1859.] HUNT CHEMICAL GEOLOGY. 493 



greatly over the soda. At the same time these sediments contain 

 silica in excess, and but small portions of lime and magnesia. These 

 conditions are readily explained when we consider the nature of 

 the soluble matters found in the mineral waters which issue from 

 these argillaceous rocks. I have elsewhere shown that, setting aside 

 the waters charged with soluble lime- and magnesia-salts, issuing from 

 limestones and from gypsiferous and saliferous formations, the springs 

 of argillaceous strata are marked by the predominance of bicarbonate 

 of soda, often with portions of silicate and borate, besides bicarbon- 

 ates of lime and magnesia, and occasionally of iron. The atmospheric 

 waters filtering through these strata remove their soda, lime, and 

 magnesia, leaving behind the silica, alumina, and potash — the elements 

 of granitic and trachjtic rocks. The more sandy clays and argiUites 

 being most permeable, the action of the infiltrating water will be 

 more or less complete ; while finer and more compact clays and marls, 

 resisting the penetration of this liquid, will retain their soda, hme, 

 and magnesia, and, by their subsequent alteration, will give rise to 

 basic felspars containing lime and soda, and, if hme and magnesia 

 predominate, to hornblende or pyroxene. 



The presence or absence of iron in sediments demands special 

 consideration, since its elimination requii'es the interposition of organic 

 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 described by Bischoff 

 and many other writers, particularly by Dr. J. W. Dawson* in a 

 paper on 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-measm-es, and in the white clays asso- 

 ciated with great beds of green sand (essentially a silicate of iron) in 

 the cretaceous series of New Jersey. Similar alternations of white 

 felspathic 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, afibrds 

 evidence of the existence of organic life at the time of the deposition 

 of these old crystalline rocks. 



The agency of sulphuric and mmiatic acids, from volcanic and 

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

 oxide of ii'on and other metallic oxides. The oxidation of pyrites, 

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

 subsequent decomposition of w^hich 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. In 

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

 alumina with oxide of iron, have doubtless been formed. 



Waters deficient in organic matters may remove soda, lime, and 

 * Quart. Journ. Geol. Soc. rol. v. p. 25. 

 VOL. XV. PART I. 2 k 



