SEGREGATION. 217 



explain tlie formation of the numerous layers of nodular 

 flints throughout the body of the chalk. 



There is, however, an exjiilanation which will readily 

 account foi' all the features presented by the chalk-flints, 

 and requires the aid of no foreign factors in the process. 

 This explanation is based on the phenomenon known as 

 segregation. Now segregation is the tendency presented 

 by a small quantity of one substance, when diffused 

 through a much larger quantity of another substance, to 

 collect together in nodules or strings, which generally 

 accumulate either around some fragment of tlieir own 

 nature or some foreign body — especially an organic one — 

 as a nucleus. We have well-known examples of this 

 segregating process in the huge lenticular calcareous masses 

 termed " septaria," found in the London and Kimeridge 

 clays, and also iu the iron-nodules of other formations. 

 Premising that soluble silica has a peculiar affinity not 

 only for any kind of silica, but likewise for gelatinous 

 organic substances (both of which were presented by the 

 sponges of the cretaceous seas), it will be obvious that if 

 we can only satisfy ourselves that at the time of its 

 deposition the chalk contained diffused among its substance 

 a sufficient amount of soluble silica, we shall at once be able 

 to account for the formation of its flint. Now, as we pass 

 upwards in the cretaceous system from the lower green- 

 sand to the upper chalk, we find a gradual change from 

 a completely siliceous to a calcareous rock. Moreover, 

 while in the gault, upper greensand, and lower chalk (in 

 which in the south of England there are no flints) there 

 is a large but decreasing amount of soluble silica, varying 

 from 46 per cent, in the upper greensand to 31 per cent, 

 iu the chalk- marl, when we reach the upper chalk with 

 flints such soluble silica is reduced to a mere trace. As 

 it is at the base of the white chalk that the sponges attain 

 their greatest development, and as it is also here that 

 flints first commence, the disappearance of the soluble 

 silica may be safely attributed to its segregation by means 

 of the sponges and other bodies, and its conversion into 

 flint. Prof. Prestwich, writing on this subject, observes 

 that in presence of the siliceous spicules of the cretaceous 



