276 ELEMENTARY GEOLOGY 
carbonate of lime. These spherical grains, usually smaller than 
the head of a pin, are odlitic grains (p. 95), and sometimes rocks 
are made of them, which are then called odlites (Fig. 41). 
Besides such concretionary masses, there are some which are 
formed in the rocks after these have been deposited. They are 
called claystones or clay ironstone concretions (Figs. 156 and 157), 
and are particularly common in strata of clay in which there is 
considerable iron or carbonate of lime. From the cliff of clay 
Fic. 156. 
A group of claystone concretions slightly reduced. 
rock one may often see them projecting as spherical, oval, or disk- 
shaped masses, sometimes tiny, but often many pounds in weight. 
They project beyond the rest of the cliff because they are more 
compact than the enclosing stratum. They may, indeed, be solid 
rock-like masses, enveloped in soft clay. Then their forms are 
sometimes very irregular and even fantastic. Not uncommonly 
