ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 133 



effect of gravity on the northward lying rocks will be to open the 

 suitably situated joints or fissures, so producing one series of 

 fissures (Fa) dipping away from the granite, and another series 

 (Fc), parallel in course, but dipping in an opposite direction, i.e. 

 towards the main mass of granite. 



The first series will be almost certainly faulted by " descent 

 of the hanging wall," the second may or may not be faulted at 

 this stage. * 



Whenever a re -action sets in, and the elevated region 

 begins again to subside, there will be a re-opening of the same 

 main fissures in preference to the formation of new ones, the 

 somewhat faulted northern fissures (Fa) will probably remain 

 faulted, while the little faulted southern ones (Fc) will now be 

 liable to considerable faulting. At the same time new subsidiary 

 fissures are likely to be opened, so subdividing the prisms in 

 the manner indicated in the two diagrams to wbich the reader is 

 referred. 



The strains occasioned by these various movements will be 

 mainly in directions parallel to the axes of elevation, but the 

 whole rock mass must necessarily be in a state of strain, and the 

 strains remaining after the longitudinal faultings will be best 

 relieved by the production of a second set, as different in 

 direction to the first as possible, i.e. at right angles. These will 

 form the "perpendiculars" to the longitudinal systems. There 

 may still be two sets dipping in opposite directions, as in the 

 case of the longitudinal systems, but the dip on the whole is 

 likely to be greater (more nearly vertical) than in the case of 

 those fissures. 



Mountain Systems. As the "systems of elevation" with 

 their corresponding depressions and longitudinal fissures follow 

 approximately straight lines, we may regard them as portions of 

 great circles; all folds, fissures, &c, which are parallel to each 

 other,! being regarded as belonging to the same "mountain 

 system." 



*If there should be no suitably situated planes of weakness (divisional planes 

 actual or potential) the tendency will be still to rupture the rocks in these direc- 

 tions. 



fParallel, that is as meridians are parallel. 



