En Op ATOR EE RET ON gee ene 
ORIGIN OF CRYSTALLINE ROCKS. 505 
The first results of a very long series of experiments and inquiries 
-into the history of gypsum, were published by me in 1859, and 
farther researches, reiterating and confirming my previous conclu- 
sions, appeared in 1866.* In these two papers, it will, I think, be 
found that the following facts in the history of dolomite are es- 
tablished, viz. : first, its origin in nature by direct sedimentation, 
and not by the alteration of non-magnesian limestones; second, 
its artificial production by the direct union of carbonate of lime 
and hydrous carbonate of magnesia, at a gentle heat, in the pres- 
ence of water. As to the sources of the hydrous magnesian car- 
bonate, I have endeavored to show that it is formed from the 
magnesian chlorid or sulphate of the sea or other saline waters in 
two ways :— first, by the action of the bicarbonate of soda found 
in many natural waters ; this, after converting all soluble lime-salts 
into insoluble carbonate, forms a comparatively soluble bicarbon- 
ate of magnesia, from which a hydrous carbonate slowly separates : 
second, by the action of bicarbonate of lime in solution, which, 
with sulphate of magnesia gives rise to gypsum ; this first crystal- 
lizes out, leaving behind a much more soluble bicarbonate of mag- 
nesia, which deposits the hydrous carbonate in its turn. In this 
way, for the first time, in 1859, the origin of gypsums and their in- 
timate relation with magnesian limestones were explained. 
It was, moreover, shown that to the perfect operation of this re- 
action, an excess of carbonic acid in the solution, during the evap- 
oration, was necessary to prevent the decomposing action of the 
hydrous mono-carbonate of magnesia upon the already formed 
gypsum. Having found that a prolonged exposure to the air, by 
permitting the loss of carbonic acid, partially interfered with the 
_ process, I was led to repeat the experiment in a confined atmos- 
phere, charged with carbonic acid, but rendered drying by the 
presence of a layer of dessicated chlorid of calcium. As had 
been foreseen, the process under these conditions proceeded unin- 
rruptedly, pure gypsum first crystallizing out from the liquid, 
and subsequently, the hydrous magnesian carbonate. f This ex- 
periment is instructive as showing the results which must have 
attended this process in past ages, when the quantity of carbonic 
acid in the atmosphere greatly exceeded its present amount. 
T 
* Amer. Jour. Sci., II, xxxviii, 170, 365; xlii, 49. 
Proceedings Royal Institution, May 30, 1867, and Canadian Naturalist, new series, 
Til, 231. 
