Professor Favre on the Geology of the German Tyrol. 89 



disengaged in great abundance during volcanic eruptions ; 

 and from this chloride of magnesium must be formed, and 

 added to that which ah-eady exists naturally in marine 

 waters in greater quantity than the sulphate. 



This remark has suggested the idea to Professor Mari- 

 gnac, to endeavour to ascertain whether the chloride of 

 magnesium, brought into contact with carbonate of lime, 

 might form dolomite in certain circumstances. For this 

 purpose, he has made an experiment analogous to that of 

 M. Haidinger — that is, he has placed a certain quantity of 

 carbonate of lime, prepared chemically, and a dissolution of 

 chloride of magnesium in excess, in a tube of thick glass. 

 The tube was closed after the expulsion of air, and subjected 

 to a temperature of 200° C 



An analysis of the produce was made on O^'-TTO of the 

 matter taken from the tube. The magnesia was brought to 

 the state of phosphate, and the following obtained : — 



Carbonate of lime, . . . 0370 

 Phosphate of magnesia, . . 0-533 



We find, then, as the result of the operation — 

 Oa. C.48{^''-^J 



which shews, that a double decomposition took place in the 

 tube ; that there is formed a double carbonate of lime and 

 magnesia, and of chloride of calcium, which remained in 

 dissolution. Not only has the decomposition been sufficiently 

 complete to form dolomite, but, further, a double carbonate 

 is formed, containing a quantity of magnesia exceeding that 

 of true dolomite. This kind of rock is frequently met with 

 in nature.* The chloride of magnesium may therefore form 

 dolomite with the limestone, when it is subjected to the same 



* In Dauphiny. fiuoyraard (Bulletin de la Societe Geologique de France, Ire 

 Serie, xi., p. 438.) 



