84 On the Constitution of the Territory of Rome, with 



III. Agency of Fresh Water. 



The plain of Rome, or the portion of the city intersected by the Tiber, 

 and which is bounded by the marine formation to the north, and by the vol- 

 canic hills to the south, belongs to the products of stagnant fresh water to a 

 considerable height on the sides of the valley, and pretty far up the lateral 

 valleys which separate the seven hills from one another. These waters over- 

 flowed this region at a period when, after the retreat of the sea, and after 

 the cessation of volcanic irruptions, the present river dug its bed. The pre- 

 vailing substances are loose unconnected masses of clay, sand, and boulders, 

 which were left behind, covering a considerable surface, after the retiring of 

 the water ; yet its presence has, in many points, formed a beautiful stone 

 of a firmer consistence, peculiarly characteristic of this country ; of which 

 are formed all the ornamental parts of the masterpieces of ancient architec- 

 ture, and the constant production of which can even now be observed : it has 

 obtained the name of Lapis Tiburtirius, or Travertine. The argillaceous strata 

 of the valley, whose general distribution has been shewn by the laborious 

 researches of Brocchi, by means of numerous borings, are particularly im- 

 portant on this account, because they cannot be penetrated by the waters 

 which issue forth from the adjacent hills, and are consequently the means of 

 supplying the numerous wells in the lower part of the city. The clay is 

 constantly mixed with a small portion of carbonate of lime, and, as it always 

 effervesces with acids, is a true argillaceous marl, (Mania argillosa). Its 

 colour is yellowish grey. It is constantly interspersed with small silver- 

 white scales of mica, and contains here and there small pieces of augite and 

 small quartz fragments ; it greedily absorbs water ; is plastic, and hardens 

 in the fire. Treated with acids, it gives an insoluble residium, which, when 

 not mixed with quartz, is chiefly composed of ferruginous alumina. This 

 clay is useful in pottery work. Brocchi has shewn, that in the most ancient 

 periods use was made of it. 



sembles a porphyry, the latter sandstone, and similarly aggregated strata. 

 The wacke-like basis seldom changes its ash-grey colour. The tufaceous stone 

 at Rome is never so light. Its fracture is fine earthy, but uneven, fine grain- 

 ed, soft ; the tufa, again, is almost friable, which is never the case with the 

 lithoidal tufa. Immense numbers of small micaceous scales are found in it,, 

 partly as individual black plates, partly as elongated masses, from an inch to 

 the size of a cannon ball. These masses are a collection of micaceous plates, 

 mixed with augite crystals, and often containing magnetic ironstone. Lustre 

 and colour is always wanting in similar plates in the tufa. On the contrary, 

 leucite and augite are more rare in the Peperino than in the tufa, but more 

 frequently small angular white pieces, which are granular limestone. 



The Gabine and Alban stone form immense beds, so that they appear one 

 mass ; e. g. round the Gabine Lake and at Marino. They often include 

 lumps of basaltic lava. 



The Alban and Gabine stone is found much more frequently in the an- 

 cient buildings than the indigenous tufa. Yet the only certain remnant of 

 the ancient kings is of the latter rock. It appears that later the former 

 rock was preferred on account of its greater fineness, or more beautiful colour. 

 The outer upper walls of the Tabularium are built of Gabine stone. 



