Prof. Grenville Cole — Rhythmic Deposition of Flint. 67 



easiest direction of movement for permeating waters. The geological 

 importance of the- phenomena of diffusion was set before us by H. J. 

 Johnston-Lavis 1 as far back as 1894, and the rhythmic deposition 

 of silicates in limestone was described by him in association with 

 J. W. Gregory 2 in the same year. These observations have of late 

 received full recognition, and may well prepare us to regard con- 

 cretionary zones in rocks as lying perpendicular to the direction of 

 movement of solutions. 



Cracks may arise in which the materials that elsewhere form 

 concretions may become deposited as the solution concentrates through 

 evaporation ; but the main surfaces of deposition are likely to be 

 parallel with the bedding- planes of the sediment in which the con- 

 cretionary growth occurs. 5 



In sandstones concretions usually imply a mere cementing process, 

 or the local chemical replacement of a material already present as 

 a cement. Cases are now known to us where iron-ores form pseudo- 

 morphs of quartz-grains ; but the typical examples of concretionary 

 growth by pseudomorphic substitution occur in limestones. Here, of 

 course, there must be a diffusion in two directions ; the calcium 

 carbonate must pass out while something is being precipitated in its 

 phice. In the case of flints, it has often been supposed that layers 

 more rich in the siliceous spicules of sponges or radiolarians attracted 

 additional silica derived from the solution of similar skeletons in other 

 portions of the rock. While it seemed improbable that the lumps of 

 flint which replace the limestone around casts of siliceous skeletons, 

 or which, in other cases, include mere scattered spicules, owed the 

 whole of their silica to the nucleus around which they grew, it was 

 thought that abundant nuclei were requisite. On the hypothesis of 

 rhythmic deposition, however, the difficulty raised by Dr. Wallich 

 and Mr. Bulman as to a periodic abundance of sponges disappears. 



The residual skeletons and spicules in a zone of rhythmic deposition 

 may have acted as nuclei and may thus have aided Hint-formation ; 

 but for the most part they are preserved merely as casts, like those 

 in the adjoining parts of the chalk that are devoid of flints. A very 

 general solution of the hard parts of diatoms, radiolarians, and 

 siliceous sponges appears to have taken place in the body of the chalk, 

 and the traces of sponges within the flints represent so much material 

 added to the invading solution, until, by exchange with calcium 

 carbonate, silica began to be deposited. 



We may note that Mr. Bulman's suggestion that animal matter 



1 " The Causes of Variation in the Composition of Igneous Eoeks " : Natural 

 Science, vol. iv, p. 139 : 1894. 



2 "Eozoonal Structure of the Ejected Blocks of Monte Somma," Sci. 

 Trans. E. Dublin Soc, vol. v, p. 264 : 1894. 



3 The concentric rings of flint in the chalk of Cromer, described by Clement 

 Keid on p. 4 of The Geology of the Country around Cromer (Mem. Geol. 

 Surv. : 1882), seem a case of diffusion of silica outwards from certain centres 

 in a stratum about a foot in thickness. One of these rings has a longer 

 diameter of 15 feet ; others are perfect circles 9 feet in diameter. The two 

 concentric circles figured in the memoir certainly suggest rhythmic deposition. 

 It would be interesting to know what physical character of the beds above and 

 below has prevented the rings from developing into spherical shells. 



