EIVERS OF THE REGION AND THEIR ACTION 391 



by a series of scarps, to be described later. The covering of soil over the 

 chalk of the uplands is in general only a few feet, in depth, except where 

 it consists of the wind-blown sands of the coastal belt or, in exceptional 

 localities, of Tertiary and other thicker deposits. The chalk itself has 

 afforded easy passage to percolating waters from a time early in its his- 

 tory. Possibly due to shrinkage in the process of hardening after dep- 

 osition, it is traversed in all directions by numerous joints, which with 

 well-marked bedding planes divide it into blocks which are seldom of 

 large size. The writer does not remember seeing a block equal to a two- 

 foot cube at any time, and joint blocks of less than a foot in dimensions 

 are much the most usual. While the chalk itself is very absorptive, being 

 capable of retaining water up to about 30 per cent of its bulk when dry, 

 it holds this water very tenaciously and is not easily permeable except 

 along bedding planes and other fissures. Along these numerous crevices 

 ground-waters charged with silica in solution found their way, probably 

 while the formation was still submerged in the sea, Avhen bedding planes 

 and probably joints had already been formed, certainly before it had been 

 elevated to its present position or dissected by erosion, and while still 

 below the ground-water level. The silica was likely derived from the 

 surface material (including chalk) removed during the first cycle of 

 erosion following emergence, or even later, or possibly from ground-waters 

 under ^^head^^ which entered the chalk from underlying sands or percolated 

 through it from parts exposed on shore before complete emergence. 

 J. Smith Flett^ holds that the origin of the silica was siliceous organisms 

 deposited with the chalk, which were dissolved within the substance of the 

 chalk (how, he does not say) and filtered through the "porous medium'^ 

 to the fissures, where changed conditions caused the precipitation of the 

 silica. He describes the process as "metasomatic'' and also as '^concre- 

 tionary" and posits a long time for its accomplishment. This theory of 

 the origin of the silica is based on the ascertained presence of siliceous 

 sponge spicules in the flints and on the facts that some fossil sponges 

 occur, and that the large flints called potstones are often shaped like a 

 species of gigantic sponge, Spotigia patera, now living in the seas of 

 Sumatra. The presence of diatoms is also thought probable. The flints 

 can only rarely be shown to be fossil sponges, and if they were must have 

 involved the addition of much more silica to the original skeleton. 

 Moreover, the fissures, and not the presence of dead sponges, etcetera, 

 determine the location of flints. They in general show no organic struc- 

 ture, whereas the calcareous shells replaced by flint are recognizable in 

 detail. The presence of undissolved spicules may be also regarded as 



3 See Flint, Encyclopedia Britannica, eleventh edition. 



