120 W. A. Richardson—The Relative Age of Concretions. 
V. THE SHAPE OF CONCRETIONS. 
H. C. Sorby ? considered that the shape of nodules depended on 
that of their nuclei, and gave a series of measurements to demon- 
strate the point. In some cases this dependence is doubtless a fact, 
but a broad survey of concretions does not warrant a general 
application of the principle. Some examples will be given in the 
following paragraphs to illustrate the subject. 
L. Moisey ? examined several thousand ironstone nodules from the 
Notts—Derby coalfield. These nodules, contaiming fossils flattened 
vertically, are, on the whole, small, and ellipsoidal in shape. The 
length and depth of the nodules corresponds fairly well with the 
length and depth of included fossils, but their width seems 
independent of that of the fossil. Moreover, some 35 per cent of the 
nodules examined contained no fossil, yet were similar in shape to 
those that did. 
I have gone over the tip-heaps and exposures afforded by quarries 
in the Lower Lias of Lincolnshire to collect evidence of the influence 
of inclusions in nodules. A few of these Liassic doggers (often 
septarian) may have a large ammonite at their centre. In the 
majority either no fossils were found, or the fossils were distributed 
sporadically through the nodules. The general shape of these nodules 
was also a flattened elipsoid. 
A colleague, Mr. H. 8. Holden, recently brought me a small 
concretion in Coal-measure shale from Gedling. The concretion 
formed in the neighbourhood of a Carbonicola, and its final stages 
of deposition just embraced the tip of the shell. This small con- 
cretion was almost spherical, and the shell had evidently no influence 
on the origin or location of the segregation. The lamination planes 
were disturbed by the concretion, but not by the fossil, thus 
indicating a secondary origin. 
Chalk flints occasionally show in their shape the influence of their 
contents. The majority are, however, highly irregular. This 
irregularity might be due to local differences in the Chalk, that 
enabled replacement to take place more easily in certain directions 
than in others. 
Most concretions that have been dredged from the ocean appear 
to form around some object. And if they are comparable in size 
with the nucleus, their shape seems to be strongly influenced by it. 
Liesegang ? emphasized the general tendency of concretions to 
assume a globular form. He demonstrated partly by experiment and 
partly by field observations, that if the segregation be forced initially 
to depart from the circular form by commencing at an irregular 
centre, the concretion tends to approximate more and more to that 
shape as growth progresses. 
1 Sorby, Quart. Journ. Geol. Soc., vol. |xiv, 1908, p. 216. 
2 Moisey, Grou. Maa., Dec. V, Vol. V, 1908, p. 222. 
3 Liesegang, Geologische Diffusionen, Dresden and Leipzig, 1913, chap. vi. 
