130 ORIGIN AND DEVELOPMENT OF OKE DEPOSITS. 



contorted district, it is obvious that the lamination would assist 

 and not retard the production of the true cleavage. Again, if a 

 mud containing numerous flakes of mica were to be converted into 

 a slate, the flakes being forced to take up positions such that the 

 basal planes were coincident with the cleavage, the tendency for 

 the rock to split along those planes might be increased. A 

 similar result would follow in the case of a foliated rock, where, 

 if the induced cleavage tendency coincided with the cleavage 

 planes of one set of minerals, there might be a very good 

 cleavage produced, while if the directions were different the 

 cleavage tendency would be obstructed or retarded. 



In leaving this part of the subject, it may be observed that 

 cleavage in all its forms differs from every form of jointing, in 

 the fact that the rock is physically continuous. A rock may be 

 minutely and regularly jointed, but there will be no special 

 tendency to fracture observable parallel to the joints, while in 

 the case of truly cleavable rocks the finer the particles of which 

 the mass is composed the more practically unlimited is the 

 cleavage. In the best instances, in fact, cleavage is only limited 

 by the transverse weakness of the separated plates. Thus joints 

 may be called actual, while lamination, foliation, quarry cleavage, 

 and true slaty cleavage only give rise to potential divisional 

 planes. 



Besides jointing and cleavage there are other potential 

 divisions produced in rocks, which are sometimes of geological 

 and mineralogical importance. Thus, for instance, concretionary 

 structure in its many modifications. Some of these will be more 

 particularly alluded to in a future chapter. 



Geological importance of jointing. From a geological point of 

 view the importance of jointing is very great. A rock having 

 few joints, and those lying horizontally or dipping "into the 

 hill," is almost indestructible by natural agencies ; whereas, if 

 the joints are numerous, and especially if they are highly inclined, 

 the rain can freely enter, frost has great power to act, and rapid 

 denudation takes place. This is especially noteworthy in the 

 case of limestones. So long as there are few joints the rock 

 remains almost unchanged. True it is slowly dissolved if water 

 falls upon it, and if moving water holding in suspension sand is 



