122 W. A. Richardson—The Relative Age of Concretions. 
It will, therefore, be seen that contemporaneous concretions would 
be favourably placed for the development of spherical form. 
Examples of such are manganese nodules and “ marlekor”. The 
latter are aggregations of spheres and have been imitated in un- 
stressed gels by Liesegang.t The spherical shape may also be 
attained by concretions of subsequent age, providing there is a 
homogeneous environment. On the other hand, the flattened 
ellipsoidal shape seems confined to displacive, or partially displacive 
subsequent nodules. Replacements are capable of very irregular 
development as many economic deposits show. 
Fic. 2.—Shape and dimensions of two boundaries reached by silver nitrate 
diffusing in an unequally stressed gel. 
VI. Tue DistrisputTion or NoDULES. 
The following modes of distribution may be distinguished :— 
1. Sporadic distribution appears to be characteristic of most 
concretions to be found on the ocean-floor or in recent alluvial 
deposits. 
2. The majority of phosphatic nodules are located at a definite 
level. The nodules themselves are frequently rolled, bored by 
molluscs, and carry attached contemporaneous animal remains. 
They are contemporaneous and have been formed where there is 
a pause in sedimentation. 
It may be remarked in this connexion that so far as the evidence 
goes, modern concretions seem to form very slowly. It follows that 
they must occur where sedimentation is also slow. 
3. Some nodules occur in definite lines, parallel to the bedding. 
The spacing of the lines follows apparently some law or “‘ rhythm ”’, 
sometimes in a very marked way. Moreover, several observers 
have called attention to a tendency for the distance between the 
individual nodules of a given bed to be uniform. Failures to find 
competent external causes of this arrangement have led to the 
suggestion that the rhythmic precipitation of diffusing solutions 
is the cause. In some cases there is a strong resemblance to the 
sequence of Liesegang banding. For example, closely spaced bands 
may be followed by a region where the bands are progressively more 
1 Liesegang, loc. cit., chap. xviii. 
