PHYSICAL ORIGIN OF CERTAIN CONCRETIONS 



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the super-concentrated or overloaded water carrying fine clay particles 

 along a smooth bottom, an adhesion of those particles naturally 

 results. They are pressed together as are finely disseminated par- 

 ticles of butter in the everyday illustration of churning. They may 

 unite with or without a nucleus. A soft nodule will form, grow, and 

 become round by being rotated along its different axes as boys roll 

 snowballs. It will be propelled by the current, gathering as it goes. 

 It will pass over slightly different characters of materials and may 

 gather at intermittent periods; hence different concentric shells will 



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Fig. I. — Clay balls in the bed of the Rio Chaco, New Mexico. 



result. Should it pass over sandy particles or small pebbles, it will 

 gather them up and may later cover them with additional coatings 

 of clay. At eddies or acute bends in the stream the concretions 

 aggregate and may become sHghtly welded together. There is a 

 limit to their size depending on the strength of current flowage; 

 they grow until the current is no longer able to transport them, then 

 settle to become covered by subsequent deposition. It may occur 

 that the upper or exposed portion while lying on the bottom receives 

 additional material from the depositing sediments, resulting in an 

 orbital form with a partly inclosing shell which, with modifications, 



