288 MATERIALS AND THEIR ARRANGEMENT 



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Fig. 280. Nodule of chert, about half natural size. (Photo, by Church.) 



In general, concretions are made by the deposition of mineral 

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



Fig. 281. Irregular calcareous concretions. Ryegate, Vt. (Photo, by Church.) 



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

 The material of the concretion probably comes, in most cases, 

 from the immediately surrounding rock. Concretions are generally 



