266 Rev. 0. Fisher — On Faulting, Jointing and Cleavage. 



V. — On Faulting, Jointing, and Cleavage. 



BytheEev. 0. Fisher, M.A., F.G.S. 



[Continued from ^. 2\Z.) 



PART III. 

 Keveesed Faults. 



WHEN the horizontal stress is a compressing stress, and exceeds 

 the vertical stress, if motion of ^ D B is caused, it will take 

 place in the direction B A ; and our formula becomes 

 ^(P_7F) sin2 0=;(t. (1.) 



Where P is the horizontal and W the vertical compressing stress 

 and /A the force along B A which resists motion, all per unit area. 



This condition must be satisfied in order that motion along B A 

 may take place by shearing. 



If, as before, v be the coefficient of friction, the friction upon 

 unit area will now be, 



,. (Pfcos- e+Psin^e). (2.) 



Since P is always greater than W in the case of reversed faulting, 

 this will be least when sin has the least ratio to cos 6 which the 

 conditions admit of. Hence the lesser angle which satisfies the con- 

 dition (1) will, when other circumstances are favourable, determine 

 the hade of the fault. We have already seen that a direct fault 

 must have a higher hade than 45°. We may therefore conclude, 

 on both accounts, that any fault having a lower hade than 45° must 

 be a reversed fault. 



But the converse conclusion does not hold, viz. that a reversed 

 fault will necessarily have a lower hade than 45°. It will depend 

 upon the room available for the motion, as well as upon other 

 circumstances which cannot be easily appreciated. It is, however, 

 clear that a fault cannot be formed unless the shearing force along 

 B A\& greater than the friction, or 



(P_ W) sin e cos e > V {TF cos^ 6 -\- P sin'' 0), 



or tan < ..— ^+ ^ [^-^-^ ) 



2v P 



W 



P ' 



The highest possible value of will be given when P is infinitely 

 greater than TF, and then we must have tan < — 



or cot > V. 



