136 ORIGIN AND DEVELOPMENT OP ORE DEPOSITS. 



Conditions necessary to faulting. From what has gone before 

 it will be evident that, in the great majority of cases, if not in all, 

 faulting is to be regarded as resulting from the action of gravity 

 on an insufficiently supported rock-mass. Plainly, then, there 

 can be no faulting (a) while the mass is still plastic, or (b) while 

 the acting force is less than the cohesion of the material or the 

 friction on its divisional planes. With the case of rock- 

 substance still plastic, in the ordinary sense of the word, we have 

 no more to do, but it is evident that the existence of bed- 

 joints and master- joints must facilitate faulting, since they 

 destroy the strong cohesion between the particles. 



Faults, then, are due, directly or indirectly, to the force of 

 gravity. Yet Mr. Wilson very truly says " Faults are the 

 inevitable result of the elevation of a curved surface."* There is 

 no contradiction here, since elevation and depression are only 

 relative terms, and we have seen that depression produces 

 elevation. The opposition, therefore, so far as there is any, is 

 one of time rather than of agency, and we may say that depression 

 occasions contortion and elevation, and is followed by faulting. On 

 already fractured rocks we have seen that faulting may result 

 from the direct action of gravity, when the gravitating force of 

 an insufficiently supported mass overcomes the support afforded 

 by lateral pressure (itself an effect of gravity), and the friction 

 of the divided surfaces. On unfractured rocks faulting may 

 result when the action of gravity is so great as to overcome the 

 cohesive force of the rock-matter in its weakest part. 



Faulting may also result from the pressure due to the 

 gravitation of a large mass exceeding that of a smaller one, plus 

 the resistence due to the friction of the adjacent surfaces. Thus, 

 in fig. 8a, Plate IX, if the mass A be very great, and the lateral 

 pressures acting in the direction of the arrows, as well as the 

 friction of the surfaces b, c, be small, faulting will take place, 

 and the result will be as in fig. 8b. But if the pressures in the 

 direction of the arrows be very great, the friction on the surfaces 

 b, c, small, and the weight of the mass A also small, it may be 

 forced up, as in the position fig. 8c. It is obvious that faulting 

 by descent of the hanging walls, as in the former instance, will, 



*Op. cit. 



