M. Brongniart o?i the Fresh-water Formation of Rome. 175 



These considerations have appeared to us of sufficient im- 

 portance to arrest our attention lor a short time, as they con- 

 tribute to complete the history of a formation first recognized 

 in the environs of Paris. 



Tlie travertine or fresh-water limestone of the plains of 

 Tivoli is not, however, entirely without the remains of shells. 

 I have observed them at Villa Adriana, at the foot of the hill, 

 in a limestone bed, the position of which, relatively to the other 

 rocks, is very clearly shown. 



The fundamental rock of the Tivoli hills is a fine compact 

 limestone, containing interrupted beds or nodules of chert, 

 and which appeared to me to bear the greatest resemblance 

 to the Jura limestone * ; the fresh-water limestone sometimes 

 rests immediately on this older rock, "at others it occurs on a 

 volcanic brecciola which rests on this compact limestone. The 

 place last mentioned shows this superposition in the most evi- 

 dent manner. Proceeding from the surface downwards, we 

 see: 1. a compact travertine, with tubular sinuosities and a 

 few shells ; 2. a mixture of friable travertine, and the debris 

 of volcanic brecciola ; 3. a thick bed of this brecciola. 



The facts observed by M. Brocchi, which I have also veri- 

 fied with him, and those which 1 have myself noticed, esta- 

 blish the relative position of the rocks analogous to those of 

 Paris, as well at Rome as in its environs ; the following order 

 of succession proceeding from the lowest to the highest. 



1. A compact limestone, analogous to the Jura limestone*, 

 or even, perhaps, to chalk. The fossils alone, when found, 

 and they are very rare, can remove these doubts. 



2. Calcaire grassier, composed at its base of blue, shelly, 

 and argillaceous marl ; and at its upper part, of reddish sandy 

 limestone, and sometimes even of mai'ine sandstone, as is 

 clearly seen at Rome, at the foot of Monte della Grita. 



3. Volcanic brecciola in ail its modifications, covering the 

 latter rock, as is well seen at Mount Marius. 



4. The fresh-water formation. It would therefore be here 

 in a different position from that which I have observed in the 

 Cantal, in the department of the Puy de Dome, and in that 

 of the Allier. These may be referred to the middle or gyji- 

 seous fresh-water formation, and those of the Roman states to 

 the upper fresh-water formation, above the second marine 

 formation ; and this agrees still moi'e perfectly with the posi- 

 tion M. Prcvost has assigned to the calcaire grassier of the 

 Apennines. 



I have already mentioned that there was a considerable 



• Equivalent to the Oolite formation of the English series.— Tran*. 



quantity 



