174 M. Brongniart on the positiori 



Let us now pass on to the third example of the superpositiori 

 of the serpentine jasper formation on the calcareo-marly 

 formation. Tiiis seems the most complete : all the rocks 

 appear clearly above each other. It is no longer at the foot 

 of the Apennines, but placed on the crest of that chain of 

 mountains. It occurs in a place of easy access, I would say 

 more, in a place frequented by all travellers, and conse- 

 quently by all the geologists who pass from Florence to 

 Bologna. 



As I visited this place some time after those preceding, I 

 then considered that the superposition that I had remarked 

 at Rochetta, and regarded as a fact not much known, ought 

 to be described in geological works ; and it was with aston- 

 ishment, and even yet mistrusting the exactitude of my. 

 researches, that I presume that it had escaped them, or ra- 

 ther that, seduced by false appearances, and led by a kind 

 of prejudice on the antiquity of the diallage rocks and ser- 

 pentines, they have not been willing to refer to this forma- 

 tion the rocks of a serpentine base that form the summit of 

 the mountains. 



It is to the south of Pietramala, on the side of the great 

 road, close to a place celebrated from the hydrogen gas that 

 Is perpetually disengaged from the earth, and which is 

 almost always lighted, that an example of this superposition 

 occurs, as clear and more evident, if possible, than that of 

 Rochetta : for here the rocks are nearly horizontal, as is 

 seen pi. 5. fig. 4. 



I should omit mentioning the rocks which are met with 

 before the point is reached where the superposition of the 

 serpentine formations begins to be clearly seen ; not because 



to the summit. Yet in other parts of the same mountains, the jasper 

 rests on gabbro ;" which would be equally possible, if these two rocks 

 are, as it appears, of contemporaneous formation. But not being able to 

 see this superposition in the visit made to this mountain by Count Bardi, 

 M. Nesti, and myself, the former appeared to adopt my opinion on the 

 superposition of the serpentine to the jasper. 



He elsewhere remarks that pieces of compact limestone are found in 

 these two rocks ; but he regards them as produced by crystallization and 

 of contemporaneous formation with the jasper and serpentine. 



