496 GEOLOGY. 



to have wedged segments of the original concretion farther and farther 

 apart, until the outer surface of the septarium is made up more largely 

 of vein-matter than of the original concretion (Fig. 377). Such con- 

 cretions are often popularly known as ''petrified turtles/' 



Concretions of the sort indicated above often develop after the 

 enclosing sechmentary rock was deposited. This is shown, among other 

 things, by the fact that numerous planes of lamination may sometimes 

 be traced through the concretions. 



Concretions also form in water during the deposition of sedi- 

 mentary rock. Exceptionally, sedimentary rock is made up chiefly of 

 concretions. The chemical precipitates from the concentrated waters 

 of certain enclosed lakes sometimes take the form of minute spherules 



A 



,_;»►■ ^-^ 



i 



\ 



Fig. 378.— Pisolite. Half natural size. (Photo, by Church.) 



which resemble the roe of fish. From this resemblance the resulting 

 rock is called oolite (Fig. 357). Oohte is now forming about some coral 

 reefs, presumably from the precipitation of the hme carbonate which 

 was temporarily in solution. Considerable beds of hmestone are some- 

 times oolitic. The calcium carbonate of such rock may be subsequently 

 replaced by silica, so that the oolitic structure is sometimes found in 



