STRUCTURAL FEATURES 



279 



In hori/ontal n>rks the joints approach verticality, but where 

 tin- rocks have been deformed notably, the joint planes may have 

 any position. In igneous and metamorphic rocks they may simu- 



Fig. 265 Fig. 266 Fig. 267 



Fitf. 265. A possible interpretation of Fig. 264. (Dana.) 



j(>6. A possible interpretation of Fig. 264. (Dana.) 



Fig. 267. A possible interpretation of Fig. 264. (Dana.) 



late bedding planes (Fig. 269). Joints do not ordinarily show them- 

 selves 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 penetrated, they are be- 

 lieved to be relatively superficial phenomena. They must be limited 

 to the zone of fracture, and most of them are probably much more 

 narrowly limited. Many joints end at the plane of contact of two 



Fit,'. 268. Jointed rocks. Cayuga Lake, N Y. (Hall.) 



sorts of rock. Thus a joint extending down through limestone may 

 end where shale is reached. Joints may be offset at the contact of 

 layers or formations, and a single joint may give place to many 

 smaller ones. All these phenomena may be explained by the vary- 

 ing elasticity of various sorts of rock. Generally speaking, rigid 

 rock is more readily jointed than that which is more yielding. 



