48 GEOLOGY 



replaced by silica, leaving the oolitic structure in siliceous rock. 

 Beds of iron ore are likewise sometimes concretionary. Thus 

 there are widespread beds of "flaxseed" iron ore made up of con- 

 cretions of iron oxide which, individually, resemble the seed which 

 has given the ore its name. 



One of the most extraordinary features of some concretions 

 of complex form is their symmetry. This may be of various 

 phases; in exceptional cases there is a bilateral symmetry almost 

 as perfect as in the higher types of animals. 



Fig. 27. Oolitic texture. About natural size. (Photo, by Church.) 



Concretions sometimes develop cracks within themselves, and 

 these may then be filled with mineral matter differing in compo- 

 sition or color from that of the original concretions (Fig. 28). 

 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 filling of the cracks appears to have wedged segments of the 

 original concretion farther and farther apart, until the outer sur- 

 face of the septarium is made up more largely of vein-matter 

 than of the original concretion. The development of concretions 

 in rock is not commonly looked upon as metamorphism, but it 

 is really a metamorphic change in the broader sense of that term. 



