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M. Brongniart on the Fresh-isoater Formation of Rome. 173 



M.Omalius d'Halloy recognized these rocks at the entrance 

 of the Pontine marshes near Cisterna, at the foot of the vol- 

 canic hills of Velletri, in a low plain. The limestone is white, 

 compact, and solid, pierced by numerous tubular cavities, and 

 contains Limnece and globular Helices; he presumes that it is 

 in many points covered, as in Auvergne, by volcanic breccias. 

 It would appear that this limestone also occurs to the south 

 towards Calabria; for it is stated that the temples of Ptestum, 

 in the gulf of Salerno, are constructed with a concretionary 

 stone, which is very certainly a travertine. 



This formation appears at first as a trace at Monte Verde, 

 S. of Rome, is afterwards found well-characterized in Rome 

 itself, and then in considerable extent and thickness, at some 

 distance from this city, towards the E. in the direction of 

 Tivoli, and on the N. W, in the direction of Civita Vecchia : 

 it was while examining these different points that I became 

 acquainted with the various circumstances attending its posi- 

 tion. 



At Monte Verde it occurs only as a thin interrupted bed, 

 deprived of its essential characters ; it rests on a siliceous sand, 

 mixed with some augite, which covers an earthy and very ho- 

 mogeneous volcanic tufa. 



At Rome, M. Brocchi, with whom I had the great advan- 

 tage of visiting these places, pointed out to me the fresh-water 

 limestone at the eastern foot of the Aventin, on the banks of 

 the Tiber, at the spot named the Cavern of Cacus; it is com- 

 pact, contains some fresh-water shells, and rests on a red and 

 earthy volcanic breccia : it is not covered by any rock. 



The plain which extends from Rome to the mountains where 

 Tivoli is situated, is in a great measure covered by a thick de- 

 posit of travertine, commencing at Martellone on the route 

 from Rome to Tivoli, and being continued to the foot of the 

 mountains of the latter place. This plain, in which the quar- 

 ries of Ponte Lucano are situated, which furnish the travertine 

 employed in building, may be considered, as M. Omalius 

 d'Halloy has observed, as the bottom of a great lake, at pre- 

 sent traversed by the Teverone, bordered by a volcanic breccia, 

 and filled up and rendered nearly dry by calcareous deposits; 

 for it is not entirely dried, and we may, with M. Omalius 

 d'Halloy, consider the small lakes of Tartari, the Soltiitara, 

 &c. as the remains of this vast mass of water. 



Geologists who have examined this formation, and especially 

 MM. Von Buch and Omalius d'Halloy, have remarked; 

 1. That the lower and ancient travertine, the formation of which 

 does not now take place, was that used lor buiklings, as alone 

 prc.'jcnting sufiicienl coini)actncss and solidity; that at present 



formed 



