PRINCIPLES CONTROLLING ORE DEPOSITS 



741 



below is practically impervious. However, an impervious limit- 

 ing stratum may exist at depths far less than the bottom of the 

 zone of fracture. An impervious limiting stratum, perhaps a 

 shale, may be found at a depth ,of 100 meters or.less, or at any 



Fig. 2. 



depth intermediate between this and the bottom of the zone of 

 fracture for the strongest rocks. Where there are one or more 

 pervious strata which are inclined and above, below, and between 

 which are impervious formations, there may be two or more 



