THE CHALK 49 



of Dresden, to explain the occurrence of the flints in the 

 bands with clear interspaces between, which are such a 

 marked feature of the Upper Chalk. He has shown how 

 " a solution diffusing outward and encountering something 

 with which it reacts and forms a precipitate, moves on 

 into this medium until a concentration sufficient to cause 

 precipitation of the particular salt occurs. A zone of 

 precipitation is thus formed, through which the first 

 solution penetrates until the conditions are repeated, and 

 a second zone of precipitate is thrown down. Zone after 

 zone may thus arise as diffusion goes on." He suggests 

 that the zones of flint may be similar phenomena, water 

 diffusing through the masses of chalk taking up silica till 

 such concentration is reached that precipitation takes 

 place, the water then percolating further and repeating 

 the process.* 



The precipitation of silica and replacement of the chalk 

 occurs irregularly along the zone of precipitation, forming 

 great irregular masses of flint, which enclose the sponges 

 and other marine organisms that lay in the chalk strata. 

 Where a deposit of silica has begun, it will probably have 

 determined the precipitation of more silica, in the manner 

 constantly seen in chemical precipitation ; and it would 

 seem that siliceous organisms as sponges have to some 

 extent served as centres around which silica has been 

 precipitated, for flints are very commonly found, having 

 the evident external form of sponges. 



It will be well to say something here of the history of 

 the flints as the chalk which contains them is gradually 

 denuded away. Rain water containing carbonic dioxide 

 has a great effect in eating away all limestone rocks, chalk 

 included. A vast extent of chalk, which formerly covered 

 much of England has thus disappeared. The arch of 

 chalk connecting our two ranges of downs has been cut 

 through, and from the top of the downs themselves a 

 * See Common Stones, by Grenville A. J. Cole, F.R.S. 1921. 



