398 



THE GEOLOGIST. 



the influence of long persistent chemical actions in nature, which we 

 have already noticed before. If the anticipations of Leopold Yon 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 Trance. M. Hugard 

 has recently shown that sulphate of baryta is mixed up with the 

 dolomites in the Yalley 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. 



M. 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 ocean and rivers 

 than sulphates, are, nevertheless, to le found in dolomites, in such quantity 

 (more than the 1-1 000th 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 Yalley of Passa,"^-' especially in those 

 from Eosengarten, and, again, in the dolomites of Seefeld, in the Tyrol, 

 in those of the variegated marls of Eribourg, 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. Gothard, have never shown traces 

 of either sulphates 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-kno-wn English savant, Dr. Percy, formerly 

 suggested that the circumstance of the production of Gehlenite, at a high tempera- 

 ture, in 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 Yalley of 

 Fassa (or Fassathal) in the Tyrol, mentioned here. — T. L. P. 



t Silicate of Alumina.— T. L. P. 



