OTTAWA VALLEY CONCRETIONS 631 



Dr. J. I. Northrop*^ applied to the root-induced structures described 

 above the term rhizomorphs. This name was applied by Northrop to 

 designate cylindrical root-like masses of limestone in the Bahama Islands 

 concentrically arranged about a tubular axis "formerly occupied by plant 

 roots and rootlets." These Bahaman structures differ from those just 

 described in representing only lithification of the calcareous sand or very 

 soft limestone, while the Pleistocene sand structures represent segrega- 

 tion as well as induration of the material composing them. Northrop, 

 however, made this term embrace a variety of tubular structures, includ- 

 ing the tubular ferruginous masses found in the New Jersey clays and 

 elsewhere. 



Todd used at an earlier date the term incretions for the same class of 

 structures included by Northrop's rhizomorphs. Since the name incre- 

 tion implies a particular method of growth which may not entirely coin- 

 cide with the facts, a name which refers to the shape of the structures 

 alone appears preferable. There is certainly need of a general term for 

 the class of concretions having the distinctive features characterizing the 

 pipe-like clay and calcareo-siliceous concretions. The term rhizomorph 

 which Northrop proposed for these structures is not available, because it 

 has been previously used by the botanists. The preemption of this name 

 by botanical technology leads the writer to suggest as a substitute for it 

 the term rhizocretion. This term may conveniently be made to include 

 concretions having a root-like shape or having had a root as a nucleus. 



In the lower part of the sand beds the calcareous root concretions are 

 found in certain localities associated with vast numbers of hardened clay 

 fillings or casts of tubular cavities of highly irregular shape with a ver- 

 tical trend. These tubular clay casts, many of which approach roughly 

 the corkscrew shape, remind one somewhat of Barbour's*^ "D^emonelix 

 cigars or fingers," although they are more slender and lack the blunt 

 rounded ends of the "cigars." The sand beds holding these curious clay 

 casts lie directly above beds of clay of the same type as that found in the 

 tubular clay casts. The extraordinary irregularity in shape of these clay 

 fillings of the vertical tubular openings into which they have been forced 

 is the most puzzling feature which they present. These irregular clay 

 structures change abruptly from a semi-columnar to a semi-spiral form 

 and frequently bifurcate into fine and coarse branches. Frequently 

 needle-like perforations often filled by the roots which made them 

 traverse longitudinally these clay structures. These structures are not 



^' J. I, Northrop : A Naturalist in the Bahamas, 1910, p. 40. 

 ^^Bull. Geol. Soc. Am., vol. 8, 1897, pi. 34. 



