Observations un the Geognostk Character of Italy. 85 



111 several points of the plain, the clay is conjoined with collections of dif- 

 ferent kinds of sand. It is mostly a yellow calcareous sand, more or less in- 

 termixed with argillaceous marl, and at times including limestone fragments, 

 as, e. g. was seen by Brocchi in a pit at St. Guiseppe a Capo, No. 11. It is 

 partly, also, a siliceous sand, which is usually limited to the base of the hills, 

 and which, in the plain itself, is only exposed by one pit on the Campo Vac- 

 cino, by the side of the Temple of Peace, towards St Francesca Romano. It 

 has also been found on the slope of the Palatine, towards the Colosseum, ac- 

 cording to the chart of Brocchi ; and it has been met with on the edge of the 

 Cselius, in some pits which were made to find out the ancient Cloaca of the 

 Amphitheatre. The colour of this sand is yellow, with numerous silver-white 

 scales of mica, and pieces of augite. With the aid of the glass, there is visible 

 between the transparent grains of quartz, small white prisms, probably of 

 felspar. It is always mixed with clay, but destitute of any lime. Hence it 

 does not effervesce with acids, and melts before the blowpipe into a black slag. 

 The derivation of this clay and sand from fresh water is principally evinced, 

 according to Brocchi's observations, by our finding in it porous and tubular 

 calc-tuff, containing remains of lacustrine snail-shells. In the sand in the 

 Campo Vaccino, we find the Helix palustris and planata, Lin., both of which 

 only thrive in fresh and quiet waters. In the calcareous sand, on the sides 

 of the Janiculus, Brocchi mentions the existence of Cydostmna obiusum, Drap. 

 probably the Helix piscinalis of Gmelin. 



Strata of the same substance are also found in more elevated situations, 

 far above the level of the plain of Rome, which are evidently owing to the 

 same original cause. Brocchi, e. g. found an argillaceous marl of a yellow 

 colour, which belongs to this series, on the Capitoline Hills, in the cellars of 

 the Palace of the Conservators, lying on a volcanic tufa. It is here divided 

 into three beds, the lowest of which, indurated and full of augite crystals, 

 also contains many portions of an orange-coloured pumice lava. The others, 

 again, are whiter, and without volcanic fragments. They commonly contain, 

 vegetable remains, and bits of the shells of the Tellina cornea and Helix teniacu- 

 laia, or Ct/clostoma impurum, Drap., and their delicate opercula. These remains 

 are more scanty in the two upper beds than in the lower ; but the former, 

 ugain, are richer in concretions of a muddy yellow limestone. In a still more 

 striking manner is the same appearance presented at the Esquiline, in the 

 subterranean passages of St Pietro in Vincoli, where, 140 feet above the 

 Tiber, and above the lithoidal tufa, is a yellow clay, full of calcareous con- 

 cretions, and rectilineal streaks of a very friable granular tufa, which agrees 

 in all its characters with the fresh-water clay of the j)lain. There is also 

 seen on the slope of the Aventine, under the bastion of Paul III., opposite 

 the Porta di Testaccio, a bed of a yellow-grey sandy marl, in which are 

 many of the helices of the Campo Vaccino, covered by a considerable deposile 

 of porous calc-tuff. 



The Travertino, undoubtedly the most important of all the fresh-water 

 rlcposiles of this region, has been fully and learnedly described by Leo- 

 liold Von Buch, as it occurs here. It is, for the most part, a chemical de- 

 positc of carbonate of lime, which the ancient waters, held dissolved in an ex- 

 •■es.<! of carbonic acid, and deposited here, as is often the case at the foot of 



