492 PHOCEEDIFGS OF THE GEOLOGICAL SOCLETT. [Jan. 5, 



would give rise to chlorides and sulphates with separation of silica ; 

 and the accumulation of the atmospheric waters would form a sea 

 charged with salts of soda, lime, and magnesia. The subsequent 

 decomposition of the exposed portions of the crust, under the influ- 

 ence of water and carbonic acid, would transform the felspathic 

 portions into a silicate of alumina (clay) on the one hand and a!^aline 

 bicarbonates on the other ; these, decomposing the lime-salts of the 

 sea, would give rise to alkaline chlorides and bicarbonate of lime — 

 the latter to be separated by precipitation or by organic agency as 

 limestone. In this way we may form an idea of the generation, 

 from a primitive homogeneous mass, of the siliceous, calcareous, and 

 argillaceous elements which make up the earth's crust, while the 

 source of the vast amount of carbonate of lime in nature is also 

 explained. 



When we examine the waters charged with saline matters which 

 impregnate the great mass of calcareous strata constituting in Canada 

 the base of the Silurian system, we find that only about one-half of 

 the chlorine is combined with soda ; the remainder exists as chlorides 

 of calcium and magnesium — the former predominating, while sul- 

 phates are present only in small amount. If now we compare this 

 composition, which may be regarded as representing that of the 

 palaeozoic sea, with that of the modern ocean, we find that the 

 chloride of calcium has been in great part replaced by common salt, 

 — a process involving the intervention of carbonate of soda and the 

 ■formation of carbonate of lime. The amount of magnesia in the sea, 

 although diminished by the formation of dolomites and magnesite, is 

 now many times greater than that of the lime ; for, so long as chloride 

 of calcium remains in the water, the magnesian salts are not precipi- 

 tated by bicarbonate of soda*. 



"When we consider that the vast amount of argillaceous sediment- 

 matter in the earth's strata has doubtlessly been formed by the 

 same process which is now going on, viz. the decomposition of fel- 

 spathic minerals, it is evident that we can scarcely exaggerate the 

 importance of the part which the alkaline carbonates, formed in this 

 process, must have played in the chemistry of the seas. We have 

 only to recall waters like Lake Van, the natron-lakes of Egypt, 

 Hungary, and many other regions, the great amounts of carbonate 

 of soda furnished by springs like those of Carlsbad and Yichy, or 

 contained in the waters of the Loire, the Ottawa, and probably many 

 other rivers that flow from regions of crystalline rocks, to remind 

 us that the same process of decomposition of alkaliferous silicates 

 is still going on. 



§ Y. A striking and important fact in the history of the sea, and of 

 all alkahne and saline waters, is the smaU proportion of potash-salts 

 which they contain. Soda is pre-eminently the soluble alkali, while 

 the potash in the earth's crust is locked up in the form of insoluble 

 orthoclase : the soda-felspars readily undergo decomposition. Hence 

 we find, in the analyses of clays and argillites, that, of the alkalies 

 which these rocks still retain, the potash almost always predominates 

 * See Eeport Geol. Surv. Canada, 1857, pp. 212-214. 



