398 
TllK GEOLOGIST. 
the influence of long persistent chemical actions in nature, which we 
have already noticed before. If the anticipations of Leopold Von Buch 
are exact, says M. Ch. Deville — if really dolomite has been formed in 
nature by the substitution of one equivalent of magnesia for one equivalent 
of lime, in limestone — the magnesia having been brought in one form 
or another from the interior, one ought to find in the transformed rock 
traces of the substances that have occasioned the transformation. In 
the memoir referred to this is proved to be the case. A great number 
of dolomites, as is well known, are found in nature associated with 
anhydrite or gypsum, and these are sometimes intimately mixed, as we 
find, for instance, in the dolomite of I'Ariege, in France. M. Hugard 
has recently shown that sulphate of baryta is mixed up with the 
dolomites in the Valley of Binn, and that this salt appears to be nothing 
less than the traces of the sulphuretted agents which have determined 
the formation of these dolomites. 
jVI. Ch. Deville has proved satisfactorily to himself that chlorides, 
notwithstanding their solubility in water, which accounts for their 
being rarer in rocks, more frequent in the waters of the ocyan and rivers 
than sulphates, are, nevertheless, to he found in dolomites, in such quantity 
(more than the 1-lOOOLh part of the rock) as may be determined by 
careful chemical analysis. The author assures us that he has found 
chlorides in dolomites from the Valley of Fassa,* especially in those 
from Kosengarten, and, again, in the dolomites of Seefeld, in the Tyrol, 
in those of the variegated marls of Fribourg, in dolomites from the 
carboniferous strata in the province of Liege (Belgium), and, finally, in 
the Tertiary dolomite of Beyne. When treated with pure water, at 
boiling point, these rocks yield chlorides of calcium and magnesium. 
Others, such as the dolomite of St. Guthard, have never shown traces 
of either suljihates or chlorides, which singular fact tends to prove that, 
in the manufacture of dolomites, nature has evidently employed more 
than one process. 
Clays. — The author's experiments on clays are similar to those 
described above. Taking a fragment of pure kaolin,f he soaks it in a 
* We should remember that a well-known English savant, Dr. Percy, formerly 
suggested that the cii-cumstunce of the production of Gehleni/e, at a high tempera- 
ture, iu an iron furnace, may possibly be available by geologists in explaining the 
formation of the rocks in which the natural mineral occurs, as in the Valley of 
Fassa (or Fassathal) in the Tyrol, mentioned here. — T. L. P. 
t Silicate of Alumina. — T. L. P. 
