Observations on the Geognuntic Character of' Italy. 83 



Monte Mario, of which Brocchi (tab. ii. ji. 1, 4.) has given a very instrue- 

 tive section. It is principally the Tufa lerroso which is here predominant. 



On the left bank of the Tiber, where the granular and lithoidal tuffs occur 

 together, the latter is superimposed on the former. There are examples of 

 this on the Esquiline, where the subten-anean passages of the convent of St 

 Francesco de Paola have afforded a very instructive section ; also of the Ca- 

 pitoline Hills under the Tarpeian Rock. Yet Brocchi expressly mentions 

 that this relative position is by no means of regular occurrence. The reverse 

 is seen before the gates of Rome, in the rocks round the tomb of Naso, as 

 well as elsewhere. 



Tliere is no undoubted superposition of the volcanic tufa on the marine 

 formation on this side the Tiber. The only point where, in this part of the 

 city, a foreign deposite is found under it, is the singular discovery of Brocchi 

 at the Tarpeian Rock. We there see, in the large subterranean passages of 

 the Hospital della Consolazione, lowermost, a thick stratum of brown mica- 

 ceous clay, in which a compact limestone of the same colour forms some even 

 beds one or two feet thick. To this succeeds a mass of sand and clay six 

 feet thick, and over it ten feet of granular tufa, above which, to the sununit 

 of the rock, is the lithoidal tufa. Brocchi is much inclined to refer the fun- 

 damental bed to a marine formation ; on the grounds advanced by him, it is 

 certainly very probable. Other local appearances besides this, would indi- 

 cate that the proper fundamental stratum of the Seven Hills of Rome is a sub- 

 terranean prolongation of the marine formation, from the right to the left 

 bank of the Tiber. It is the sounding for wells in this part of the city which 

 lead to this general result from a comparison of their depths, although now 

 we can hardly draw any conclusions regarding the strata through which they 

 have been pierced. From Brocchi's observations, which we have collected 

 for that purpose, it follows that the most of these wells, some of which are 

 even placed on the summit of the hill, reach the water almost universally at 

 a depth which comes near to the level of ancient Rome, from ten to twenty 

 teet under the level of the modern city. The volcanic tufa itself can hardly 

 retain the water, on account of its porous structure ; and it, therefore, must 

 meet at this depth with a stratum of clay or marl, which prevents its sinking 

 deeper ; similar to the strata on the Vatican and Janiculus, whose abundant 

 springs are mentioned by all who describe this place, appear at the surface. 

 The position of the volcanic tufa, with respect to the fresh.water formations, 

 which we will now describe, is also remarkable. • 



• In passing, we may be permitted to touch upon two minerals foreign 

 to the Roman soil, but which, often alternating with the lithoidal tufa, 

 hold an important place in the architecture of the ancients. These are 

 the Gabine and Alban stone. We will comprise both best under the 

 name Peperin, ( Feperino, Pepperstotie. J The Gabine differs from the 

 Alban only in containing less augile and mica, and consists of a collection 

 of angular pieces of grey and reddish brown lava, traversed by calc-spar, 

 and at times conlaiiiing small rolled limestone masses. Both it and the 

 Alban Peperino are well distinguished from the Roman tufaceous rock. In 

 the Peperino (says Von Buch) every thing is almost fresh, entire and un- 

 broken, splendent ; in the tufa dull, and broken down ; the former more re- 



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