32 FAULTS. 



at the upper side twelve or fifteen feet, immediately 

 on entering the adjoining limestone," 1 thereby giv- 

 ing proof that the contraction in the limestone at 

 Caudelaria was much greater than in the hornblende 

 rock. It should, however, be pointed out, that part of 

 the vacancy in the limestone might possibly have been 

 formed by a subterranean stream prior to the deposi- 

 tion of the metalliferous vein, and that consequently 

 the vacancy need not have been wholly due to contrac- 

 tion. If in rock-masses contraction takes place in a 

 vertical direction, there must be unequal subsidences, 

 unless all the rocks are severally homogeneous in the 

 horizontal direction, which is rarely the case. These 

 unequal subsidences must take place along the lines 

 of greater weakness, which will be the master joints ; 

 and such movements must break the continuity of 

 the strata, and change the joints into dislocations or 

 faults. 



Henwood's observations remarkably confirm the 

 connection between faults and the uneven shrinkage of 

 rocks. He states, in reference to the recorded examples 

 of lodes in Cornwall and Devon intersected by the 

 same cross vein : " There is no instance in which 

 motion in one direction, and of the same extent, will 

 restore the continuity of every lode so intersected ; 

 and such uniform motion will, in fact, produce, in 

 their relations on opposite sides of the cross-vein, 



1 Henwood, ibid., p. 83. 



