VIIL] CHEMISTRY OF DOLOMITES AND GYPSUMS. 83 



his theory, excepted the limestones of primitive formations, 

 "but these are regarded by modem geologists as also sediment- 

 ary formations, and consequently offer no exception to the 

 general view. The different sources of carbonate of soda indi- 

 cated by Mr. Cordier may moreover be reduced to a single 

 one, inasmuch as both the salts of alkaline springs, and those 

 of what he calls plutonic emanations, are probably derived 

 from the decomposing feldspathic minerals of sedimentary 

 rocks. The argillaceous rocks, deprived of a large proportion 

 of the alkali which they once contained in the form of feld- 

 spars, are the equivalents of the limestones which have been 

 formed at the expense of the chloride of calcium of the primi- 

 tive ocean. 



EXTRACTS FROM A PAPER IN THE AMERICAN JOURNAL OF 

 SCIENCE FOR NOVEMBER, 1859, ON THE SALTS OF LIME 

 AND MAGNESIA AND THE FORMATION OF GYPSUMS AND 

 MAGNESIAN ROCKS. 



The theory of the formation of magnesian sediments will be 

 readily understood from the experiments which have been de- 

 scribed in the earlier parts of this paper ; but before proceeding 

 to its consideration I wish to call attention to the results of 

 the concentration by evaporation of natural waters in basins 

 without an outlet. If such a basin contain sea-water, the 

 gypsum, being insoluble in a saturated brine, will be entirely 

 deposited before the crystallization of the sea-salt, and there 

 will remain a liquid containing no lime-salts, but chlorides of 

 sodium and magnesium, with a large amount of sulphate of 

 magnesia. Such are the waters of Lake Elton and many of the 

 brine-pools of the Eussian steppes ; while on the contrary the 

 saturated brines of the Dead Sea and some other salt lakes 

 contain little sulphate, but abundance of chloride of calcium, 

 and if they are the residues of sea- water, have been modified 

 by additions of this salt, which has converted the sulphate of 

 magnesia into chloride of magnesium and gypsum, the calca- 

 reous chloride remaining in excess. 



