92 Professor Favre on the Geology of the German Tyrol. 



lime reached a certain depth, that is to say, a cei'tain pres- 

 sure. 



According to this mode of viewing the great phenomenon 

 of the formation of the dolomites of the Tyrol, we perceive 

 why these rocks approach, to a certain point, the eruptions 

 of pyroxenic porphyries, without, however, being completely 

 connected with them ; in fact, the sea, where these volcanic 

 eruptions took place, and in which the circumstances fitted 

 for forming dolomite were united, extended to a great dis- 

 tance. Yet, the dolomitic sediments must have been made 

 with greater activity in the neigliboui'hood of the centres of 

 eruption. We may thus explain why the secondary chain, 

 placed on the northern declivity of the central chain, is like- 

 wise dolomitic, although there was no pyroxenic eruption in 

 that place ; for, at this remote epoch the central chain was not 

 yet elevated, and the formations which at a later period were 

 to form the secondary chains of the Tyrol were deposited in 

 the same sea. 



From the decomposition of the sulphate of magnesia by the 

 carbonate of lime, the sulphate of lime must have resulted, 

 and this salt has been precipitated in a warm state ; for a 

 more elevated temperature than ordinary was a consequence 

 of the submarine eruptions of which we have spoken. Now, 

 Mr Forbes has remai'ked, that the sulphate of lime so pre- 

 cipitated was anhydrous.* Here then, we have the explana- 

 tion of the formation of anhydrite, a substance which, accord- 

 ing to the observations of M. de Charpentier, has furnished 

 the gypsum of the Alps. We know that this latter rock is 

 frequently met with in the Tyrol, near Vigo, at the Seisser- 

 Alp (De Buch), and in the valleys of St Vigile and Unter- 

 meyer ; besides, the gypsum being a soluble rock, it is pro- 

 bable that it is now found in fewer places than it was formed. 

 However this may be, the presence of this rock indicates 

 that sulphuric acid has not been a stranger to the formation 

 of dolomite. 



We have distinguished two species of dolomite ; their geo- 



* Letter to M. Morlot (Comptcs Rendus, already quoted) 



