30 



C. R. VAN HISE 



ing is inclined to the surface of the earth. The shortening 

 involves an equivalent elongation in another direction. This 

 elongation is at right angles to the direction of shortening, and 

 is inclined to the surface of the earth in a direction opposite to 



FfCQ 



D C 



Fig. ii. — Inclined cleavage produced by shearing motion parallel to bedding 

 without crustal shortening, and vertical cleavage produced with crustal shortening. 



the direction of shortening. The forces producing a shear 

 involve two couples, which at any given moment produce a ten- 

 sion in the direction of elongation and a compression in the 

 direction of shortening. Thus, as a result of the work of the 

 two couples in the production of cleavage by shearing parallel 

 to the bedding, the direction of tension and the direction of 

 shortening being inclined to the surface of the earth in opposite 

 directions, are in such relations to each other at any given time 

 and place that the total effect is neither elongation nor shorten- 

 ing of the crust of the earth. 



This is illustrated by figure II. The rectangle ABCD is 

 deformed into the parallelogram A'B' C D' , by shearing motion 

 parallel to the bedding. The cleavage is parallel to the flat- 

 tened ellipsoids. The area of the rectangle and parallelogram 

 are the same, and also the line A'B' at the surface of the 

 cleaved rock is of the same length as the line AB of the original 

 rock before it was deformed and cleavage produced. The Ocoee 

 slates of the Hiwassee river, west of McFarland for several 

 miles, show a cleavage dipping to the southeast at an angle 

 averaging about 30 . The beds are easily recognizable, and are 

 very nearly horizontal. They show no bendings which can be 

 dignified by the name of folds. However, even in this case the 



