ROTATORY DISPLACEMENTS 189 



ment. We can not look upon such a large mass as acting like a rigid 

 body; there is a certain amount of distortion as the fault dies out near 

 its ends, and the rotation is probably not constant along the fault or at 

 right angles to it, the differences being permitted by small plastic distor- 

 tions. The folded strata of the Sierras would make it impossible to de- 

 termine, by their positions, the variations in the rotation of different 

 parts of the mass; but physiographic methods might yield more definite 

 results. The Wahsatch mountains offer another example of a large 

 tilted block; its western boundary fault must be a surface of revolution. 



A landslide where the mass holds its form and is not broken is an 

 example of this kind of rotation on a small scale. 



Another fairly common example of apparent rotation with the axis 

 parallel with the fault surface is the upturning of the edges of the strata 

 on the downthrow side of a fault, which usually extends but a short dis- 

 tance from the fault-plane. This may be due to a general shear, to a 

 large number of minute faults parallel with the main fault, or to the 

 bending up of the individual strata accompanied by a slight slipping of 

 each stratum upon its neighbor, just as cards slip upon each other when 

 a pack is bent. The last method seems to me the one we should expect 

 to occur most frequently. This is not a true rotation of a block as a 

 whole, but is a distortion of the rock-mass, suggesting a rotation on ac- 

 count of the tilting of the strata. It is better to look upon such a quasi- 

 rotation as a disturbance in the neighborhood of the fault and, as already 

 noted, to make our observations for displacements at a greater distance, 

 beyond the zone of upturning. 



Where a simple rotation has occurred it is easy to determine its axis 

 and amount, if we know the plane of rotation. Let figure 11 be a section 

 in a plane at right angles to the axis ; 

 let a and a' be the disrupted parts 

 of the same stratum before and after 

 rotation; extend the directions of 

 these parts until they meet in 0' ; 8 yl *~"' 



will be the angle of rotation. Erect @ ,'f'^"5~~~ 

 a perpendicular to the middle point "^ 



a ,, ,. ' .. _ . ., Figure 11. — Simple Rotation 



ot the line connecting a and a , and 



find a point on it at which the line aa! will subtend the angle 8; this 



point, 0, will be the axis about which the rotation took place. 



ROTATION WITH TRANSLATION 



Where both rotation and translation have taken place we may either 

 suppose a particular point of the rock to have been moved directly to its 



>?---. 



