78 



PHYSICAL GEOLOGY 



posed of minute concretions (Fig. 57). Limestone of this origin (p. 249) 

 is often widespread and many feet in thickness. It is, however, held 



by some investigators that the 

 most of the oolitic limestone is 

 the product of microscopically 

 small algae (plants) capable of 

 secreting lime. 



Geodes. — Geodes differ from 

 concretions in that they are 

 formed in cavities of the rock 

 and from without inward (Fig. 

 58). When lava contains steam 

 cavities, silica may be deposited 

 on the walls of the cavities and, 

 by slow addition, may in time 

 fill them. In this way agates 

 are formed, the colored layers 

 of which are due to coloring 

 matter carried in and deposited 

 with the silica. Other geodes 

 are formed by the force of 

 crystallization in the following 

 way : if silica begins to crystallize in the cracks of a crushed fossil 

 embedded in a rock, — a shell, for example, — the fragments of the 

 shell may be forced farther and farther apart by the force of crys- 



Fig. 57.— 

 limestone. 



A hand specimen of oolitic 

 (U. S. National Museum.) 



I [G. 58. — A geode broken in two. Cheyenne River, South Dakota. 



