PHYSIOGRAPHY. 



[chap. 



abruptly to an end, that their continuity is suddenly broken, 

 and that one set of beds abuts upon another along a sharply- 

 defined plane. The beds have, indeed, been fractured, and 

 have slid one over another. Such a fracture, accompanied 

 by displacement of the strata, constitutes what geologists call 

 z. fault. Thus the set of beds represented in Fig. lo have 

 been broken along a plane represented in section by the 

 line D E ; and, though once continuous, are now dislocated ; 

 the bed A having been thrown down to A', the bed B 

 having slipped to the position B', and the bed C to C. 

 The drainage received by the surface of the porous stratum 

 B, will flow down until it reaches the fault, where it will be 



Fig. io. — Effect of fault on position nf spring or well. 



preventejd from escaping by the clay wall of A'. If there- 

 fore a bore-hole be put down to F, the water which has 

 percolated through the bed down to this point will be 

 forced upwards by the pressure of the water in the sur- 

 rounding rock, and will therefore rise in the hole to nearly 

 the level which it occupies in the bed B. Or, in the absence 

 of a bore-hole, the water will escape from the saturated 

 bed B, by oozing out at the surface, near the junction of 

 the adjacent strata. It is obvious, from this illustration, 

 that faults must be of great importance in determining the 

 position of springs and wells. 



