152 THE FORMATION OF FLINTS 



nating with the black flint, thus giving rise to a banded 

 appearance. Such "banded" flints have excited a good 

 deal of curiosity, which seems somewhat disproportionate 

 to their importance ; but they are difficult to explain. The 

 banding recalls that of certain agates, and points to some 

 rhythmic or periodic action, probably connected with 

 surface tension, or such a process as gives rise to the 

 " tears " of strong wine. A somewhat analogous banding 

 may be obtained by allowing oil to saturate sheets of 

 paper, as shown in the illustration (Fig. 44). To discuss 

 \ this experiment in detail would occupy us too long,* and 

 we must take the blame of a flagrant attempt to explain 

 one difficulty by comparing it with another. 



The store of silica which was eventually to form our 

 flint was accumulated at the time when siliceous sponges 

 were flourishing at the bottom of the deep chalk sea ; so 

 much we may regard as definitely ascertained ; but at 

 what period and under what conditions the transforma- 

 tion of this material into flint took place is another 

 question, and not so easy to answer. The lie of the 

 irregular nodules conformably with the bedding, and 

 their rounded, flowing outlines, so singularly organic in 

 appearance, might well give rise, as they have often done, 

 to the notion that the flints were formed either contem- 

 poraneously with the growth of the sponges, or not long 

 after their death. But all ideas of this nature will sooner 

 or later be dispelled by an impartial study of almost any 

 chalk quarry, in which the flint is well displayed : the 

 rounded forms prove far from universal, and flint of 

 precisely the same characters as that forming the nodules 

 will be found lining and filling the joints of the chalk. 

 The joints are cracks which almost certainly owe their 



-' Much light is thrown on this subject by a very instructive paper 

 on the " Separation of Solids in the Surf ace -layers of Solutions," 

 by Dr. TV. Ramsden, Proc. Eoij. Soc., vol. Ixxii., p. 156. 



