43fe 



GEOLOGY. 



are characterized by this structure. In most cases these accretions prob- 

 ably grew in depositing waters that gently rolled the grains while layers 

 were being added. They thus do not fall under the head of cementation 

 after the beds were formed; but concentric additions to the grains ap- 

 pear sometimes to have taken place after they were formed into beds. 



Cavity filling. — When cavities of some size occur in rocks and the 

 percolating waters are in a depositing state, the interiors of the cavities 

 are sometimes lined with concentric 

 layers of deposit. Here, instead of 



Fig. 357. — Oolitic texture. About 

 natural size. (Photo, by Church.) 



Fig. 358. — Agate structure. The cavity 

 was first coated with mineral matter de- 

 posited from solution. The contracted 

 cavity was then nearly filled with the 

 same sort of material deposited in layers, 

 apparently over the bottom, until the 

 cavity was nearly obliterated. 



building out from a nucleus, 

 the waters build in from the 

 walls of the cavity. The agate 

 structure (Fig. 358) is a case of 



this kind, in which the successive layers are commonly silica in 

 the form of chalcedony and differ from each other in color and tex- 

 ture. Often before the cavity is entirely filled, the deposit changes 

 from chalcedony, to crystals of quartz, which grow with their 

 bases on the walls and their pyramidal points toward the center 

 of the cavity. Geodes are examples of a similar process in which 

 the cavity is but partially filled with crystals which have their bases 

 set on the walls of the cavity and their points directed inwards 

 (Fig. 359). The crystals of geodes are most commonly quartz orcalcite, 

 but they may be any other mineral that the waters are capable of 



