290 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [Feb. 6, 



to that posterior date, and there can be no doubt, that from the pe- 

 riod when the lower grounds were still much under brackish or fresh 

 water, and when travertine was formed in broad ancient depressions 

 then occupied by waters which are now reduced to the rivers Tiber 

 and Anio, there has been a more or less continuous formation of tra- 

 vertine. In order, however, to draw a clear distinction between the 

 former energy of nature in producing travertine under conditions 

 different from those which now prevail, let us appeal to Tivoli and its 

 environs. There we have fortunateh'- exact chronometers. There we 

 can compare the enormous ranges of this calcareous deposit which 

 were elaborated lono; before man could have trodden the soil, with 



o ... 



those small additions v,'hich have been made by the river Anio since 

 the foundations of the Etruscan and Roman buildings. 



Fig. 2. — Section at Tivoli. 



Volcanic tuff. Travertine. Volcanic tuff. Apennine limestone. 



All Tibur or Tivoli, with its temples, stands on what may be desig- 

 nated the ancient travertine, fine lofty scarps of which hang in un- 

 dulating and contorted layers with their splendid concretions on the 

 flanks of the cretaceous or hippuritic limestones of the Sabine Hills, 

 where not a rill of water now descends, and where consequently not 

 an inch of travertine is ever added. He who would convince him- 

 self that the great mass of travertine belongs to a remote anti- 

 quity when the configuration of the land was very diiferent from the 

 present, has indeed only to ascend by. the small old road from the 

 Villa Adriana to Tivoli. He will there see the truncated edges of 

 that finely laminated rock associated with water-worn pebble beds of 

 a former epoch, when these deposits must either have shelved away 

 from eminences which they now occupy, into waters which then 

 bathed the flanks of the Apennines, or when great barriers, since re- 

 moved, pent up lakes at higher levels, xlgain, if he should descend 

 from the walls of Tivoli to the ancient Via Tiburtina, he "\vill succes- 

 sively pass over, first some beds of travertine on the summit, next beds 

 of pebbles chiefly of apennine limestone, and thirdly a great mass of 

 travertine. The whole of these repose distinctly on volcanic tuff, 

 with much disseminated leucite, in which caverns have been exca- 

 vated, and the lower portions of which have been channeled out by 

 the Teverone or Anio (see fig. 2). 



The quarries out of which ancient Rome was in a great part built, 

 situated in the country below, and entirely separated from the Tivoli 

 hills, ofl'er another magnificent proof of the grandeur of the phseno- 

 menon wliich produced the old or ante-historic travertine. 



These quarries have been made in one of the faces of a vast 

 flattened dome occupying many square miles, the highest points 



