198 



STRUCTURAL GEOLOGY. 



Concretionary or Nodular Structure. 



This, also, is a superinduced structure simulating an 

 original structure. As slaty cleavage simulates stratifi- 

 cation, so concretions or nodules simulate and are apt to 

 be mistaken for fossils. 



In many strata, especially calcareous sandstones and 

 shales, we find rounded masses often of curious shapes, 

 separable from the general mass of the strata, and differ- 

 ing a little from it in hardness and composition. These 

 are called concretions, nodules, septaria, etc. They have 

 evidently been separated out of the general mass after the 

 latter was deposited. This is shown by the fact that the 



planes of stratification often 

 run right through them 

 (Fig. 115). 



Forms and Structure. 

 Inform they are sometimes 

 FlG 115 perfectly spherical, like can- 



non-balls, and vary in size 



from that of a marble to many feet or even yards in diame- 

 ter ; sometimes flattened ellipsoidal, and these, when 



FIG. 116. Nodules, from strata. 



