W. A. Richardson—The Relative Age of Concretions. 123 
widely spaced, as indicated in Fig: 3a. The close recular banding may 
be both followed and preceded by such regions (Fig. 3b). Widening 
of the spacing is often accompanied by a reduction in the average 
size and number of nodules, and by the tendency for the banding to 
give way to “ scattered zones ”’. 
The whole series may represent a single rhythm. There is, then, 
a presumptive case that all the concretions were formed by one 
process of precipitation. As diffusion continued the concretionary 
layers were deposited progressively—the whole process extending 
over a period, as to the duration of which nothing is known. 
Yertically- Depth from Surface. 
Horizontally - Space between Bands. 
Fic. 3.—A, Liesegang rhythm. B, Another type of Liesegang rhythm. 
C, Three Liesegang rhythms in one deposit. 
On the other hand, it may sometimes be possible to divide up the 
layers of a given deposit into sets, each showing the characteristics 
of asingle rhythm. The case is presented diagrammatically in Fig. 3c. 
If only a single rhythm can be detected it may be taken that the 
concretions are the result of a single process spread over a time- 
interval commencing at the earliest after the complete deposition 
of the rock. 
The detection of two or more such rhythms in a single series 
indicates that after the overburden of sediment had reached a certain 
thickness precipitation and concretionary growth took place. Such 
may be the conditions under which concretionary liassic limestones 
were deposited according to the hypothesis of W. D. Lang.’ This 
hypothesis has recently been further investigated by E. F. Davis,” 
and called by him “ colloidal segregation ”’. 
1 Lang, Proc. Geol. Assoc., vol. xxvi, 1915, p. 117. ui 
2 Davis, Bull. Dept. Geol. California Univ., vol. Lu, vol. u, vol. 11, No. 3, 
1918, p. 394. 
