88 CHEMISTRY OF DOLOMITES AND GYPSUMS. [VIII. 



no other bicarbonate than that of lime; but bicarbonate of 

 soda is often present in large proportion in natural waters, and 

 the addition of this salt to sea-water or other solutions con- 

 taining chlorides and sulphates of lime and magnesia will, as 

 we have shown, separate the lime as carbonate, and give rise 

 to liquids, which, without being concentrated brines, as in the 

 previous case, will contain sulphate of magnesia, but no lime- 

 salts. A further portion of bicarbonate of soda will produce 

 bicarbonate of magnesia, by the evaporation of whose solutions, 

 as before, hydrated carbonate of magnesia would be deposited, 

 mingled with the carbonate of lime which accompanies the alka- 

 line salt, and in the case of the waters of alkaline springs, the 

 compounds of iron, manganese, zinc, nickel, lead, copper, arsenic, 

 chrome, and other metals, which springs of this kind still bring 

 to the surface. In this way the metalliferous character of many 

 dolomites is explained. 



As the separation of magnesian carbonate from saline waters 

 by the action of bicarbonate of soda does not suppose a very 

 great degree of concentration, we may conceive this process 

 to go on in basins where animal life exists, and thus explain 

 the origin of fossiliferous magnesian limestones like those of 

 Dudswell and the palaeozoic formations of the western United 

 States, whose organic remains, as I am informed by Professor 

 James Hall of Albany, are generally such as indicate a shallow 

 sea. To the intervention of carbonate of soda is, I conceive, 

 to be referred the origin of all those dolomites which are not 

 accompanied by gypsums, and which make up by far the larger 

 part of the magnesian limestones ; nor will the dolomites thus 

 derived be necessarily marine, for the same reagent, with waters 

 like those of the Danube and Arve, would give rise to dolomites 

 and magnesites in fresh- water formations, which wmild not be 

 accompanied by gypsums. 



To the first stage of the reaction between alkaline bicarbon- 

 ates and sea-water I am disposed to ascribe the formation of 

 certain deposits of carbonate of lime which, although included 

 in fossiliferous formations, are, unlike most of their associated 

 limestones, not of organic origin, but have the characters of a 



