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POPULAR SCIENCE MONTHLY 



vertical crack. The principal displacement was not vertical, but hori- 

 zontal. If one thinks of the land to the east of the crack as stationary, 

 then the change may be described as a northward movement of the land 

 west of the crack. If the land to the west be thought of as stationary 

 then the land to the eastward moved toward the south. It is probable 

 that both tracts shared in the movement, the eastern shifting toward the 

 south and the western toward the north. Perhaps the nature of the 

 change can be more readily understood by reference to Tig. 4, which 

 represents an ideal block of the earth's crust, 100 feet square on the 

 surface and 25 feet deep, before and after its division and dislocation 

 by the earthquake-causing fault. 



Wherever a fence, road, row of trees, or other artificial feature 

 following a straight line was intersected by the fault its separated parts 

 were offset, and an opportunity thus afforded for measuring the 

 amount of change. The measurements range in the main from 6 to 15 



Fig. 5. A Faulted Road near the Head of Tomales Bay. The nearer and more dis- 

 tant parts of the road were originally in one line— a continuous, straight road. The present t ff- 

 set is twenty feet. 



