II.] ON SOME POINTS IN CHEMICAL GEOLOGY. 13 



more compact clays and marls, resisting the penetration of this 

 liquid, will retain their soda, lime, and magnesia, and by sub- 

 sequent alteration will give rise to basic feldspars containing 

 lime and soda, and if lime and magnesia predominate, to horn- 

 blende or pyroxene.' 



The presence or absence of iron in sediments demands es- 

 pecial consideration, since its elimination requires the interpo- 

 sition of organic matters, which, by reducing the peroxide to 

 the condition of protoxide, render it soluble in water, either 

 as bicarbonate or combined with some organic acid. This 

 action of waters holding organic matter upon sediments con- 

 taining iron-oxide has been described by Bischof and many 

 other writers, particularly by Dr. J. W. Dawson * in a paper on 

 the coloring matters of some sedimentary rocks, and is applica- 

 ble 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 associat- 

 ed with great beds of green-sand (essentially a silicate of iron) 

 in the cretaceous series of New Jersey. Similar alternations 

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

 Green Mountain rocks of Canada, and on a still more remark- 

 able 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 crystal- 

 line rocks. 



The agency of sulphuric and muriatic acids, from volcanic 

 and other sources, is not, however, to be excluded in the solu- 

 tion 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 perhaps to characterize the 

 iron-ores of organic origin, f In this way the deposits of emery, 



* Quar. Jour. Geol. Soc., Vol. V. p. 25. 



f The occurrence of hydrated mixtures of oxide of iron and alumina, like 



