2 52 O. W. WILLCOX 



such cases the concretion is not a closed tube, but is a more or less 

 corrugated sheet, the corrugations, however, having the usual orienta- 

 tion. Such a corrugated sheet may be observed in the upper part of 

 Fig. 4. 



The formation of the secondary chambers of the compound con- 

 cretions is to be ascribed to the same tendency to change the center 

 of curvature. Ordinarily, new molecules coming in contact with the 

 wall of a concretion simply went to increase the thickness of that wall, 

 but not always. At any time some innovating molecule was likely to 

 align itself so as to form a starting-point for a new arched surface, 

 which would compsl additional molecules to arrange themselves in 

 conformity to its general scheme, until succeeding arches cut it off 

 from further growth. 



