90 Professor Favre on (he Geology of the German Tyrol. 



conditions as the sulphate of magnesia. M. Marignac, having 

 made the same expei-iment by exposing the tube only two 

 hours to a temperature of 200°, found that the result was 

 only a magnesian limestone but little charged with magnesia. 

 Thus, then, the length of time during which combination 

 may take place, is one of the numerous circumstances which 

 have an influence on the formation of dolomite. By sup- 

 posing it to be the only agent, we may be able to compre- 

 hend how it is that we find in nature, magnesian limestones, 

 dolomites, and surdolomilic limestones. 



After these different considerations, we have no doubt that 

 hydrochloric acid, the different acids of sulphur, and in par- 

 ticular, sulphurous acid, which have accompanied the sub- 

 marine dejections of the pyroxenic tufa, have acted upon this 

 rock only in producing different salts of magnesia, which, 

 under the double action of a pressure of about 15 atmospheres, 

 and a temperature of 200°, have formed dolomite and mag- 

 nesian limestone by means of the lime, whose presence, in seas, 

 is attested by the corals and shells still found in dolomite. 



It is necessary, however, to remark, that dolomite presents 

 a particular character not found in rocks as at first formed, 

 and which seems to indicate that it has undergone modifica- 

 tions since its formation. This character is given to it by the 

 numerous cells or small cavities, and by the multitude of 

 pores or empty places which it presents. 



These cells, according to M. Elie de Beaumont,* are the 

 result of the difference which exists between the atomic vo- 

 lume of the magnesia and that of the lime, and prove that the 

 dolomite is an altered limestone, in which an atom of lime 

 has been replaced by an atom of magnesia. M. Morlot has 

 confirmed this view, by shewing that the empty spaces or cells 

 of the dolomite exactly represent the difference of the volume 

 of an atom of magnesia and that of an atom of lime. 



This character, the cavernosity of dolomites, is therefore 

 important ; it must be taken into account. Now, the follow- 

 ing is the way in which things may have proceeded in nature. 



* Bulletin de la Societe Geol. de France, viii., p. 174 ; 1837. 



