HAMILTON ON THE GEOLOGY OF TUSCANY. 279 



bourhood of Pomerance, are capped with thick beds of this same 

 calcareo -arenaceous deposit. Pomerance itself is built on a mass 

 of it, and it closely resembles that which I shall hereafter describe 

 in the Sienna district, and on which the towns of S. Quirico, 

 Pienza, and Sienna are also built ; although here it is perhaps 

 rather more calcareous. It also occurs to the S. E. of Leghorn, 

 where it forms the substratum of the plain and the beach. It is, 

 however, certainly a remarkable feature that it is seldom quite 

 horizontal, but appears to follow the slope of the hills, lapping 

 over them like a covering. It everywhere contains numerous 

 marine shells, chiefly Pecten and Ostrea ; the latter sometimes con- 

 stituting whole beds, while the material is in many places sand 

 and pebbles, with a hard calcareous matrix. Although this form- 

 ation is always found as a capping of the hills, and never occurs 

 in the valleys, it is not seen on the high hills of Monte Catini and 

 Monte Vaso, which were probably already elevated into islands 

 before it was deposited ; but it may, I think, be traced in a few 

 places circling round the eastern portion of these hills, opposite 

 Volterra. Of these the localities most worthy of observation 

 are where the new road leading to M. Catini and La Cava has been 

 cut through thick beds of conglomerated pebbles, sand, and cal- 

 careous bands containing large Ostrea and Pectens, and evidently 

 resting against the unconformable beds of the secondary formation, 

 and of the gabbro rosso. 



But the most extensively developed feature in this marine ter- 

 tiary formation is the blue marl which immediately underlies the 

 last-mentioned beds, and has in the great basin of Volterra a 

 thickness of nearly a thousand feet. It may, however, be a ques- 

 tion whether this blue marl does not belong to an older tertiary 

 period, — Eocene, perhaps, instead of Miocene, as the limestone 

 capping, instead of being almost horizontal, generally follows the 

 slopes and undulations of the hills, as if deposited after the valleys 

 had been scooped out. 



At Volterra fossil shells are of rare occurrence in the blue marl ; 

 but as we approach the borders of the formation towards the north, 

 they become more abundant, and in some cases, as near the junction 

 of the Sterza and the Era, may be said to constitute nearly one 

 half of the whole mass of the formation. They are, however, so 

 broken and fragile, that it was difficult to extract entire speci- 

 mens. Dentalium, Cardium, Venus, Cerithium, Pleurotoma, 

 Turritella, and large Ostrece, were most numerous. With them 

 were associated small crystals of selenite, which, abounding in 

 many portions of the blue marl, give it a glittering and sparkling 

 appearance. 



Below the borgo of Monte Catini is found a singular spotted 

 argillaceous rock, resting against the erupted mass of igneous 

 rock on which Monte Catini is built. Its appearance is that of a 

 trap rock ; but, on further search, it proved to be full of shells, 

 chiefly Cardium. It is of a greenish grey colour, and at first shows 



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