98 CHEMISTRY OF NATURAL WATERS. [IX. 



thus beds of white clay and sand are formed. The waters thus 

 charged with proto-salts of iron absorb oxygen when exposed 

 to the air, and then deposit the metal as hydrated peroxide, 

 which, when the organic matter is in excess, carries down a 

 greater or less proportion of it in combination. Such organic 

 matters are rarely absent from limonite, and in some specimens 

 of ochre amount to as much as fifteen per cent.* The condi- 

 tions under which hydrous peroxide of manganese is often 

 found are very similar to those of hydrous peroxide of iron 

 with which it is so frequently associated ; and there is little 

 doubt that oxide of manganese may be dissolved by a process 

 like that just pointed out. A portion of manganese has been 

 observed in the soluble matters from decaying peat-moss ; and 

 it seems to be generally present in small quantities with iron 

 in surface-waters. 



9. There is reason to believe that alumina is also, under 

 certain conditions, dissolved by waters holding organic acids. 

 The existence of pigotite, a native compound of alumina with 

 an organic acid, and the occasional association of gibbsite with 

 limonite, point to such a reaction. That it is not more abun- 

 dant in solution, is due to the fact, that, unlike most other 

 metallic oxides, alumina, instead of being separated in a free 

 state by the slow decomposition of its silicious compounds, 

 remains in combination with silica. The formation of bauxite, 

 a mixture of hydrate of alumina with variable proportions of 

 hydrous peroxide of iron, which forms extensive beds in thu 

 tertiary sediments of the great Mediterranean basin, indicates 

 a solution of alumina on a grand scale, and perhaps owes its 

 origin to the decomposition of solutions of native alum by 

 alkaline or earthy carbonates. Emery, a crystalline anhydrous 

 form of alumina, has doubtless been formed in a similar man- 

 ner. (American Journal Science (2), XXXII. 287, and ante, 

 page 13.) The existence in many localities of an insoluble sub- 

 sulphate of alumina, websterite, in layers and concretionary 

 masses in tertiary clays, evidently points to such a process. 

 Compounds consisting chiefly of hydrated alumina are frequently 



* Geology of Canada, p. 512. 



