12 ON SOME POINTS IN CHEMICAL GEOLOGY. [II. 



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

 by the same process which is now going on, namely, the de- 

 composition of feldspathic minerals, it is evident that we can 

 scarcely exaggerate the importance of the part which the alka- 

 line carbonates, formed in this process, must have played in the 

 chemistry of the seas. (Page 2.) 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 reminded 

 that a similar though much slower process of decomposition of 

 alkaliferous silicates is still going on. 



A striking and important fact in the history of the sea, and 

 of most alkaline and saline waters, is the small 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 still 

 retain, the potash almost always predominates greatly over the 

 soda. At the same time these sediments contain silica in ex- 

 cess, and but small portions of lime and magnesia. These con- 

 ditions 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 Hint 

 (setting aside the waters charged with soluble lime and mag- 

 nesia-salts, issuing from limestones and from gypsiferous and 

 saliferous formations) the springs from argillaceous stmt 

 marked by the predominance of bicarbonate of soda, often 

 with portions of silicate and borate, besides bicarbonates of 

 lime and magnesia, and occasionally of iron. The atmospheric 

 waters filtering through such strata remove soda, lim- 

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

 elements of granitic and trachytic rocks. The more sandy 

 clays and argillites being most permeable, the action of the in- 

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



