164 



MESSRS. KIRKBY AND DUFF ON THE 



a fracture are scarcely altered, while 

 on the other they are very much dis- 

 turbed for several yards from it. 

 We at first thought that it was al- 

 ways the low side that was most 

 disturbed, but additional experience 

 showed us that sometimes one and 

 sometimes the other was affected, 

 and in certain cases both; while again 

 in other instances the strata on nei- 

 ther side were much altered. The 

 amount of throw has little to do with 

 it, nor yet the inclination of the plane 

 of fault. It is perhaps likely that 

 the most broken side represents the 

 side offering the least resistance to 

 the disturbing force, which in most 

 cases would be that in motion. It 

 is also probable that in many cases 

 the movement of a mass of strata in 

 the act of displacement by faulting, 

 would be rather that of a tilting than 

 of a uniform depression or elevation 

 of the whole ; that is, while it moved 

 downward along one plane of dislo- 

 cation it would move upward along 

 the plane of another, hence forming 

 faults of depression and elevation 

 contemporaneously, the position of 

 the centre or axis on which the move- 

 ment took place determining the rela- 

 tive amount of throw of each. Hence 

 we have a possible explanation, not 

 only of the broken sides of faults, but 

 of the well-known facts that the in- 

 clination of strata is generally to a 

 downthrow, and their declination to 



