STRUCTURAL GEOLOGY, 



497 



silicious rock. If the concretions become larger, say as large as peas, 

 the rock is called pisolite instead of oolite (Fig. 378). 



Fig. 379. — Columnar structure, ''Devil's Post Pile." Upper San Joaquin Canyon, 



Sierra Nevada Mountains. 



Beds of iron ore are likewise sometimes concretionary. Thus in 

 the Clinton formation there are widespread beds of '' flaxseed '^ ore 

 made up of concretions of iron oxide which, individually, resemble the 

 seed which has given the ore its name. The nucleus in this case is 

 usually too small for identification. 



Secretions. — ^When cavities in rock are filled by material deposited 

 from solution, the result is sometimes called a secretion. Secretions 

 therefore grow from without toward a center, while concretions follow 

 the opposite order. Crystal-Hned cavities {geodes, Fig. 359) and agates 

 (Fig. 358) are examples of secretions. Crystal-lined cavities and veins 

 are the same in principle. 



