536 A TREATISE ON METAMORPHISM. 



which life plays in weathering has already been pointed out. But when 

 the processes of weathering have gone so far that a good soil has formed 

 and abundant life has taken possession of the little weathered rocks, the 

 conditions for weatheringj, including disintegration and decomposition, are 

 probably most favorable. 



DISTRIBUTION OF DISSOLVED MATERIALS. 



We have seen that all the elements of the rocks are dissolved, but 

 with exceedingly variable rates, and that in consequence of this there is a 

 concentration in the belt of weathering of a few minerals and fewer 

 elements. Where only the end products of weathering are joresent the 

 important elements there concentrated are but six in number — oxygen, 

 silicon, aluminum, iron, magnesium, and hydrogen. Before taking up the 

 question of what becomes of the dissolved materials, it is advisable to recall 

 the elements dissolved, their relative rates of solubility, and the fact that 

 under ordinary conditions of weathering and erosion but a portion of any 

 of the elements in the belt of weathering are rapidly dissolved. 



It has been seen (p. 518) that the common bases are taken into solution 

 in the following order: 



(1) The alkalies. Of these the sodium minerals are more readily 

 decomposed than the potassium ones and hence a much greater amount of 

 sodium than potassium is dissolved. 



(2) The alkaline earths. Of these the calcium minerals are certainly 

 more readily decomposed than the magnesium minerals, and therefore 

 calcium is taken into solution in much greater quantity than magnesium. 



(3) Iron, especially that in the ferrous form. 



(4) Aluminum, which, while dissolved to a less extent than the other 

 bases, is dissolved in large amounts. 



It has also been seen (p. 485) that the dominant acids which go into 

 solution are silicic and carbonic. Common subordinate acids in solution 

 are hydrosulphuric, sulphuric, nitric, hydrochloric, and phosphoric. In vol- 

 canic districts hydrofluororic and hydrobromic acids are frequently present. 



Within the zone of observation by far the greater quantity of the 

 substance is carried as carbonate, and, as is well known, the salts are usually 

 not carried as normal carbonates but as bicarbonates." 



"Letts, E. A., and Blake, E. F., The carbonic anhydride of the atmosphere: Sci. Proc. Royal 

 Dublin Soc, new series, vol. 9, pt. 2, 1900, p. 160. 



