THE WORK OP WATERS UNDERGROUND 121 



times dissolves material scattered through rocks and brings it 

 together and deposits it in nodular masses. Such concretions 

 (Fig. 113) are usually of material different from the dominant 

 material of the rocks in which they occur. Thus, concretions 

 in limestone are frequently of silica, and concretions in clay 

 or shale are often of calcium carbonate or an iron compound. 

 Concretions vary in form from nearly perfect spheres to nota- 



FIG. 113. Ironstone concretion in shale. South shore of Lake Huron, about 



25 miles northeast of Sarnia, Out. (Lee.) 



How does the picture prove that the concretion was formed after 

 the deposition of the shale ? 



bly irregular lumps and masses, and range in diameter from 

 a fraction of an inch to 10 or 15 or more feet. A fossil 

 sometimes forms the nucleus about which a concretion grows. 

 While many concretions have been made after the formation 

 of the inclosing rocks, others have developed during the 

 deposition of the sediments. When concretions are made 

 in soft sediments, they often press the surrounding material 

 away as they grow. 



Small cavities in rocks may be partly or wholly filled by 

 material deposited from solution in ground water, forming 



