PRINCIPLES OF ROCK WEATHERING 7^9 



not int'ixHiucntly shortly fell away to the condition of sand. 

 Closer inspection revealed the fact that the blocks, when 

 brought to the surface, were in a hydrated condition, giving 

 forth a dull instead of clear, ringing sound when struck with a 

 hammer, and showing a lusterless fracture, though otherwise 

 unchanged. That such had not previously fallen away to the 

 condition of sand, it is assumed, was due to the vise-like grasp 

 of the surrounding rock-masses. 



Solutio/i. — It is, however, the solvent power of water that 

 most concerns us here, though solution alone plays a compar- 

 atively unimportant part upon rocks not first subjected to physical 

 and oxidizing agencies, excepting in the case of those composed 

 essentially of carbonate of lime or magnesia. Rain and nearly 

 all superficial waters as already noted contain traces of carbonic 

 and other organic acids,' which act upon the material of the 

 rocks, carrying it away invisibly, but none the less surely. 



As long ago as 1848 the Rogers brothers showed^ that pure 

 water partially decomposed nearly all the ordinary silicate min- 

 erals which form any appreciable part of our rocks. The action 

 of carbonated water upon the minerals in a finely pulverized 

 condition was recognizable in less than ten minutes, but pure 

 water required a much longer time before its effect was sufficient 

 for a qualitative determination. So pronounced was the action 



' Under this head are included the complex, unstable and little understood 

 products of plant decomposition known as humic, ulmic, crenif and apocrenic acids. 

 These act not merely as reducing agents (from their tendency to themselves undergo 

 oxidation) but are energetic solvents asvirell, attacking not merely the lime carbonates 

 but also silica and phosphates, arsenates and sulphates of the alkaline earths and the 

 metallic sulphides. 



Berthei.ot and Andre (Comptes Rendus Academic de Paris, 1 14, 1892, pp. 41-43) 

 have shown that the brown substance of humus and analogous compounds undergo 

 direct oxidation under the influence of the air and sunlight, forming carbonic acid. 

 These reactions are purely chemical, taking place without the intervention of microbes, 

 and are accompanied by a change in color of the original humus. The oxidation is 

 rendered more active through the division and mellowing of the humus by cultivation. 

 Through chemical union of the carbonic acid with certain bases, as lime soda and 

 potash there are formed soluble carbonates which may be leached out by meteoric 

 waters. 



' American Journal of Science, Vol. V, 1848. 



