96 On the Constitution of the Territory of Rome, with 



ed slaggy pumice lava, whicli the Roman tufa only contains in 

 very small fragments. It is even the universal rule, that the 

 minuteness, and also the firmer cohesion of the ingredients, ge- 

 nerally go together, the more we approach it on the NW. to 

 the hills of Rome, where this mass seems to terminate. 



If we have hitherto required, for the explanation of the geo- 

 logical phenomena presented by the oldest formations of Rome, 

 and the volcanic cover which succeeds them, a distribution 

 of the sea and land different from the present ; yet, on the 

 other hand, in the newest strata of Rome, in the formations of 

 marl and river sand^ and in the thick beds of travertino, we 

 meet with evidence of a state of things, which, in local limita- 

 tion, comes very near to the present constitution of the country. 

 The volcanoes of the neighbourhood were extinguished then, as 

 well as now. When these strata were formed, the internal com- 

 motion of the earth's crust had already ceased, the sea had 

 nearly retired within its present bounds, and perhaps its last 

 ebbings had contributed to excavate the broad furrow of 

 the principal valley, and of its lateral ones. The great basin of 

 the Tiber, as well as the lesser valleys which separate the hills 

 of Rome from one another, were also covered by these fresh- 

 water formations ; the former must, therefore, have existed pre- 

 vious to the production of the latter. The condition of organ- 

 ized beings must have been the same as at present ; for the re- 

 mains, which once lived in them, agree completely with those 

 now existing on the spot ; yet the formation of the valley could 

 not be quite completed, as is indicated by the extension of 

 fresh-water formations into places which they could no longer 

 reach. The Tiber, in times previous to the historical epoch, 

 must have been elevated more than 130 feet above its pre- 

 sent level. The circumstances of its flow must have also been 

 different in ancient times. The modern Tiber forms neither 

 marl nor sand, for the level of ancient Rome neither covers, nor 

 does it shew, a stone which can be compared with die Traver- 

 tino. The shell remains, which exist in these formations, are 

 never those which thrive in its bed ; they have all been 

 inmates of stagnant, or very slow flowing waters. The river 

 water must have, therefore, formerly existed in a still state to 



