DISTRIBUTION OF DISSOLVED MATERIAL. 541 



that iron is abundantly transported as a carbonate, and aluminum as a 

 sulphate, as in the natural alums Also, both the alumina and the ferric 

 oxide may be partly transported to the places of precipitation as ahuninates 

 and ferrates. 01 



What proportions of these materials have been dissolved and precipi- 

 tated and what parts are simply residual undissolved substance is probabh T 

 not usually determinable, but in many places solution and reprecipitation 

 appear to be the chief cause of the concentration. Not infrequently these 

 concentrates are so rich in iron and alumina as to be ores. This phase 

 of the matter is much more fully developed on pages 842-846 and pages 

 983-989. 



As much less abundant important insoluble compounds which accu- 

 mulate under the same principle as iron oxide may be mentioned the 

 manganese oxides, pyrolusite, mang-anite, etc. The abundant precipitation 

 of quartz in the belt of weathering, as explained on page 540, while largely 

 due to its plentifulness, is also controlled in an important degree by its 

 relative insolubility. 



Still another set of compounds which well illustrate the principle are 

 the phosphates. It is well known that the manufactured soluble phosphates 

 used as fertilizers largely "revert" or are transformed to relatively insoluble 

 phosphates, and as such are precipitated in the soil." 



(c) To a certain point . the less humid a region the more likely are 

 precipitates to form in the belt of weathering. The reason for this is 

 obvious. If the amount of rainfall be so small that it is largely brought to 

 the surface by capillary and other forces, as explained on pages 419-423, 

 and evaporated, the material it holds in solution will be precipitated; but 

 if a region be so arid that there is not sufficient precipitation to dissolve 

 material abundantly, it can not be abundantly precipitated. Indeed, it has 

 been seen (pp. 496-498) that in regions of extreme aridity disintegration is 

 the rule and decomposition takes place only to a small extent. Therefore 

 we conclude that the climatic conditions for abundant precipitation of 

 minerals in the belt of weathering are those of moderate rainfall for a 

 certain period of the year, with aridity for the remainder of the year. 



"Cameron, F. K., Application of the theory of solutions to the study of soils: Kept. Div. of Soils, 

 U. S. Dept. Agric, No. 64, 1899, p. 169. 



& Wyatt, Francis, The phosphates of America, Scientific Pub. Co., New York, 1S92, pp. 22-26. 



