7 6 



PHYSICAL GEOLOGY 



Concretions are masses varying greatly in shape and in size from less 

 than a pinhead to more than 10 feet in diameter, and are formed by 

 the gradual segregation of mineral matter. The shape, as has been 

 said, varies greatly. Some concretions are spherical, some are flat, 

 and others curved. The odd shapes which resemble animals (Fig. 55) 

 are usually produced by the growth of two or more concretions until 

 they join. The center of attraction may be a fossil or a bit of mineral, 



Fig. 55. — Clay-stone concretions of various shapes. They are composed largely 

 of lime carbonate and occur in clay. 



but in the majority of specimens no nucleus can be detected. In 

 some formations (for example, the Arikaree, Miocene, in Nebraska) 

 they may, by their abundance, so strengthen the loose sands and clays 

 containing them as to form a resistant bed which stands as cliffs 

 wherever cut by streams. 



Concretions usually occur in definite beds in a formation, and it 

 is sometimes possible to trace such beds for several miles. They 

 occur in rocks of every age, from the most ancient to those now 

 forming on the bottoms of lakes and seas. 



