422 University of California Publications. [Geology 



comes indefinitely thin and the number of cards indefinitely large, 

 is called a shear. 



When we notice the curvature of the broken line in figure 2, 

 we are inclined to think that the rock was bent like a beam ; but 

 when we reflect that the breadth of the beam would correspond 

 to the length of the fault, 270 miles, and the length of the beam 

 to the distance from the fault to which the elastic strain extended, 

 and which was probably not more than six, and certainly not 

 more than ten miles, we see that the length of the beam would 

 be so very short in comparison with its breadth, that the charac- 

 teristics of a beam would be entirely lost. We must, therefore, 

 look upon the elastic strain as a shearing strain alone, parallel 

 with the fault. 



Fig. 3 Fig. 4 



We can imitate the movements experimentally. Two short 

 pieces of wood were connected by a sheet of stiff jelly one-half 

 inch thick, and two inches wide, and about three inches long, as 

 shown in figure 3. The jelly was cut through along the line, W , 

 by a sharp knife, and a straight line, AC, was drawn in ink on 

 its surface. The left piece of wood was then shifted about one- 

 half inch in the direction of t', and a gentle pressure was applied 

 to prevent the jelly from slipping on the cut surface. The 

 jelly was sheared elastically and the line took the position AC 

 shown in figure 4. On relieving the pressure so that the friction 

 was no longer sufficient to keep the jelly strained, the two sides 

 slipped along the surface W and the line AC broke into two 

 parts, AB and DC. (The broken lines represent positions im- 

 mediately before the slip, and the full lines immediately after it.) 



