IXTRODUCTIOX 611 



AVherever careful attention has been given to concretionarv inclusions 

 in sedimentary- rocks, the limited stratigraphic range of particular types 

 has been evident. This feature and the relation of form to kind of sedi- 

 ment are the main facts to which attention will be given in the following 

 observations on these specimens. Direct response to the physical features 

 of the beds in which concretions occur is shown in their shape and size. 

 C'oncretions frequently reflect in their peculiarities certain contrasts in 

 the composition and texture of successive beds of a terrane. They may 

 therefore often serve as an index to differences which on casual observa- 

 tion are not always obvious. This characteristic response of concretions 

 to the character of the matrix appears to be illustrated in the Diemonelix 

 beds which led Barbour, by the increasing diversity' of forms between the 

 bottom and top of his section ranging from the Dtemonelix cakes to 

 ^'devil's corkscrews." to suggest that they represented ''possible steps in 

 the phylogenetic history of a new fossil.^* '^ Illustrations of this response 

 to physical features of the sediments will ))e pointed out in some of the 

 observations which follow. 



XnMEXrLAXrRE OF CoXCRETTi )y^ " 



X< I elaborate or exhaustive discussion of the nomenclature of concre- 

 tions can be attempted here, but some reference to the subject seems 

 essential to a clear understanding of the detailed descriptions which 

 follow. 



Concretions consist of a^^^rreofates of r<x-k-formintr materials differiuL'^ 

 in structure or composition from the inclosing rock and frequently 

 amorphous or colloidal in character. Organic agencies may or may n<.t 

 play a part in their formation. In place of the ordered arrangement of 

 molecules present in a crystal, each surrounded by others <>f the same 

 kind, the concretion generally shows amorphous structure frequently 

 associated with varying chemical composition in different parts. 



Certain classes of concretions with well detined and uniform ty|>es of 

 symmetry, like oolites and cone-in-cone, have received widely accepted 

 distinctive names and are not generally referred to mnler the name of 

 concretions, although they belong in tliis general category. 



Holtedahl- has proposed that we use for structures which appear to be 



" E. H. Barbour : Nature, strucrure, and phylogenv of r»iemoiielix. Bull. «T«^:»i. S«>c. 

 Am., vol. S. 1S97. p. 30H. 



" The author is indebted to r>r. George Abbott for the loan of several papers used in 

 the preparation of this section and in the bibliography at the end of the paper. 



* O. Holtedahl : Am. Jour. Sci.. vol. 47. 1910. p. 9-5. and vol. 1. February. 19-21. p. 19.x 



