On Some Points in Chemical Geology. 419 



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

 chlorid 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 chlorid 

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

 cipitated by bicarbonate of soda.* 



When we consider that the vast amount of argillaceous sediment- 

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

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

 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 Vichy, or 

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

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

 ed that the same process of decomposition of alkaliferous silicates 

 is still going on. 



V. A striking and important fact in the history of the sea, and < if 

 all alkaline and saline waters, is the .-mall 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-feldspars readily undergo decomposition. Hence 

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

 which these rocks -till retain, the potash almost always predominates 

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

 silica in excess, and bul small portions of lime and magnesia. Th< -•• 

 conditions are readily explained when we consider the nature of 

 the soluble matters found in the mineral water- which issue from 

 these argillaceous reeks. 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 from argillaceous strata are marked by the predominance of 

 bicarbonate of soda, often with portions of silicate and borate, he- 

 sides bicarbonates of lime and magnesia, and occasionally of iron. 

 The atmospheric waters filtering through such strata remove 

 soda, lime and magnesia, leaving behind the silica, alumina and 



* See Report Geol. Surv. Canada, 1857, pp. 212-214, and Am. Jour. 

 Science (2) xxviii. pp. 170 and 305. 



