STRUCTURAL GEOLOGY. 



511 



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 are to be explained on the basis of the different constitution 

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

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



Joints may remain closed, or they may gap. In the latter case, 

 they may be widened by solution, weathering, etc., but they are quite as 





Fig. 411. — This diagram might represent either isoclinal or monoclinal structure. 

 In the former case the strata might have the structure shown in any one of the 

 following Figures, 412 to 414, so far as dip and strike show. (Dana.) 





V..-- 



^-..y 



'r~:\\ 



\ \ u \u\\\\\\ w '. \ • 



Fig. 412. Fig. 413. Fig. 414. 



Fig. 412. — A possible interpretation of Fig. 411. (Dana.) 

 Fig. 413. — A possible interpretation of Fig. 411. (Dana.) 

 Fig. 414. — A possible interpretation of Fig. 411. (Dana.) 



1^^^ 



Fig. 415. — Jointed rocks. Cayuga Lake, N. Y. (Hall.) 



likely to be filled by detritus from above, or by material deposited from 

 solution (veins). It is along joint-planes that many rich ore-veins are 

 developed (pp. 478-484). 



Joints have been referred to various causes, among which tension^ 



