Professor Favre on the Geology of the German Tyrol. 91 



It is not necessary to suppose that the beds which form the 

 great dolomitic masses of the TjtoI were at first deposited in 

 the state of limestone, and that they were then changed into 

 dolomite at a period more or less remote from the time of 

 their deposit ; that is to say, after these beds had attained 

 the enormous thickness which they now present. It is by 

 no means probable that these beds of cellular dolomite were 

 deposited in the state of dolomite, for the rock would be 

 compact ;* but we may conceive an intermediate between the 

 two modes of formation in order to explain the origin of the 

 Tyrolese dolomites, which are cellular throughout the whole 

 of their enormous mass, and admit that as fast as the lime- 

 stone was precipitated, in a form more or less pulverulent, it 

 was changed into dolomite ; and this kind of metamorphism 

 of the limestone, which took place after its formation, well 

 explains the cavernosity of the dolomites, and enables us to 

 understand their stratification. 



Saline substances may have been more abundant in ancient 

 seas than in the present ones, without organic life being 

 thereby destroyed ; this is proved by an observation of M. 

 de Verneuil who saw in the Crimea a species of cardium and 

 other shells living in lakes where the saline substances were 

 so abundant that they ft-equently crystallised in summer.f 

 This is the reason why we find fossils in the dolomites of the 

 Tyrol, although they are not very abundant, and the mode in 

 which dolomite is formed, explains why the shell of these fos- 

 sils is frequently dolomitic. + 



In these ancient seas, as in the present ones, shells and 

 corals lived at a small depth below the surface of the water ; 

 there they secreted lime, and, it is probable, that the trans- 

 formation into dolomite only took place when the precipitated 



* I may say, however, that compact dolomites are found in sedimentary for- 

 mations of different ages, and, consequently, we may sujipose that there is a cer- 

 tain class of these rocks which were at first deposited in the state of double car- 

 bonate of lime and magnesia. 



t Verneuil (Mem. de la Societc Gcolog. de France, iii., p. 9). 



X Collogno {Mem. dc In Soc. (Jeolog. de France, x., p. 310). 



