86 Professor Favre ott the Geology of the German Tyrol. 



authors think that they terminated during the tertiary epoch, 

 which may be the case. With regard to the epoch at which 

 they commenced, M. de Buch says, that the elevation of 

 pyroxenic porphyry is posterior to the secondary formations, 

 because it pierces the different beds of it. (^Annates de 

 Chimie et de Physique, 1823, xiii., 293). I think that it is 

 necessary to make a distinction here. The eruptions of these 

 melaphyres have perhaps continued a very considerable time, 

 and even though a portion of these rocks has reached the 

 surface after the deposition of the secondary formations, ac- 

 cording to the learned geologist of Berlin, it appears to me 

 certain that these eruptions have commenced at the period of 

 the muschelkalk. Indeed the observations which I have al- 

 ready referred to, and which I recapitulate here, demonstrate 

 that there were eruptions of melaphyre contemporary with 

 the muschelkalk, and anterior to the formation of dolomites. 



As a proof, I have indicated, \st, The superposition of the 

 dolomites on the pyroxenic formation, a fact clearly deter- 

 mined by the general examination of the country at Grodner- 

 Thal, and in the neigbourhood of St Cassian. This super- 

 position is seen at the ravine of Pufl, at Palat-Spitz, at Lang- 

 kogl, and in the sections given by M. de Buch {Annates de 

 Chimie et de Physique, 1823, xxiii.) 2d, The pyroxenic rocks, 

 as I have stated, have furnislied the materials of certain beds 

 of muschelkalk inferior to the dolomites. 3</, It may well be 

 that the pyroxenic rocks have been erupted before the forma- 

 tion of dolomites, since their stratified tufa alternates with 

 dolomitic beds, situate at the inferior portion of the great 

 masses of dolomite. 



This is a point which I wished to establish in a positive 

 manner, for that was necessary in order to understand the 

 origin of the dolomite. 



The formation of dolomite is an important question, and 

 one which has given rise to so much writing and discussion, 

 that it is difficult not to touch upon the notions which have 

 been advanced on the subject ; the more so, as certain 

 authors have treated of it in a manner so general and 

 vague, that they seem to have wished to include in their 

 system the greater part of former theories, as well as the 



