W. A. Richardson—The Relative Age of Concretions. 115 
(a) Displacive. The original material has been pushed aside to 
make room for the later. 
Or (b) Replacive. The original material has been carried away in 
solution, and the new deposited progressively in its place. 
There is also the possibility that a replacive concretion may cause 
some displacement of the strata, if 1ts volume be greater than that 
of the original material. 
Workers who wish to determine the relative age of segregations 
appeal mainly to facts gathered by field and microscopic study. 
Yet it is safe to say there is hardly a class of fact that has not been 
adopted by advocates of both contemporaneous and subsequent 
origin in support of their very different contentions. It is surprising 
on this account that no attempt to examine critically the significance 
of fundamental tests of relative age appears to have been made. 
II. Benpine or LAMINATION PLANES. 
The bending of planes of lamination so that they conform to the 
shape of a concretion is often reported. Some observers consider 
this feature a proof of the secondary displacement of the strata ; + 
whilst others see in it only evidence of the resistance of an object, 
harder than the main deposit, to the weight of overlying material.? 
On reflection, however, it will be seen that there are three distinct 
ways by which such a structure may have arisen, namely :— 
1. Accumulation of sediment in layers over a contemporaneous 
segregation. 
2. Differential consolidation of the sediment in the neighbourhood 
of a contemporaneous segregation. 
3. Progressive displacement of rock-layers by a subsequent 
concretion. 
The effect of sedimentation may be considered before the other, 
possibilities. The arrangement of successive layers with respect 
to an object already on the sea-floor will be determined largely by 
the following factors :— 
1. Gravitation. 
2. The angle of repose of the material—a measure of its internal 
friction in water. 
3. The sensitiveness of the deposit to water-currents. 
4. Friction between the object and the deposit. 
The last is doubtless negligible as its influence is theoretically 
limited to a layer one particle thick, and beyond only the internal 
friction between particles of the deposit comes into play. 
The general operation of these factors was made the object of 
simple experiments. A rectangular glass vessel was provided with 
a plasticene floor, the slope of which to the horizontal could be varied 
readily by tilting the vessel. Two types of material were used, and 
the results may be summarized thus :— 
? Dean, Amer. Journ. Sci., vol. xlv, 1918, p. 411. 
* Gardner, ibid., vol. xvi, 1908, p. 457. 
