Physical Structure of' the Site of Rome. 7 



by the evaporation of that water and by pressure. It does not 

 appear that in the eruption of the year 79, by which Hercula- 

 neum and Pompeii were destroyed, any tava flowed from Vesu- 

 vius. At Herculaneum, the substance which fills the interior 

 of the houses must have been introduced in a state of mud, but 

 streams of lava have flowed over the site of the city in modern 

 times, at different periods. Masses of hard lava and beds of 

 ashes are accumulated to a depth of nowhere less than seventy, 

 and in many places of 112 feet. Besides these stony bodies, 

 there are others of a less aggregated texture, pumice, scoriae, 

 lapilli, and ashes. There are varieties of the tufa, which it is 

 also necessary to distinguish, viz. stony tufa and granular titfa. 

 The stony titfa is compact, of a reddish-brown colour, with specks 

 of an orange tint, and is of sufficient hardness to be used as a 

 building stone, and it was so employed to a great extent by the 

 ancient Romans. The granular tiifa is light, friable, and com- 

 posed of largish grains weakly cohering, with fragments of vol- 

 canic minerals and rolled pebbles of compact lava. It is an aggre- 

 gate of lapilli. There is, moreover, a variety of lava of such fre- 

 quent occurrence, as to require to be described. It is known by 

 the name of Peperino ; the whole substance is fresh, undecom- 

 posed, and bright to the eye, whereas, in tufa, the greater part 

 is dull, and appears withered. The peperino resembles a por- 

 phyry, the tufa a sandstone. 



The general form of Latium is undulating and hilly. Rome 

 stands in a spacious valley, flanked on both sides by hills, with 

 the Tiber flowing through it. On the right bank are Monte 

 Mario, the Vatican, and the long ridge of the Janiculum ; on 

 the left bank the Pincian, Quirinal, Viminal, and Esquiline, 

 which can scarcely be called separate hills ; then the Ccclian and 

 Aventine ; and in the plain surrounded by these, rise the insu- 

 lated Palatine and Capitoline Hills. The breadth of the valley 

 from the summit of the Esquiline to the summit of the Janicu- 

 lum is about two miles, its length from the Pincian to the 

 Aventine nearly the same. The breadth of the Tiber as it 

 flows through Rome is from 180 to 190 feet, and its average 

 depth twenty feet. 



Of the hills of the right bank, Monte Mario is considerably 

 the highest, being 468 feet above the level of the sea, or 446 



