part 1] AI^I^IVERSARY ADDRESS OF THE PEESIDEIJ^T. IxXXl 



be so ^Drodueecl if there were no such thing as friction ; but 

 it is no less certain that when friction along the surface of the 

 fault is taken into consideration it would be impossible for hori- 

 zontal compression alone to give rise to displacement along a 

 fault-surface, where the hade did not exceed 30° from the vertical, 

 and doubtful if the hade were much less than 45°. 



The reasoning is quite simple, clear, and conclusive ; there is a 

 certain angle of inclination marking the extreme slope at which 

 one body will rest on another : if the slope is less no movement 

 will take place, if steeper the upper one will slide over the lower 

 down the slope separating the two. This limiting inclination is 

 known as 'the angle of repose,' and varies according to the substance 

 and nature of the surface; for highly finished and well-lubricated 

 metallic surfaces it is only a few degrees, for dressed stone it is 

 not far short of 30°, for an undulating surface, such as is found 

 in even the cleanest-cut fault, the angle would be still higher. 

 N"ow what is true where the bodies are affected by the vertical 

 force of gravity is equally true of an}'' other force acting in any 

 other direction, the angle being measured from a plane at right 

 angles to the direction of pressure, that is from the vertical, when 

 compression takes place in a horizontal direction. Hence it re- 

 sults that horizontal pressure could not, by itself, give rise to 

 movement along the fault-surface unless the hade were at least 

 30° from the vertical ; where the hade is less, pressure would only 

 lock the two surfaces more closely together, and increase the 

 resistance to movement. 



So far the reasoning is clear and conclusive ; it is an error, how- 

 ever, to draw, as has been done, the conclusion that reversed faults 

 of lower hade than this limiting angle could not have originated 

 as such, but must have been formed as normal faults, to become 

 apparently reversed through tilting subsequent to their formation. 

 This is one, though not the only, possible deduction from observed 

 facts ; for the alternative is open to us that the forces which pro- 

 duced movement along the fault were either vertical, or possessed 

 a considerable vertical component, in the direction in which they 

 acted. 



A similar conclusion would result from field-observations of 

 reversed faults. I have myself repeatedly found reversed faults in 

 soft tertiary shales and sandstones, and on the rare occasions when 

 it was possible to find the actual fault-plane unobscured by surface- 

 debris or weathering, it was frequently a clean-cut surface, along 



yoL. i,xxvii, / 



