Action of the Atmosphere upon neichj-deepencd Soil. 455 



from the atmosphere. The bleached and whitened appearance of 

 stones that have lain amongst peat is familiar to many ; this may 

 be supposed to arise from the acids of the peat dissolving out the 

 iron or colouring-matter of the mineral. 



Professor Johnston, of Durham, drew some attention to this 

 action of the organic acids in a communication to the British 

 Association, without however doing much to the elucidation of 

 the subject ; he adduced the instance of the mineral pigotite, 

 formed in the caves of Cornwall by water dripping from the 

 roof. This water contains a peculiar organic acid, derived from 

 the soil of the moors, which dissolves the alumina of the granite 

 and combines with it. Scarcely anything, however, appears to 

 be known about the matter, and it therefore need not longer 

 detain us. 



As the nature of the changes wrought upon the various oxides 

 and earthy salts of which the soil is composed must depend 

 greatly upon the constitution of the mixture of mineral substances 

 from wliich tliis earthy matter has been derived, it will be pro- 

 per to examine some of the geological and mineralogical features 

 which the surface of this country presents. 



It is found that the whole of Scotland, and the north and 

 middle divisions of England and Ireland, with the exception of 

 the higher hills, are covered more or less deeply with a super- 

 ficial deposit of clay, gravel, and loam (containing in many places 

 large blocks and stones), out of which the agricultural soil of the 

 country is derived. This soil consists of the debris of minerals 

 In a greater or less state of disintegration, together with fragments 

 of undecomposed minerals and a variable amount of organic 

 matter. The soil, therefore, is generally just the subsoU plus 

 organic matter. This organic substance, or earthy mould, is 

 chiefly carbonaceous material derived from the decay of vegetable 

 and animal bodies. 



The superficial deposit to which I have adverted is known 

 geologically by various names : as the nortlicrn drift, boulder 

 formation, erratic tertiaries, diluvium, and till ; and recent inves- 

 tigation has demonstrated that at the time of its deposition the 

 land now covered by it was submerged below a glacial or icy sea, 

 above which rose the summits of our highest hills in the shape 

 of islands.* The physical conditions of this area have been 

 thought very similar to those that at present prevail on the north- 

 eastern coasts of America, within the line of summer floating ice. 

 The limits of th<? region have not everywhere been ascertained, 

 but the late Professor Edward Forbes draws the boundary across 



* It will be seen that I am spe:ikiiij^here in a general st-nse. Jlany river-valleys 

 contain superficial acciiinulatious newer than the drift ; and in other localities it 

 may not always occupy the surface. The plienomcua of ice, also, may not have 

 prevailed during the whole time of its deposition. 



