually secondary to cleavage developed 

 in the normal planes. In those cases in which the folding is 

 extreme, both the primary and secondary structures may 

 become nearly accordant upon the limbs of folds, but are trans- 

 verse to each other on the anticlines. If the turns in the 

 strata are very sharp at these places, the discrepancy between 

 the two structures may be overlooked, and it maybe concluded 

 that bedding and secondary structures are everywhere accord- 

 ant. Such a mistake may result in a great over-estimate of the 

 thickness of strata, and give erroneous ideas of structure. 



When cleavage and fissility develop there are many slightly 

 separated movements of small degree. When a thrust fault 

 develops there is a single major movement. After a secondary 

 structure has formed in a region, within the cracks of fissility 

 or along the planes of weakness of cleavage, there may be sec- 

 ondary impregnations or injections, and thus banded rocks 

 may owe their structure to fissility and secondary impregna- 

 tions or injections, or both, and the bands may or may not 

 accord with an original structure. 



