380 On some Relations of the Salts of Lime and Magnesia, 
tion of the gas is often mingled with sedimentar psums.* 
(See Bischof, Lehrbuch, ii, 189-185.) This author has suggested 
the decomposition of chlorid of magnesium by alkaline or earthy 
sulphurets as a source of sulphuretted hydrogen and hydrate of 
magnesia, into which sulphuret of magnesium is readily resolved 
in the presence of water. (Chem. Geology, i, 16.) If a salt of 
calcium were present, this reaction could only take place in the 
absence of carbonic acid, for carbonate of magnesia is incom- 
patible with chlorid of calcium. The direct reduction and 
decomposition of sulphate of magnesia by organic matter and 
carbonic acid may, however, yield sulphuretted hydrogen and 
carbonate of magnesia, and thus, in certain cases, give rise to 
magnesian sediments. 
69. In the preceding sections, we have supposed the waters 
mingling with the solution of sulphate of magnesia to contain 
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 containing 
chlorids and sulphates of lime and magnesia, will, as we have 
seen, (§ 1) separate the lime as bicarbonate, 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 farther portion of bicarbonate of soda will produce 
bicarbonate of magnesia, by the evaporation of whose solutions 
as before, hydrated carbonate of magnesia would be = oe 
mingled with the carbonate of lime which accompanies the aika- 
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 also the frequent association of metals, 
such as copper, nickel, cobalt, chrome and titanium, with ser- 
pentine, steatite, diallage, olivine, and other magnesian silicates, 
which owe their origin to the alteration of magnesian sediments 
such as we have described. 
* On certain modes of decomposition of the sulphates, see Jacquemin, Comptes — 4 
ap ag ae Se dae ake eae 
2. ne 
