STRUCTURAL GEOLOGY 



495 



One of the most extraordinary features of some concretions of com- 

 plex form is their s}iiimetry. This may be of various phases ; in excep- 

 tional cases there is a bilateral s^^nmetry almost as perfect as in the 

 higher types of animals. This is especially true of certain calcareous 

 concretions developed in plastic clays (Fig. 373). 



Fig. 377. — Septariiim from Cretaceous clays near the east base of the Rocky 

 Mountains in ^lontana. (Photo, by Church.) 



Concretions sometimes develop cracks within themselves, and these 

 may then be filled T\ith mineral matter differing in composition or color 

 from that of the original concretions (Figs. 375 and 376). Concretions 

 the cracks of which have been filled by deposition from solution, are 

 called septaria. They are especially abundant in some of the Cretaceous 

 shales and clays. In not a few cases the fiUing of the cracks appears 



