1 7i M. Brongniart o?i the fresh-'water Formatio7i of Rome. 



formed by the lakes of the Solfatara and Teverone not being 

 sufficiently dense. 2. That fresh-water shells are very rare in 

 it; for not only does M. Omalius d'Halloy mention that he 

 had not seen any, but he considers that their absence is to be 

 accounted for by sulphuretted hydrogen gas dissolved in these 

 waters, and which prevents any fresh-water Mollusca from 

 living in them. The very different state of the lakes Tartari 

 and of the Solfatara accord very well with this theory. 



The first contains limpid water, its banks are covered with 

 calcareous incrustations possessing a crystalline structure; 

 these are seen to be ancient, and it appeared to me that the 

 present waters do not possess the property of depositing any : 

 the bottom of this lake is covered with vegetables of various 

 kinds, with Batrachian reptiles, insects, &c. 



The lake of the Solfatara, situated nearer the foot of the 

 hills, is altogether different : it is formed of a considerable mass 

 of whitish watei', perpetually disengaging bubbles of air, and 

 a marked odour of sulphuretted hydrogen gas ; the water de- 

 posits a thick bed of white compact limestone, a true traver- 

 tine, on the vegetables which grow on its banks, and in the 

 channel at which it escapes. When the bottom of this lake is 

 agitated, a considerable disengagement of gas is produced ; in 

 the line through which the gas passes, the water acquires a 

 limpidity, owing without doubt to the dissolution of the lime- 

 stone by the carbonic acid disengaged. There is no living 

 animal either on the shores of this lake or in its waters ; at 

 least we have never observed any. 



The differences presented by these two lakes seem to be in 

 relation to the differences often seen in the lower and upper 

 parts of fresh-water formations; that of the Solfatara shows 

 the formation of the limestone in activity. The waters are too 

 highly charged with carbonic acid and earthy matters to allow 

 animals to exist in it ; consequently the first calcareous de- 

 posits should not contain any of their remains, at least in the 

 vicinity of the spring : but in proportion as the mass of mine- 

 ral matters diminishes, or in proportion as the channels are 

 obstructed by these deposits, the waters become less charged 

 with gas and lime, the deposit less rapid and less crystalline, 

 the animals begin to appear, and this deposit charged with 

 their remains would be above the first. This is probably the 

 present state of the Lago de Tartari ; and this relative position 

 of the fresh-water formation without shells, and that with shells, 

 is precisely that observable in all those places where these two 

 rocks have been observed : thus, in the Paris basin, the sili- 

 ceous limestone without shells occurs beneath the shelly mill- 

 stone, &c. 



These 



