NOMENCLATURE OF CONCRETIONS 613 



sider true concretions. Liesegang, Ricliters, and Wilson regard similar 

 forms as representing true concretions.-^ 



Todd^^ divided concretions into four classes based on their method of 

 growth. These are accretions, intercreiions, excretions, and incretions. 

 The first grow from the center outward in a regular manner. The incre- 

 tions are the forms commonl}^ knoAvn as septaria, which result from 

 irregular and interstitial growth, causing internal fissures. In excre- 

 tions, according to Todd, growth proceeds from the exterior inw^ard. 

 Incretions are those cylindrical forms with a hollow core. 



Dr. George Abbott^^ has published in tabular form a classification of 

 concretions showing the form and composition of the varieties recognized. 



J. I. ISTorthrop,^'^ a zoologist, introduced the term rhizomorphs, which 

 was preoccupied by the botanists for the same class of structures called 

 incretions by Todd. The substitute proposed for these terms by the 

 writer will be indicated elsewhere in this paper. 



William HilP^ has, on the basis of structure and texture, recognized 

 seven types of siliceous concretions, wdiich are included under the names 

 flint, chert (four types), opalite, and hornstone. 



Professor Grabau has recognized two types of concretions, distin- 

 guished from the point of view of origin and relationship to the inclosing 

 beds: "(1) Those forming as contemporaneous accimiulations, after- 

 w^ards buried by clastic or other strata; and (2) those forming within the 

 strata after their deposition.'^ Chemically formed oolites and pisolites 

 Grabau includes under contemporaneous concretions.-*^ 



W. A. Tarr-^ in a recent paper has classified concretions as to origin 

 in essentially the same way as Grabau, using for the two classes recog- 

 nized the terms syngenetic and epigenetic. Concretions of the first- 

 iiamed class were formed at the same, time as the inclosing beds, while 

 epigenetic concretions wTre developed subsequent to the adjacent beds. 

 Richardson,-^ considering concretions from the standpoint of the rela- 



21 a. R. F. Liesegang : Geologisclie ditfusionen, Dresden and Leipzig, 1913, p. 159. 

 h. F. Richters : Ueber Marlekor, Prometheus, 23. 1912, p. 697. 



c. M. E. Wilson : Geol. Surv. Can., Mem. 39, 1913, p. 105, pL xxvii. 



22 J, E. Todd : Concretions and their geological effects. Bull. Geol. Soc. Am., vol, 14, 

 pp. 353-360. 



23 George Abbott: Concretions. S. E. Natl. (Eng.) for 1907, pp. 66-81, pis. 13-18. 

 2* J. I. Northrop : A Naturalist in the Bahamas, 1910, p. 40. 



23 William HiU : Flint and chert. Proc. Geol. Assoc, vol. 22, 1911, pp. 61-94 (93-94). 



2« A. W. Grabau : Principles of stratigraphj^ 1913, p. 718. 



2' W. A. Tarr : Syngenetic origin of concretions in shale. Bull. Geol. Soc. Am., vol. 

 :^}2, 1921, p. 373, 



-^ W. Alfred Richardson : The relative age of concretions, Geol, Mag., vol, Iviii, 1921, 

 pp. 114-124. 



