118 W. A. Richardson—The Relative Age of Concretions. 
with the penetration, the action may be considered as a reasonable 
imitation of the mechanical conditions that prevail as a nodule 
grows gradually into laminated stratum. 
The lamination planes were seen when the model was sectioned 
to be bent conformably to the shape of the ball (Fig. ld). Therefore, 
a concretion exerting pressure on a medium by gradually forcing itself 
outwards during growth is also competent to produce conformity of 
stratification lines to its own shape. 
Now, comparing these two cases from the mechanical standpoint, 
they are related inasmuch as the generating force of the one is the 
reaction in the other. The general tendency will be to compress 
layers of the matrix above and below, causing a sideway flow of 
material into interspaces between the nodules. The net result in 
both cases is bending of the layers, with vertical thinning and the 
simultaneous production of subsidiary pressure effects. 
It is now essential to determine any features that may be used to 
distinguish between the two cases. The foregoing discussion has 
already made it probable that with contemporaneous origin of the 
concretion the layers will probably settle to a gravitational surface 
around the nodule. If these layers are thin compared with the 
nodule, they may be expected to terminate abruptly against its 
sides, and under the influence of pressure, as in the model, will 
wedge upwards against the side of the nodule, after the manner of 
““catenary ’’ bedding (Fig. lc). If, on the other hand, the deposit 
is thicker bedded it is difficult to see how the question of relative 
age could be determined from the phenomenon itself. It may be 
remarked that 1f consolidation pressure produced this effect, one 
might expect it to be at least as frequent over undoubted con- 
temporaneous objects, such as fossils, as it is over nodules. But it 
certainly is not. Indeed, I have sought long for something of this 
kind in the field (and I am indebted to many others who have also 
kept watch in their field work for the same evidence), but hitherto 
without success. Moreover, there does not appear to be a recorded 
case. This is on the whole rather unexpected, and I am gradually 
coming to the opinion that conformity of stratification lines is 
caused in every case by concretionary growth, and that the date of 
formation is at the earliest later than the first compacting of the 
matrix. 
III. RepPLAceMENT. 
Replacement of fossils, of the grain and structural features of the 
original rock has always been regarded as undoubted evidence of 
secondary origin. An excellent application of the study of pro- 
gressive replacement to the problem of relative age may be found in 
Miss Raasin’s ! investigation of the Portland Cherts. 
W. A. Tarr,? however, has suggested a different explanation of 
1 Raisin, Proc. Geol. Assoc., vol. xviii, 1903, p. 75. 
2 Tarr, Amer. Journ. Sci., vol. xliv, 1917, p. 428. 
