TEERANES. 



97 



banks, and is common among the old limestones of the world. Calcareous 

 concretions are most common. Those of pyrite, limestone, and quartz 

 are also common; and many other minerals take the concretionary form. 

 ISTodules of flint or chert in rocks are often concretions, and frequently have 

 a fossil as a center. 



83. 



81. 



82. 



The consolidation of a concretion is sometimes followed by further drying 

 from the outside inward, and in this process the interior often becomes 

 much cracked, as in Figs. 72, 73; and the cracks may be afterward filled 

 with calcite or some other material, and make septaria, the name alluding to 

 the division or septation of the interior. These septaria concretions occur at 

 times in very large flattened forms, even one to three feet in diameter, when 

 they are sometimes popularly called petrified turtles, from the resemblance 

 to the back of a turtle in the divisions ; the more beautiful kinds are often 

 sawn into circular slabs and polished for table-tops. 



Solidification /rom /wston often produces concretions in the mass which 

 sometimes consist of more or less distinct concentric layers of different 

 minerals, or, it may be, of a single mineral. Fig. 84 illustrates this structure 

 in a granite-like rock, the '^ orbicular dioryte " of Corsica. The pudding- 

 granite of Craftsbury, Vt., contains large black, ovoidal concretions, consist- 

 ing chiefly of black mica. 



Concretions are also made by growth radially from a center, but this kind 

 is of inferior geological importance. The process makes attached spheres 

 and hemispheres, radiately fibrous or colum- 

 nar within. An example — in a reversed 

 position, in order to exhibit the interior 

 structure — is shown in Fig. 76. 



Spheres and irregular spheroids or balls 

 in rocks, when hollow within and lined 

 with crystals, are not concretions, but in- 

 stead geodes; and any cavity so lined, 

 whatever the shape, takes this name. 

 Geodes are often quite large, as in the 

 Keokuk limestone of Iowa and Illinois, 

 where they have been supposed to occupy 

 the centers of sponges that were at some time hollowed out by siliceous 

 solutions, like the hollowed corals of Florida, and then lined with crystals 

 Dana's manual — 7 



84. 





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