470 Action of the Atmosphere upon newly-deepened Soil. 



soluble salt of either lime or sodium, e.y. sulphate of lime or chlo- 

 ride of sodium. The most crystalline are the most durable. 

 {Mem. of Geolocjical Survey, ii, 701.) The "principal effect of 

 the atmosphere on calcareous matter is to dissolve it through the 

 action of the rain-water containing carbonic acid. Carbonate of 

 lime is but slightly soluble in pure water, but absorbing more 

 carbonic acid it is converted into bicarbonate, which is soluble 

 to a considerable extent. " In some very earthy limestones," 

 says De la Beche, " the disappearance of the calcareous matter 

 in the upper portion of the rock for many feet down is sometimes 

 so complete, and the peroxidation of the iron so extensive, that 

 a rusty-looking porous substance is all that is left." The re- 

 mains of shells in such rocks are often also dissolved out, leaving 

 only the empty spaces or casts. Lime appears to have the effect 

 of decomposing some of the silicates by turning out the alkaline 

 base and instead thereof entering itself into union with the silicic 

 acid.* Soda may be liberated by this process, and also potash. It 

 probably also decomposes in some similar manner the salts of 

 organic acids. 



Oxides of Iron. — Combinations of iron are so widely and gene- 

 rally dispersed in all rocks and soils that it would be a great 

 omission to neglect taking some particular notice of them. 



1. Carbonate of Iron, or Siderite. — Its chemical composition, 

 when pure, is : — 



Protoxide of h'on 62'1 



Carbonic acid 3T"9 



100-0 



but part of the iron is generally replaced to some extent by man- 

 ganese, magnesia, or lime. It is soluble in acids with efferves- 

 cence. Occurs in beds of great extent in the coal formation of 

 this country, the clay ironstone and black -band being but impure 

 varieties of this mineral, and from it most of the British iron is 

 manufactured. 



2. Phosphate of Iron, or Vivianite. — It is chemically a combi- 

 nation of phosphoric acid with oxide of iron and a considerable 

 amount of water ; easily soluble in acids. It is frequently found in 

 the neighbourhood of decayed animal matter, occurs also in some 

 peat mosses, forming a blue crust on the dried peats. It occa- 

 sionally fills the interior of fossil shells, but is found also in 

 some igneous rocks, and doubtless is a frequent product in the 

 soil. 



* According to G. Bischof, bicarbonate of lime and silicate of potash, when 

 mixed together in solution, give rise to carbonate of potash and carbonate of lime, 

 silica being liberated ; and he states that this decomposition takes place in nature. 



