378 
TUE GEOLOGIST. 
crystalline, but contain no magnesia. Delanoue lias pointed out examj)les of a 
similar error in the crystalline limestones of the calamine mines in Belgium, 
where in cases of su])posed dolomitization by contact with igneous rocks, he 
found no increase in the proportion of magnesia. 
These facts show that doloniities liave oeen formed under conditions where 
the theory of the intervention of \ olcanic and metamorphic agencies is inad- 
missiljle, and we are to conclude that they have been deposited as magncsian 
sediments in seas, or sometimes in lacustrine basins, from waters which often 
permitted the development of animal life. The contUtions ref[uired for the 
separation of carbonate of magnesia froni the sea or other waters, therefore, 
naturally clain\ our attention as a first step towards the solution of the problem 
before us. Mr. Hunt has already shown m a previous report, that the precipi- 
tate produced by carbonate of soda in water containing soluble salts of lime 
and magnesia consists in great part of carbonate of lime, the magnesium 
salts bemg decomposed only after the lime has been removed; and some ex- 
periments since made with carbonated waters serve further to illustrate this 
geologically important fact. 
If to an artificial sea-water, containing, besides common salt, elilorids of cal- 
cium and magnesium in the ])roportion of one equivalent of each, we add a so- 
lut ion of bi-earl)onate of soda ui water saturated with carbonic acid, a gelatinous 
precipitate sejiarates, which inmiediately becomes crystallme. This precipitate 
being separated after a few hours, and submitted to analysis, gave three succes- 
sive precipitations from the same liquid of 2'20, 2'00, and 1"23 per cent, of car- 
bonate of magnesia, the remainder being carbonate of lime ; the proportion of 
magnesia thus diminishing as the magncsian salt became predominant in the so- 
lution, which now gave no further precii)itate with bi-carbonate of soda, but 
deposited by evaporation to dryness, a granular residue of liydrated carbonate 
of magnesia with a little carbonate of lime. 
Bmeau has shown that if we evaporate solutions containing bi-carbonates of 
lime and magnesia in presence of svdphate or muriate of lime either at the or- 
dinary temperature or by artificial heat, the carbonate of lime is deposited 
with but a trace of magnesia. From this he concludes that tlie carbonates 
of magnesia exhibit, with all the soluble salts of bmc, the same reactions of 
incompatibility as the corresponding carbonate of potash and soda (Ann. de 
Cliim. et de Phys., vol. li., p. 302). 
Another cause which prevents the precipitation of carbonate of magnesia 
with the carbonate of lime, even when other salts of lime no longer exist in the 
solution, is found in the great solubility of bi-carbonate of magnesia as com- 
pared with the bi-carbonate of lime. According to Bischoff, canjonate of lime 
re([uiries for its solution about 1,000 parts of water saturated with carbonic 
acid ; and ^Ir. Hunt states that he has found it quite easy to obtain solutions 
containing lO'O grams of magnesia, equal to 21"0 grams of carbonate of mag- 
nesia to a litre of water, or 2'1 per cent. Binean fountl that by the aid of a 
current of carbonic acid prolonged for several days, a solution might be obtained 
containiirg 11'2 grains of magnesia, combined with nearly two equivalents of 
carbonic acid, in a litre of water. Such solution by spontaneous evaporation in 
the open air loses carbonic acid and deposits carbonate of magnesia, finally re- 
taining only O'lOS grains of magnesia in a litre, with carbonic acid sufficient to 
form a sescpu-carbonate. 
When recently precipitated hydrated carbonate of magnesia is added to a 
solution of bi-carbonate of lime, it immediately dissolves, but the transpai-ent 
solution soon after becomes troubled from the precipitation of carbonate of 
lime. This reaction is ])rccisely analogous to that produced by carbonate of 
soda, whicli, with bi-carbonate of lime, gives a precipitate of neutral carbonate. 
The results of a variety of experiments, undertaken in the hope of producuig a 
