0KIGIN AND DEVELOPMENT OF ORE DEPOSITS. 137 



in a shrinking globe, be much more common than by its ascent, 

 as in the latter, i.e. faults resulting from the direct action of 

 gravity (direct faults) will be more common than those due to 

 pressure consequent on gravity (reversed faults). 



In the case of a fissure which is vertical, we cannot, of 

 course, speak of either hanging- wall or foot- wall ; very rarely, 

 however, is this the case. In general the fissures are more or 

 less inclined, and the descent of the hanging- wall side goes on 

 until the resistance met with, from friction or wedging, is equal 

 to the force which gravitates the mass towards the centre of the 

 earth. 



Recognition of faulting. This, although sometimes plain 

 enough, is not always an easy matter. Obviously, fault effects 

 are most evident in districts where the several beds vary much 

 in colour and texture, as, for instance, in the Coal Measures ; 

 here the displacements are so evident that probably no one has 

 ever suggested that the broken portions of the beds were not at 

 one time continuous. The case is very different in the West of 

 England. The comparatively little variation in character of the 

 contiguous beds of killas in the mining districts of the West of 

 England has veiled, in many instances, notable faulting. Speak- 

 ing of the difficulty of detecting faults in such a country, 

 Delabeche says " Their very abundance renders them difficult to 

 trace with accuracy, as it is only when dissimilar rocks are 

 brought into contact that this can be satisfactorily accomplished," 

 and again, " similar slates of the granitic series being alone 

 brought into vertical contact with each other, it cannot be 

 satisfactorily traced."* 



There are, indeed, many plain indications of faulting arising 

 from the intersections of lodes, as will be seen hereafter, but the 

 practical miner, who too often looks upon all the surroundings 

 of a lode as its consequences rather than its cause, has frequently 

 and, indeed, usually misinterpreted such phenomena. 



Direction of movement The terms " downthrow " and 

 "upthrow" are, of course, relative to each other, and they are 

 not much used except in "bed" or "flat" mining, yet they 

 express a truth which is often lost sight of in vein -mining, viz., 



* Eeport, &c, pp. 293-5. 



