178 M. Brongniart on the position 



It is so brittle and so broken by atmospheric phenomena, 

 that its debris form long red slopes, which appear to cover 

 the escarpments, and partly conceal the following rocks. 



3dly. The formation that is immediately beneath it, oc- 

 curs in beds having a stratification parallel to that of the 

 jasper, and which apparently forms the very thick base of 

 this elevated part of the Apennines ; this formation I say, 

 is principally composed of a fine and compact limestone (D), 

 of a smoke grey colour, and conchoidal fracture, traversed 

 by numerous veins of calcareous spar, and of a compact 

 yellowish grey limestone without spathose veins. I here 

 notice the principal and most abundant varieties ; but 

 others occur that I have not considered worth describing. 

 Notwithstanding our research, we have not been able to dis- 

 cover any organic remains in these rocks. 



Beneath this limestone, and alternating with it, occur the 

 hard compact sandstone, and the schistose sandstone, which 

 is here often very micaceous (F). 



• From the midst of this rock rises the hydrogen gas of 

 Pietramala, and this position, it may, en passant, be re- 

 marked, is absolutely the same as that of the hydrogen gas 

 of Barigazzo, on the road from Modena to Pistoia. 



The col that is passed on the north of Pietramala is en- 

 tirely composed of this same fine and compact limestone, 

 and on the descent the same micaceous sandstone is met 

 with in thick beds inclined towards the north. 



Here then occur nearly the same rocks as at Rochetta, 

 in the same order of superposition. The varieties of very 

 little importance presented by these rocks, are those that 

 ought to be expected throughout the earth between bodies 



dary class. "In both these cases, veins of trap pass through strata of 

 secondary limestone, and where the vein is in contact with the hmestone, 

 it changes its character and becomes a serpentine, while it contains in 

 those parts the minerals usually found in that rock, namely asbestos and 

 steatite. The limestone, at the planes of contact, also contains steatite ; 

 and thus a species of regular gradation becomes established between the 

 trap and limestone. The gradation from the serpentine to the trap, 

 within the vein, is perfect and insensible." (Trans.) 



