Professoz' Favre on the Geology of the Germati Tyrol. 87 



germ of all future theories. Notwithstanding this, I have 

 endeavoured, by special reference to certain facts, to bring 

 them under one point of view, which appears to me to pre- 

 sent some novelty. Moreover, the experiment made by my 

 colleague, M. Professor Marignac, and which throws great 

 light on the formation of dolomite, is entirely new. 



In spite of the ingenious theories advanced respecting the 

 ox'igin of this rock, many doubts still remain in science re- 

 garding it, for chemical experiments have not always con- 

 firmed geological theories. 



I here speak only of the dolomites which belong to the 

 second of the geological positions I have indicated — that is, 

 the dolomites of the great secondary chain of the Tyrol ; and 

 I think that they are not a metamorphic rock, in the sense 

 usually given to that word — that is to say, that the rocks of 

 these chains have not been altered since their formation. I 

 am of opinion that these mountains have been composed, 

 from their origin, of a double carbonate of lime and magne- 

 sia — that is to say, that the formations which constitute them 

 have been deposited in the state of dolomites at the bot- 

 tom of seas, and are not limestones altered by magnesian 

 vapours. 



I rest this affirmation on tlie beautiful researches of M. 

 Haidinger, which have been brought forward by M. Morlot,* 

 and on the experiment of Professor Marignac. 



We must not overlook an important fact, well known for 

 a long period, which has even given rise to theories as to 

 the formation of dolomite, that there exists a certain con- 

 nection between the dolomitic chains and the pyroxenic 

 rocks, which indicates that it is the latter which have fur- 

 nished the magnesia, in whole or in part, to the dolomite. 



M. Haidinger has succeeded in making dolomite, by 

 heating to 200^, and under a pressure of 15 atmospheres, a 

 mixture of sulphate of magnesia and carbonate of lime — 

 that is to say, it is necessary, in order to form dolomite, 

 that there should be, \st, sulphate of magnesia and carbonate 



* Coniptes Rendus de I'Acad. des Sciences de Paris, 6th March 1848, 

 Archives de la Hib. Univ. de Geneve, April 1848. 



