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



great extent assumed its present outline and constituted terra firma 

 and natural barriers. 



But to return to Tivoli and there compare the present operations, 

 in the formation of travertine, with those of former days. 



We there see, that all the calcareous matter which the river or its 

 Cascatelle have added to these ancient deposits, since the Temple of 

 Vesta and the Villa of Maecenas were built upon them, is so mere an 

 incrustation, that if it were necessary to estimate the age of the ante- 

 historical or earlier travertine by such a scale, we should have to recede 

 millions of years to account for its dimensions. When we reflect, 

 however, upon what must have been the condition of things when 

 the whole of this coast region was the scene of powerful volcanic 

 activity, there can be no difficulty in imagining how vast quantities of 

 travertine may then have been spread out in a comparatively short 

 time ; whilst the limited and slow supply of the modern travertine is 

 perfectly intelligible, now that volcanic action is either entirely dor- 

 mant, or only exhibits here and there feeble signs of its subterranean 

 existence. 



In reference to the enormous dimensions of the old travertines of 

 Tivoli, Sir Charles Lyell has expressed his belief, that they may 

 have been formed in a deep lake, or in lakes similar perchance to one 

 of those which have been let off in the historic period along the 

 upper course of the Anio. But I cannot believe that the lake in 

 which the old travertine of Tibur was formed, could have been in 

 existence during the occupation of Italy by the human race, as my 

 friend leads us to infer by his mention of the discovery of the impres- 

 sion of a cart-wheel in an upper portion of this rock. This circum- 

 stance can, it appears to me, be much better accounted for by what 

 has recently occurred at Tivoli, and without connecting the ancient 

 travertine with the works of man. 



The river Anio has, as is well known, escaped from the flank of the 

 Apennines for ages, both by a principal channel between the great 

 plateau of travertine on which Tibur or Tivoli has been built, and the 

 adjacent ridge of secondary (cretaceous) limestone, and also by 

 numerous underground currents which find their way through the 

 cavities and recesses of the travertine to issue in the Cascatelle. In 

 fact, the hard impervious cretaceous limestone on the right bank of 

 the stream has been for ever throwing off the waters, and forcing 

 them to undermine the softer old travertine on the left bank, and as 

 the necessary result, a city built on a porous substratum so assailed 

 at high floods, has been subjected to great periodic calamities. 



It was to obviate the recurrence of these misfortunes, that the 

 Cavalier Bernini, in the 1 7th century, opened out a new and straight 

 channel through the eastern portion of the travertine, which carried 

 the chief body of water into the chasms so much admired under the 

 Temple of Vesta, whence it cascaded through the grottoes of Nep- 

 tune and the Syren down to the lower falls. At length even this 

 remedy proved inefficacious, probably owing to the channels of escape 

 getting choked up by increasing layers of calcareous matter, and 



