STRUCTURAL GEOLOGY 407 



In undisturbed rocks the joints approach vertically, but in 

 regions, where the rocks have been notably deformed, the joint 

 planes may have any position. In igneous and metamorphic rocks 

 they may simulate bedding planes (Fig. 328) . Joints do not ordi- 

 narily show themselves at the surface in regions where there is 

 much mantle rock, but they are readily seen in the faces of cliffs, 

 in quarries, and, in general, wherever rock is exposed. Though 

 some of them extend to greater depths than rock has ever been 



Fig. 328. Tabular joints in granite. Summit of Goatfell, Arran. (H. M. 



Geol. Surv.) 



penetrated, they are believed to be relatively superficial phenomena. 

 They must be limited to the zone of fracture, and most of them are 

 probably much more narrowly limited. Joints frequently end 

 at the plane of contact of two sorts of rock. Thus a joint extend- 

 ing down through limestone may end where shale is reached. 

 Joints are frequently offset at the contact of layers or formations, 

 and a single joint sometimes gives place to many smaller ones. All 

 these phenomena may be explained by the varying elasticity of 

 various sorts of rock. Generally speaking, rigid rock is more read- 

 ily jointed than that which is more yielding. 



Joints may remain closed, or they may gape. They may be 

 widened by solution, weathering, etc., and they may be filled, 



