STRUCTURAL GEOLOGY. 493 



in which case the matter originally in the place of the concretion has 

 been crowded aside; or it may involve much of the material of the 

 "imbedding rock. Thus the concretion of Hme carbonate in shale may 

 be nearly pure, or it may involve much of the earthy matter of the shale, 

 while the concretion of iron oxide in sandstone commonly includes much 

 sand. In extreme cases, indeed, the concentrated matter of the con- 

 cretion merely cements the material involved into distinct nodules. 

 Occasionally the rock substance itself takes on a concretionary form, 

 all or most of its material being involved. 





• ^ 



Fig. 374. — Irregular tubular silicious concretions in Arikaree clays. Northwest of 

 Wildcat Mountain, Banner Co., Neb. (Darton, U. S. Geol. Surv.) 



In size, concretions may vary from microscopic dimensions to huge 

 masses, 8, 10, or even more feet in diameter. The variations in shape 

 are also great. They may be spherical, eUiptical, discoid, or they may 

 assume more irregular and complex forms (Figs. 371 and 372). The 

 conditions of growth have much to do with the form. Thus a concre- 

 tion which starts as a sphere may find growth easier in one plane than 

 another, when it becomes cUscoid. Tw^o or more concretions some- 

 times grow together, giving rise to complicated forms. Some of the 

 most complex and fantastic forms are perhaps to be explained in this 

 way. Concretions sometimes take the form of tubes. Some minute 

 tubular concretions were formed about rootlets, but the larger ones 

 appear to owe their form to other influences (Fig. 374). 



