ALTERATIONS OF ROCKS 



289 



of matter unlike that of the rock in which they form. Thus con- 

 cretions of calcium carbonate (Fig. 281) are common in clay, con- 

 cretions of chert (silica) (Fig. 289) in limestone, and concretions of 

 iron oxide in sandstone. 



Many concretions develop after the enclosing sediment was 

 deposited. This is shown, in some cases, by the fact that bedding 

 planes run through the concretions. Concretions also form in 

 sediments during their deposition, and exceptionally, rock is made 

 up chiefly of them. The chemical precipitates from the concen- 

 trated waters of certain enclosed lakes may take the form of minute 

 spherules. From a fancied re- 

 semblance of these concretions 

 to the roe of fish, the resulting 

 rock was called oolite (Fig. 282). 

 Oolite is now forming about 

 some coral reefs, presumably 

 from the precipitation of lime 

 carbonate temporarily in solu- 

 tion. Some considerable beds 

 of limestone are oolitic. The 

 calcium carbonate of such rock 

 may be replaced by silica, 

 leaving the oolitic structure in 

 siliceous rock. Some beds of 

 iron ore are concretionary. 

 Thus there are widespread beds 

 of "flax-seed" iron ore made 

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

 seed which has given the ore its name. Some concretions develop 

 cracks within themselves, and the cracks may be filled with mineral 

 matter differing in composition or color from that of the original 

 concretion (Fig. 283). Such concretions are called septaria. 



In size, concretions vary from microscopic dimensions to huge 

 masses, 10 or more feet in diameter. The variations in shape 

 are also great, conditions of growth having much to do with the 

 form. A concretion which starts as a sphere may find growth 

 easier in one plane than another, when it becomes discoid. Two or 

 more concretions may grow together, giving rise to complicated 

 forms. 



None of the changes thus far mentioned (p. 285 et seq) consti- 



Fijr. 282. Oolitic texture. About nat- 

 ural si/.c. (Photo, by Church.) 



