96 



globular concretions. Augmenting through long period, these 

 layers consolidate to a greater or less extent, and gn r e consis- 

 tency to the subsoil. When examined, they always show their 

 upper portion to be the hardest and firmest, presenting some 

 openings of variable size at irregular distances, through which 

 the surface drainage alone can escape. These openings, col- 

 lecting the waters and thereby facilitating the formation of 

 subterranean channels, will be found to be where the percola- 

 tion of the water is most abundant — i.e., below the depres- 

 sions ; but also elsewhere, if the former be far aj)art, as other 

 circumstances favour the escape of surface water, and wherever 

 such exist, there in the course of time a hollow will be formed 

 by the gradual removal of part of the soil directly above the 

 place. Thus we come to the same conclusions as those arrived 

 at from a different starting point. 



The generally black colour of the soil in these mud-flats is 

 caused by the carbon of decaying organisms and imperfectly 

 oxygenised iron, as proved when exposed to red heat. Both 

 ingredients change simultaneously through the chemical 

 action of the water. The iron taking up more oxygen assumes 

 the characteristic colour, and imparts it to the soil ; the carbon 

 is turned into carbonic acid and removed. The latter occurs 

 most rapidly in the depressions. The acidulated water removes 

 a part — or the whole, in small patches — of the iron oxide, re- 

 depositing it at lower levels, viz., among buried sands and 

 gravels, cementing them into ferruginous sandstones and con- 

 glomerates (assisted, no doubt, by soluble silicates obtained 

 from the same sources), or forming beds or patches of loose 

 lapilli-like concretions with polished exterior surface, some- 

 times exploding with a tiny crash when thrown in the fire. 

 Operations like the foregoing when acting through long periods 

 may be safely assumed as quite capable to produce all the 

 results in changing the surface aspect of large areas as de- 

 scribed, and although one cannot watch them at any one place, 

 yet their respective stages can be observed at different local- 

 ities. According to this the black portions of the " Bay of 

 Biscay" soil are considered to be due to some unremoved 

 carbon, &c. ; the reddish parts to ferric oxide; the light-coloured 

 parts either to the entire absence of either, or the super- 

 abundance of lime ; both causes render them less suited for 

 vegetable growth till supplied with mould from the neighbouring 

 more productive regions. 



If downward denudation be very rapid, as in some parts of 

 the coast plain near Ardrossan, even the greater part of the 

 clay may have disappeared from the ridges, and only some 

 mixed with mould be left in the depressions. In this case the 

 former will be the more barren portions, consisting of smal 



