420 On Some Points in Chemical Geology* 



potash — the elements of granitic and trachytic rocks. The more 

 sanely clays and argillites being most permeable, the action of the 

 infiltrating waters will be more or less complete; while finer and more 

 compact clays and marls, resisting the penetration of this liquid, will 

 retain their soda, lime and magnesia, and by subsequent altera- 

 tion, will give rise to basic feldspars containing lime and soda, and 

 if lime and magnesia predominate, to hornblende or pyroxene. 



The presence or absence of iron in sediments demands especial 

 consideration, since its elimination requires the interposition of or- 

 ganic matters, which by reducing the peroxide to the condition of 

 protoxide, render it soluble in water, either as a bicarbonate or 

 combined with some organic acid. This action of waters holding 

 organic matter upon sediments containing iron-oxide has been de- 

 scribed by Bischof and many other writers, particularly by Dr. J. W. 

 Dawsonj in a paper en the colouring matters of some sedimentary 

 rocks, and is applicable to all cases where iron has been removed 

 from certain strata and accumulated in others. This is seen in the 

 fire-clays and iron-stones of the coal-measures, and in the white 

 clays associated with great beds of green-sand (essentially a silicate 

 of iron,) in the cretaceous series of New Jersey. Similar alterna- 

 tions of white feldspathic beds with others of iron ore occur in the 

 altered Silurian rocks of Canada, and on a still more remarkable 

 scale in those of the Laurentian series. We may probably look 

 upon the formation of beds of iron-ore as in all cases due to the 

 intervention of organic matters, so that its presence, not less than 

 that of graphite, affords evidence of the existence of organic life 

 at the time of the deposition of these old crystalline rocks. 



The^agency of sulphuric and muriatic acids, from volcanic and 

 other sources, is not however to be excluded in the solution of 

 oxide of iron and other metallic oxides. The oxidation of pyrites, 

 moreover, gives rise to solutions of iron and alumina salts, the 

 subsequent decomposition of which by alkaline or earthy carbonates 

 will yield oxide of iron and alumina ; the absence of the latter 

 element serves to characterize the iron-ores of organic origin. J In 

 this way the deposits of emery, which is a mixture of crystallized 

 alumina with oxide of iron, have doubtless been formed. 



f Quart. Journ. Geol. Soc, vol. v, p. 25. 



[% Hydrated alumina in the form of gibbsite is however met with 

 incrusting limOnite, and the existence of compounds like pigotite, in 

 which alumina is united with an organic substance allied to crenic acid, 

 seems to show that this base may, under certain conditions, be taken into 

 solution by organic acids.] 



