64; M. Brochx^t on the 



On the ancient Gypsum of the Alps. 



The same part of the Alps in which I have observed 

 these transition rocks, also contains gypsum, often in very 

 large masses. I have in the description of those rocks, 

 taken care to mention tliem ; but having at that time, not- 

 withstanding many years of observation, some doubts on the 

 true position of these gypsums, and on their relation to the 

 transition rocks, I abstained from pronouncing an opinion 

 on them, reserving the clearing up of their geological cha- 

 racters for another memoir, when I should have been able 

 to make some important verifications. 



The following are the principal points on which my 

 doubts were founded. The gypsum rocks are generally ex- 

 tremely crumbled, the natural consequence of the great fa- 

 cility with which this substance decomposes ; there neces- 

 sarily results a great dif&culty in observing their position re- 

 latively to the other surrounding rocks. 



Their situation in the Tarentaise, on the sides of moun- 

 tains, or on their first escarpments, often even at the bot- 

 tom of high valleys, generally in masses of little extent, 

 and always superficial, inclined me to presume that the 

 gypsum was a formation posterior to all the rocks of the 

 Tarentaise, that is, even to the transition series. 



This conjecture, moreover, appeared to me to agree with 

 the ideas, without doubt rather vague, that a number of 

 philosophers had entertained on gypsum in general, accord- 

 ing to which it would always be deposited in basins. 



I nevertheless abstained from decidedly adopting this con- 

 clusion; besides its not appearing to me sufficiently proved, 

 I had good reasons for remaining in doubt. In the first 

 place, g}psum had been met with in subterranean works of 

 the mine of Pesey : I presumed that it was covered by the 

 metalliferous rock of a later formation, but of this I was 

 not completely assured. Many persons held a contrary 

 opinion. 



Gypsum also existed in many other places in the Alps;: 

 mineralogists mentioned many as belonging to the transition 



