EXCHETIONS AND INCBETIONS 861 



calcareous particles by intercretion may accomplish the results under 

 consideration. 



It should be noted further that the first cracking of the interior takes 

 place when the breadth of the growing zone is several times that of the 

 ruptured interior, and that the cracks gradually spread toward the sur- 

 face as the concretion increases in size by surface accretion. The inter- 

 cretionary action may in later stages even extend the cracks quite to the 

 surface. This may be made more evident by the study of figures 23 

 and 16, plate 50. 



The compound concretion in figure 17 may be plausibly explained 

 by simply supposing several successive periods of growth corresponding 

 perhaps to as many rises of ground water so as to submerge it more or 

 less completely. The marked cessations in growth which divide the 

 successive portions may have been due to the effects of weathering or 

 oxidation while it was not completely covered by the ground water. 



EXCRETIONS AND INCBETIONS 



Another method of formation of concretionary nodules is so different 

 that it may be questioned whether it should be classed with the rest 

 under the head of concretions. It is the form which Dana has called 

 " centripetal " concretions.* He applied the term to signify the fact 

 that the final or permanent growth is toward the center. It follows, 

 therefore, that they are not as compact as other concretions, and that the 

 material must first be concentrated either by previous accretion or by 

 unusually concentrated sedimentation. The mineral most frequently 

 showing this action is ferric hydroxide. Ferrous salts dissolving are 

 oxidized to ferric hydroxide when they come in contact with the air on 

 the surface of the containing rock, as in figure 16a, plate 49. Such 

 action appears often in ferruginous sandstones and limestones. Such 

 cases can hardly be called concretions. Certain forms of spheroidal 

 weathering, especially those where successive concentric shells of iron 

 oxide are formed, may be explained in this way. Similar action with 

 similar resulting structure, on a smaller scale, may be found within the 

 rock around some moist spot as figures 166, c. In such a case the results 

 resemble accretions of a rhythmical structure. 



These excretions are frequently hollow. This form is simpler and we 

 therefore consider it first. If an accretion or intercretion of ferrous car- 

 bonate like figure 14, either pure or mingled with clay or sand, be 

 reached by water charged with carbon dioxide and oxygen, it will begin 

 to dissolve, and the iron will be precipitated on its surface as ferric 

 hydroxide. The impurity will remain undisturbed. Figure 14 shows 



* Manual of Geology, fourth edition, p. 98, 

 LI— Bull. Geol. Soc. Am., Vol. 14. 1902 



