66 Prof. Grenville Cole — Rhythmic Deposition of Flint. 



concretionary layers is the fact that a crystal growing in a solution 

 is surrounded by a zone of less than average concentration. The 

 crystal- growth impoverishes the solution round ahout it. If new 

 material is diffusing into the solution, the impoverished zone 

 becomes again enriched ; additions are made to the crystal, and a new 

 unsaturated zone is formed around it. 



Where a jelly that has absorbed ammonium bichromate is used as 

 a medium for the diffusion of a solution of silver nitrate, silver 

 chromate is formed. At a certain concentration Ostwald's metastable 

 boundary is attained, and crystallization of the silver chromate occurs, 

 even if there are no nuclei to serve as centres of deposition. Each 

 crystal thus formed is surrounded by an impoverished zone ; but the 

 arrival of fresh material again leads to enrichment, and the crystals 

 grow until they form a band within the jelly at the surface where the 

 necessary concentration is produced. 



The silver chromate in this experiment arises from reaction with 

 material on the far side of the band, and the distance through which 

 such material moves to reach the crystalline nuclei is limited. An 

 impoverished layer thus arises beyond that of deposition, and 

 the silver nitrate diffusing through the layer of deposition has to 

 progress for some distance before the metastable boundary is again 

 attained. Successive zones are thus built up, with intervals which 

 represent regions of weaker concentration of silver chromate. 



As we follow out Liesegang's application of these phenomena to the 

 explanation of zonal structures in agates, in concretions, and in 

 massive rocks, we come with delight upon his brief but illuminating 

 reference (p. 126) to the rhythmic layers of flint in limestone. He 

 pictures a solution of silica or of a silicate spreading through a fairly 

 uniform sediment. Or, in the case of the chalk, the silica may have 

 been at first distributed with approximate uniformity, and then became 

 affected by a progressive " one-sided " precipitation. It is pointed out 

 that drying of the rock, due to elevation, may play its part together 

 with the processes of diffusion. 



We may venture to expand this conception in relation to the 

 familiar flint-layers in the chalk. Layer-structure may be reasonably 

 expected in a rock containing concretions, if a solution has moved 

 through the mass from above or from below, or from one side to the 

 other. The movement, in a drying sediment, may be upwards; or 

 the solvent water may drain downwards when the mass is uplifted 

 above the sea. Since the consolidation of the rock usually precedes 

 folding and fracturing processes, it must be held to take place in 

 geological time with some rapidity. Mountain-building processes, as 

 we know, may overtake those of consolidation, and firm rock-masses 

 may be forced into yielding sands and muds of younger age. But the 

 chances are in favour of the occurrence of concretionary deposition 

 while the sediment remains practically undisturbed. The concretions 

 will develop along planes of bedding, because the solution to which 

 they are due has moved perpendicularly to these planes. 



This theory is directly opposed to that which many of us have 

 enunciated in our teaching work, when we regarded the concretions 

 as formed along the bedding-planes because these planes provided the 



