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O. W. WILLCOX 



interior partitions, the history of the concretion is inferred to be some- 

 what as follows : The concretion probably consisted at first only of 

 the tube A; in this condition it must have resembled the concretion 

 shown in Fig, i. As to what induced the formation of this tube, the 

 concretion itself affords no direct evidence. It may be presumed that, 

 once formed, the wall of the tube was thickened by addition of mate- 

 rial chiefly from the outside, and that the rate of addition was practi- 

 cally the same at all points on the periphery. When the wall of A 



Fig. 7. — Sand bank in Long Branch, showing tendency of the iron (originally 

 disseminated as glauconite) to segregate into anastamosing bands. 



had attained the thickness represented by a, the arched chamber B 

 barnacled itself on A; the arch a, thus cut off from outside supplies 

 of material, suffered an arrest of growth. The concretion was now 

 two-chambered, hke the one shown in Fig. 2. By the time the exte- 

 rior wall of B had attained the thickness of h, that of A had reached 

 the thickness of c. A second parasitic chamber, C, was then arched 

 across one of the sulci between A and B; b and c were thus cut off, 

 and their further growth prevented while the exterior walls of the 

 concretion, as it now appears, were taking on their present proportions. 

 The existence of still more complex individuals, in which analogous 

 relations between the several chambers and their walls obtain, shows 

 that the process need not stop with the formation of a three-chambered 

 concretion, but may be continued indefinitely. It is possible to col- 



