W. A. Richardson—The Relative Age of Concretions. 121 
Large numbers of concretions—in fact one may safely say the 
greater number of concretions—tend to have a shape that may be 
described as an ellipsoid flattened vertically. This shape may 
appear indifferently, whether there are central fossils or not. The 
form suggests a possible analogy with the ellipse of stress, or the 
ellipse of optic elasticity. If an isotropic substance is compressed i in 
one direction the ease of vibration of a ray in that direction is reduced, 
and, as for uniaxial crystals, the distribution of vibration ease can 
now be represented by an ellipsoid of rotation. 
It seems reasonable to suppose that concretionary growth should 
be similarly influenced by the state of stress of the medium. In 
a homogeneous and unstressed matrix growth should proceed at 
equal rates in all directions, and, therefore, in a given time reach 
a spherical boundary. On the other hand, a concretion forming 
subsequently to the deposition of a rock, and located at some 
distance from the surface, encounters considerable resistance to 
growth in a vertical direction. The rate of increase in this direction 
should, therefore, be less than it is horizontally, and the flattened 
ellipsoidal shape that prevails in many concretions would be 
explained. 
This phenomenon can be demonstrated by carrying out Liesegang’s 
experiment in a gel subject to greater compression in one direction 
than in another. It is rather difficult to be sure of getting an 
unequally stressed gel, but the method described below was 
successful. 
Pour a bichromate gel into a cardboard tray to a depth of about 
4in. Support a glass, or xylonite plate, provided with a small 
central hole, in contact with the surface of the gel until the latter 
has completely set. The transparent plate must fit the tray exactly. 
When setting is complete, cut away two opposite sides of the tray, 
supporting the other pair by wooden blocks. Press the plate on to 
the surface of the gel and apply weights. The gel is now free to move 
in one direction, but hindered in the other. It is, therefore, 
compressed against the remaining sides of the tray. Now place a 
drop of strong silver nitrate over the central hole, and leave in a 
damp atmosphere to diffuse. 
In all experiments where differential straining of the gel was 
successful the diffusing silver nitrate reached an elliptical boundary. 
The shape obtained in two experiments, with the dimensions of the 
axes, is given in Fig. 2 
It should be mentioned that J. Geikie! considered that the 
horizontal extension of a concretion is due to the greater ease of 
diffusion provided by lamination planes. This would no doubt 
assist the phenomenon, but cannot be considered as a major cause, 
since precisely similar ellipsoids are found in more massive beds, 
and not located along bedding planes. 
1 Geikie, Structural and Field Geology, Edinburgh, 1912, p. 121. 
