FAULTS. 35 



taken place they in some places running rudely 

 parallel to one another, in others branching off; or 

 all the movements may be confined to one narrow 

 line. That movements or faults are prone to occur 

 over and over again in nearly similar lines seems 

 proved here; as in this tract between Cleggan and 

 Dogs' Bays there are proofs of two ; while probably 

 three or four movements have taken place. The 

 first, post-silurian, but prior to the rocks having been 

 metamorphosed, as the fault-rock, after it was formed, 

 was altered along with the associated rocks. The 

 second later, but still pre- carboniferous. Of this move- 

 ment there are here no positive proofs ; as there are 

 no carboniferous rocks in the immediate vicinity ; but 

 movements during this period are known to have 

 taken place in other parts of the district. The third, 

 post-carboniferous ; the exact time of this movement 

 is uncertain. T^Q fourth, post-glacial. 



As in master-joints, so also on fault-lines, open 

 fissures may form, when deep-seated, to be filled with 

 minerals, but when at the surface, to remain open 

 until they are more or less modified by meteoric 

 abrasion. As in the recent accumulations, so also 

 among the older rocks, a vacancy of greater or less 

 magnitude will probably be produced by this agency 

 at the juncture of two or more fissures (figs. 4 and 

 5, PI. I.) The open space due to contraction may 

 not always be continuous in one line, especially in 



