ON THE FORMATION OF CAVITIES IN ROCKS. 



17 



1.02.07. The intrusion of igneous dikes may start earthquake 

 vibrations which fracture the firm rock masses. Fissures caused 

 in this way radiate from the center of disturbance or else appear 

 in concentric rings. The violent shakings which so often attend 

 great volcanic eruptions, and the sinking of the surface from the 

 removal of underlying molten material, all tend to form cracks 



FIG. 3. Normal fault at Leadville, Colo. After A. A. Blow. 



and cavities. They are possible causes which may well be borne 

 in mind in the study of an igneous district. 



1.02.08. Faults. When fractures have been formed by any of 

 the means referred to above, and the opposite walls slip past each 

 other, so as not to correspond exactly at all horizons, they are 

 called "faults," a term which indicates this lack of correspondence. 



The separation is chiefly due to the relative slipping down or sink- 

 ing of one side. The distance through which this has taken place 

 is called the amount of displacement, or throw. Faults are most 

 commonly inclined to the horizon, so that there is both a vertical 

 and a horizontal displacement. What would be the dip of an in- 

 clined stratum is in a fault now generally. known as the "hade," 

 although the word formerly had a different meaning. Experience 

 has shown that where beds or veins encounter faults and opera- 



