bearing on theoretical Speculations. 269 



the spot appeared to prove that the place of the great cascade 

 had been stationary, or nearly so, from the moment when the 

 river commenced its course through the valley of excavation 

 previously traced out for it by some cause far different from any 

 action of the river itself. The circumstances of the spot are the 

 following: The Anio above Tivoli flows gently onwards towards 

 the edge of the precipice, through a gorge of Apennine lime- 

 stone of the oolitic period. Near the entrance of Tivoli, a dyke 

 has been constructed across it, diverting a part of its waters 

 through an artificial tunnel on the left or southern side, and 

 thus conducting them so as to issue in several artificial casca- 

 telli out of the side of the hill below the main and only na- 

 tural cascade, that of the Grotta di Nettuno. With the arti- 

 ficial cascatelli we have nothing to do, further than to observe, 

 that one of them turns the machinery of an iron-foundry 

 now established within the half-ruined walls of the Villa of 

 Maecenas (Dom us antiqua heuquamdisparidominaris domino). 

 This branch, by its relations to the domain of that great pa- 

 tron, shows that no change has taken place since his time along 

 the line of its descent, from his palace to the bottom of the 

 valley beneath, excepting the deposition of travertin ; the waters 

 of the Anio while foaming in a state of precipitation have 

 always deposited travertin; and this travertin accumulating 

 on the bar of limestone over which it fell, as may especially 

 be seen at the great and only real cascade of the Grotta di 

 Nettuno, immediately became a defence against all further 

 erosive action of the river on the subjacent Apennine limestone. 

 This perpetually increasing shield of travertin would probably 

 go on accumulating more rapidly in its upper parts than the 

 agitated state of the bottom would allow below ; and hence 

 periodical breakings away of its unsupported overgrowings 

 would take place, as of the curling edges of drifted snow. But 

 this failure of support would not affect the inferior sheets of 

 travertin in immediate contact with the limestone : these once 

 formed, will have remained from the day of their formation, 

 arresting all further destruction of the stratified rock beneath. 

 From the base of the cascade to the plain of Rome is about 

 a mile ; and in this mile the river descends a valley narrow at 

 its base, and flanked on both sides by slopes of moderate in- 

 clination, most steep at the cascatelli on the left bank, and 

 on the right bank nearly opposite them. Now it is observed, 

 had this cascade been working gradually backwards through 

 the eternity of the fluvialist theory, it must have deposited 

 travertin all along the gorge it was forming at each succes- 

 sive station, which it occupied from time to time, just as it has 

 done at its actual station the Grotta di Nettuno; every part 



of 



