Physical Structure of the Site of Rome. 17 



make bricks and pottery, as it was for the latter purpose in an- 

 cient times, as appears from a passage in Juvenal *. 



The Janiculum is a long ridge of about a mile and a quarter 

 in extent, and nearly parallel with the river, its highest point 

 being 294 feet above the Tiber. In composition it is similar to 

 the Vatican and Monte Mario. Granular tufa has been found 

 in several places, and deposits with fresh water shells have also 

 been found on its sides next the Tiber -f. 



Although these three hills are chiefly composed of marine de- 

 posits, volcanic products have been found upon all of them at 

 the base and on the summit. A granular tufa is found at the 

 base of the Vatican, and in that part of the Janiculum which is 

 next the Vatican. It contains fragments of pumice. Granular 

 tufa is found on the very summit of the Janiculum, containing 

 large pieces of pumice, and lying distinctly upon the marine de- 

 posits. The same thing occurs on the summit of Monte Mario. 



The marine deposits of which these hills are composed, are 

 found to constitute a line of hills, extending northwards as far 

 as a brook called Acqua traversa, and westwards to Ostia and 

 Civita Vecchia; and as they are similar to those composing the 

 great range of ^the Subapennine hills, it is extremely probable 

 that they form the subsoil of the whole country around Rome, 

 upon which the volcanic products have been deposited, — a pro- 

 bability rendered still greater by their being actually found at 

 the base of the Capitoline Hill. In sinking wells at Rome, it is 

 always necessary to pass through the tufa ; and, as that is done 

 at various depths, it is probable that the marine strata formed a 

 hilly uneven surface, before the volcanic matter was deposited 

 upon them. 



Fresh-Water Deposits. 



Having repeatedly alluded to the deposits which lie upon the 

 sides of the hills over the volcanic matter, in so many places, 

 and form so important a feature in the physical structure of this 

 region, I shall give a brief description of their nature. 



• " Et Vaticano fragiles de Monte patellas." Sat v. 



•j- Some of the sand of the Janiculum is of a shining sparkling nature, and 

 hence that hill got the name of Mons aureus, and by corruption Montorius. — 

 Adam's Antiq. 562. 



VOL. XIV. NO. XXVII.— JANUARY 1833. B 



