ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 31 



shearing motion parallel to the surface may have been accom- 

 panied by nonrotational distortion, as a result of horizontal thrust. 

 The deformation resulting from such stresses is shown by figure 

 11. In this case the rectangle ABCD is deformed into the rec- 

 tangle A" B" C" D" . The shortening is here great, and yet the 

 beds are horizontal, although thickened. In nature the two cases 

 may be combined in any proportion. In the Hiwassee slates 

 already mentioned close observation shows slight crenulations of 

 the generally horizontal strata. These suggest that the shearing 

 motion parallel to the surface has been accompanied by horizon- 

 tal shortening, and that both kinds of deformation are here 

 combined. But the relative value of each is entirely unknown, 

 and it is therefore impossible to give any estimate of the amount 

 of crustal shortening involved in the deformation which resulted 

 in the cleavage described. 



We therefore conclude that while monoclinal cleavage over 

 considerable areas may involve no crustal shortening, it is prob- 

 able, in most cases of such cleavage, that there is a real crustal 

 shortening, although it is impossible to estimate its amount. 



After an inclined cleavage has been produced in any region, 

 the conditions of deformation may change as a result of denuda- 

 tion, and fractures may form parallel to the cleavage. These 

 fractures may be wide apart or close together. After these 

 partings are produced, displacements may occur similar to those 

 of joints (Fig. 10) or they may be closed by the falling down of 

 the overhanging material, precisely the same as in the case of 

 ordinary normal faults. The possible elongation resulting from 

 these secondary movements may partly or fully compensate for 

 the earlier movements resulting in shortening. 



Fissility. — Fissility is a name applied to an actual close part- 

 ing of a rock which results in the production of laminae. Fis- 

 sility may possibly develop as an independent structure, although 

 it is believed that it is commonly a structure secondary to 

 cleavage. It is further thought that fissility generally forms as 

 the result of ruptures along shearing planes parallel to the 

 cleavage, from compressive rather than tensile stresses. Where 



