MODE XII. TUFA. 515 



calcareous crust, two or three lines in thickness. 

 These incrustations are known by the name of 

 Comfits of Tivoli*. Before this passage was 

 opened, the overflowings, to which the lake is 

 subject, were often so considerable that the water 

 spread over the neighbouring grounds, and 

 formed on their surface a stony crust. The 

 water of the lake so charged with calcareous 

 earth, uniting with those of the Anio, in the 

 floods which their union must produce, have 

 themselves contributed to the formation of tra- 

 vertine. I do not think that the Anio alone 

 would have been capable of forming the quan- 

 tity which is found of that rock. 



" Independent of the immense quarries work- 

 ed by the ancients, there are besides others of 

 such vast extent, that they may supply the de- 

 mands for many ages. 



* " Dr. Vegni had established there a manufacture of bas-reliefs, 

 analogous to that which he possesses in Tuscany, near the Baths of 

 St. Philip. The ingenious method by which he forces the water to 

 form the bas-reliefs in a short time, which by the exactness of the 

 design and the hardness of the stone, are not inferior to the originals, 

 is sufficiently known by the relations of most modern travellers in 

 Italy. 



" I shall only add, that the colour, grain, and hardness of the 

 stone formed by the waters of the Solfatara of Tivoli, as well as the 

 neighbourhood of Rome, which furnishes so many beautiful models, 

 and skilful artists, give this manufacture a decided advantage over 

 that of Tuscany." 



2 I, 2 



