6 THE CANADIAN NATURALIST. [Feb. 



solution, is due to the fact, that, unlike most other metallic oxyds, 

 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 peroxyd of iron, which 

 forms extensive beds in the tertiary sediments of the great Medi- 

 terranean 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 

 manner. Silliman's Journal [2] xxxii, 287. 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 found in fissures of the chalk in England. 

 On the absence of free hydrated alumina from soils, see Miiller, 

 cited in Silliman's Journal [2] xxxv, 292. 



§ 10. The organic matter dissolved by the surface-waters serves 

 to reduce to the condition of sulphurets the various soluble 

 sulphates which it takes up at the same time or meets with in its 

 course. These sulphurets, decomposed by carbonic acid, which is 

 in part derived from the atmosphere, and in part from the oxyda- 

 tion of the carbon of the organic matter, give rise to alkaline and 

 earthy carbonates on the one hand, and to sulphuretted hydrogen 

 on the other. In this way, under the influence of a somewhat 

 elevated temperature, are generated sulphurous waters, whether of 

 subterranean springs, or of tropical sea-marshes and lagoons. The 

 reaction between the sulphurets thus formed and the salts or 

 oxyds of iron, copper, and similar metals which may be present, 

 gives rise to metallic sulphurets. The decomposition of sulphur- 

 etted hydrogen by the oxygen of the air, produces native sul- 

 phur ; with which are generally found associated sulphates of lime 

 and strontia. By virtue of these reactions, soluble sulphates of 

 lime and magnesia may be completely eliminated from waters,. the 

 bases as insoluble carbonates, and the sulphur as sulphuretted hy- 

 drogen, free sulphur, or a metallic sulphuret. Moreover, as Forch- 

 hammer has pointed out in the paper already cited, sulphuret of 

 potassium in the presence of ferruginous clays is also completely 

 separated from solution, the sulphur as sulphuret of iron, and the 

 alkali as a double aluminous silicate. 



§ 11. We have thus far considered the composition of surface- 



