132 OEIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



situated fissures rather than to spread it equally over the whole 

 number of divisional planes. This must be so, in fact, because 

 the fissures are altogether actual divisional planes, while the 

 unfissured cleavages and laminations are only potential. 



Conversion of fissures into faults. Fissures being once formed 

 in a rock-mass, successive pressures or shrinkages will be likely 

 to produce movements in the planes of Assuring and consequent 

 faultiDg. Ordinarily their movements will be slow, gradual, 

 propably imperceptible, except after the lapse of considerable 

 periods of time. When they occur suddenly there must be 

 earthquakes of greater or less severity, although it does not 

 follow that a great earthquake implies a great fault or vice versa. 



Lode-fissures are often spoken of as results of earthquake 

 action. It is likely enough that some fissures have been thus 

 formed, but in general it seems much more likely that they have 

 been formed by successive shrinkage and consequent faulting of 

 the rocks, as described above, so causing earthquakes. 



The fact that many mining districts are remarkably free 

 from earthquake shocks has sometimes been urged against the 

 view that faults indicate earth movements. But, in truth, they 

 only indicate that such movements have taken place in former 

 times. In many instances the numerous fractures have been so 

 solidly repaired as to render a fractured region one of the 

 specially strong parts of the earth's crust. 



We shall see hereafter why the rupture of rocks of particular 

 texture favours the production of cavities suitable to serve as 

 ore-deposits, while others are, in this respect, unfavourable. 



M. Leon Moissenet has illustrated, in a generalized way, 

 and apart from the concurrent effects of denudation, the 

 mechanical results likely to be produced in a region where strati- 

 fied rocks rest upon the flanks of a mass of granite, at a 

 moderate angle, the district being subjected to a series of 

 movements of upheaval, followed by partial subsidence.* The 

 system of upheaval is supposed to be east and west, and the 

 granite to the south is slowly elevated, the slate resting on its 

 northern flank being elevated with it, but not to the same extent. 

 If we suppose the elevatory force to be localized southward, the 



*See Moissenet, Lodes of Cornwall, 1887, p. 35, and plate III, figs. 1 and la. 



