492 



GEOLOGY. 



cretion may have come from the immediately surrounding rock, having 

 been first dissolved by water and then deposited about the nucleus, or 

 it may have been introduced from without, Hkewise by the agency of 

 water. In the first case, the mineral matter of the concretion is usually 



Fig. 373. — Calcareous concretions, some of them shoAving bilateral sj^mmetry. 

 Ryegate, Vt. (Photo, by Church.) 



one of the minor constituents of the rock. Thus the commonest con- 

 cretions in limestone are composed of impure silica (chert. Fig. 361); 

 in shale, of lime carbonate or iron sulphide ; in sandstone, of iron oxide. 

 The concretion may be made up almost wholly of concentrated matter, 



