DISTRIBUTION OF DISSOLVED MATERIAL. 537 



It must also be recalled that the dissolved silicic acid is in the colloidal 

 state, and in the belt of weathering is an inactive chemical agent, and 

 therefore is not united with the bases in solution. The acids in the solutions 

 in combination with the bases therefore do not include silicic acid. Of the 

 remaining acids carbonic is that of dominant importance. 



It is. further to be recalled that the amounts of the various bases and 

 acids which may be taken into solution are dependent not only upon their 

 relative strengths, but also upon the amounts present and upon the combi- 

 nations in which they exist. 



We are now prepared to take up the question as to what becomes of 

 the materials dissolved in the belt of weathering. These materials are 

 abstracted bv plants, transported to the belt of cementation, permanently 

 abstracted by the run-off, and dissolved, transported, and reprecipitated in 

 the belt of weathering. 



MATERIAL ABSTRACTED BY PLANTS. 



Considerable quantities of the soluble constituents produced in the belt 

 of weathering are abstracted by the plants and built into their bodies. Of 

 the bases, the plants take greater quantities of the alkalies than of the alkaline 

 earths. They take only minute amounts of iron and aluminum. The acids 

 abstracted by plants in important amounts are nitric, phosphoric, and silicic. 

 In this connection it should be remembered that under ordinary conditions 

 the amount of alkalies available is less than that of the alkaline earths, and 

 that the amounts of nitrates and phosphates available are far less than the 

 amount of silicic acid. Therefore, even if the amounts of these compounds 

 abstracted by the plants were the same, this would result in the abstraction 

 of a considerable portion of the alkalies and the nitric and phosphoric 

 acids, while the major portion of the alkaline earths, of the iron and alumi- 

 num, and of the silicic acid would remain. 



The distance of the transfer of material in the belt of weathering by 

 means of plants is limited by the depth to which roots extend. It has been 

 noted (pp. 445-446) that the roots of ordinary grains and grasses extend into 

 the soil 1 or 2 meters, and that the roots of larger plants may extend to a 

 depth of 10 meters. The vertical transfers are therefore to be measured by 

 these distances. The amount of material transferred by plants is a direct 

 function of the luxuriance of the vegetation. Luxuriant vegetation occurs 



