CONCRETIONS 



229 



from that of the stratum in which they occur. They are not peb- 

 bles, which are older than the stratum which contains them and 

 which were embedded just as we find them, but are younger than 

 the stratum and were formed subsequently. This is shown by the 

 fact that the planes of stratification may often be traced through 

 the concretion, and that fossils are sometimes found partly within 

 and partly without the nodule. In shape the concretions vary 



Fig. 86. — Large concretions, weathered out of sandstone, near Fort Buford, 

 Mont. (U. S. G. S.) 



greatly, from almost true spheres, to grotesque aggregations, but 

 always with rounded form, and almost as great a variety of mate- 

 rial is found among them. Very often a foreign body, like a fossil 

 shell or leaf, forms the centre or nucleus of the nodule, which has 

 been built up, often in concentric layers, around the nucleus. 

 One form of concretion, known as a septarium, is divided inter- 

 nally by radial cracks, which were subsequently filled up with some 

 mineral deposited from solution by percolating waters. 



The agency which produces concretions cannot as yet be ex- 



