Clay Concretions of the Connecticut River. 241 



lines of stratification running with unbroken Continuity 

 through them. The plastic clay is charged with wator con- 

 taining carbonate of lime in solution. We may suppose 

 some slight change in the conditions causes a precipitation 

 of a portion of the lime, or that certain foreign bodies 

 attract it. In either case we should have centres towards 

 which other molecules of calcium carbonate would be drawn. 

 By the law which governs the diffusion of liquids dew 

 material would be brought, and so the concretion would 

 grow. The process is one of segregation — a flocking toge- 

 ther of the molecules. Except in taking on a ciystalline 

 form, there is little difference between the building up of a 

 crystal and concretion. The .molecules segregate, and in 

 the case of the crystal crowd back the other material, while 

 in that of the concretion a part of this material enters into 

 the new form. 



If the concreting material comes from all directions with 

 equal facility, as in a porous rock, the concretions are in the 

 form of spheres, but in clay, which is more or less imper- 

 vious, it spreads laterally most freely. The tendency is, 

 therefore, to lengthen the horizontal diameter, and shorten 

 the vertical, giving the circular disk which is the normal 

 form of clay concretions. I have never seen a clay sphere 

 larger than a pea, and never found one larger than a pin's 

 head. 



When a concretion passes from one layer into another 

 poorer in carbonate of lime, the form is contracted ; thus a 

 variation in the amount of calcium carbonate results in a 

 variation in the form of the concretion. 



What the exact conditions are which cause one bed to pro- 

 duce animal forms, another lenticular, and a third cylindri- 

 cal, it would be interesting to know. Many observations 

 must yet be made upon these concretions in situ. 



