ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 131 



able to flow over it, the rock is somewhat abraded. But if there 

 be many joints, rain, frost, vegetation, gravity all get to work, 

 and the destruction is often very rapid. The origin of deep 

 ravines, vertical escarpments, and caverns, so common in lime- 

 stone centres, can be readily traced to the nature of the prevalent 

 jointing. It is probably not too much to say that the minor 

 features of surface sculpturing on the earth's surface have been 

 more determined by the nature of the jointing than by all other 

 causes taken together, at any rate, in countries where rain and 

 frost are working agents. 



Chap. II. — The Mechanical Phenomena of Faulting. 



Conversion of structural planes into fissures. "We have seen that 

 joints are, for the most part, produced by shrinkage or strain. 

 So long, therefore, as the rocks are subject to great pressure the 

 divisions will either not exist or they will be very fine and 

 close, unless there has been previous displacement by violent 

 earth-movements.* We have seen, too, that slaty cleavage is a 

 limited phenomenon. It can only be produced in fine-grained 

 rocks, for the most part, even when the necessary pressure and 

 " shear " action is present. Lamination is also a somewhat local 

 character of rocks. 



We have then to consider the case of rocks, already traversed 

 by divisional planes, either actual or potential, some being 

 either parallel in direction with a not far distant axis of 

 elevation, or at right angles to such an axis. Should now the 

 shrinkage still continue, or should the upheaval of the anticline 

 or the subsidence of the syncline be renewed, the lateral 

 pressure being in either case relieved, some of the longitudinal 

 divisional planes will be opened up into distinct fissures, since 

 they are already, by hypothesis, planes of weakness. Hence 

 it is that the lodes in a district frequently coincide in direction 

 with the main joints. These forces thus act separately or 

 together, to open the fissures more and more ; and the tendency 

 will be to concentrate the action in a comparatively few well- 



*At great depths it is probable that there are no joints, since the rocks in 

 such situations must be under great pressure, and must possess a considerable 

 amount of plasticity. Yet at still greater depths there may be rare but 

 extensive fissures gaping downwards. 



