ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 29 



However, since the hade of a reverse fault is usually flatter 

 than that of a normal fault, the shortening due to a given ver- 

 tical displacement of a reverse fault exceeds the shortening due 

 to the same displacement of a normal fault. In considering the 

 elongation or shortening of the crust of the earth by faults, this 

 factor must be considered, as well as the factor of their relative 

 frequency and area of distribution. However, reverse faults are 

 usually confined to closely-folded areas, while normal faults fre- 

 quently occur in these same areas, developing in the final stages 

 of deformation, and are also present throughout great regions 

 where reverse faults are absent, as, for instance, in the Great 

 Basin and Plateau regions. It is therefore wholly possible that 

 the amount of shortening of the crust of the earth resulting from 

 reverse faults is more than compensated by elongation of normal 

 faults, and thus the sum total of deformation by faulting result 

 in dilatation rather than shortening of the crust of the earth. 



Cleavage. — Cleavage has been supposed necessarily to indi- 

 cate an important shortening of the crust of the earth. 



It is, however, to be remarked that the shortening of cleav- 

 age must not be considered if the amount of shortening involved 

 in the folding is counted for the same region ; for cleavage is a 

 phenomenon which may result from distortion under conditions 

 of flowage, and the shortening represented by folding includes 

 that involved in the simultaneous production of cleavage. 



Moreover, cleavage is possible without any shortening what- 

 ever. I have shown in a previous paper 1 that cleavage may be 

 produced by simple shearing motion parallel to the surface of 

 the earth. The inclination of the cleavage will depend upon the 

 amount of the shearing. Shearing of a very moderate amount 

 will produce cleavage with dip as low as 30 . In the production 

 of cleavage by shearing, each individual particle is shortened. 

 Where shearing motion parallel to the surface of the earth 

 results in cleavage inclined at 30 , the amount of shortening of 

 each particle is about .4. However, the direction of shorten- 



1 Deformation of rocks, by C. R. Van Hise : JoURN. Geol., Vol. IV, 1896 

 pp. 636-637, 868-872. 



