46 GEOLOGY 



or muds, pressing the clay back as they grow. In many other 

 cases, too, kind comes to kind. 



In general, concretions are made by the deposition of mineral 

 matter which was in solution, about a nucleus. The nucleus 

 may be a leaf, a shell, or some bit of organic or inorganic matter. 

 The material of the concretion may come from the immediately 

 surrounding rock, or it may have been introduced from without, 

 likewise by the agency of water. Concretions are generally of 

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

 cretions of calcium carbonate are common in clay (Fig. 25), con- 



Fig. 24. Nodule of chert, about half natural size. (Photo, by Church.) 



cretions of silica (chert) (Fig. 24) in limestone, and concretions 

 of iron oxide in sandstone (Fig. 26). 



Many concretions develop after the enclosing sediment was 

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

 lamination may be traced through the concretions. Concretions 

 also form in sedimentary rock during its deposition, and excep- 

 tionally, the rock is made up chiefly of them. The chemical pre- 

 cipitates from the concentrated waters of certain enclosed lakes 

 sometimes take the form of minute spherules. From a fancied 

 resemblance of these concretions to the roe of fish, the resulting 

 rock was called oolite (Fig. 27). Oolite also forms in the open 

 under proper conditions. It is now forming about some coral 

 reefs, presumably from the precipitation of lime carbonate tem- 

 porarily in solution. Some considerable beds of limestone arc 

 oolitic. The calcium carbonate of such rock may be subsequently 



