Volume 4

Principles of geology : being an inquiry how far the former changes of the earth's surface are referable to causes now in operation. / By Charles Lyell ...

  • Lyell, Charles, Sir, 1797-1875.
Date:
1834
Catalogue details

Licence: Public Domain Mark

Credit: Principles of geology : being an inquiry how far the former changes of the earth's surface are referable to causes now in operation. / By Charles Lyell ... Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

    15/420 (page 7)
    there superimposed, as at Radicofani, in Tuscany, where a hill composed of marl, with some few shells interspersed, is capped by basalt. Several of the volcanic tuffs in the same place are so interstratified with the marls as to show that the eruptions took place in the sea during the older Pliocene period. At Acquapendente, Viterbo, and other places, hills of the same formation are capped with trachytic lava, and with tuffs which appear evidently to have been sub- aqueous. Yellow Sand. — The other member of the Suba- pennine group, the yellow sand and conglomerate, constitutes, in most of the places where I have seen it, a border formation near the junction of the tertiary and secondary rocks. In some cases, as near the town of Sienna, we see sand and calcareous gravel resting immediately on the Apennine limestone, with- out the intervention of any blue marl. Alternations are there seen of beds containing fluviatile shells, with others filled exclusively with marine species; and I observed oysters attached to many of the pebbles of limestone. This locality appears to have been a point where a river, flowing from the Apennines, entered the sea in which the tertiary strata were formed. Between Florence and Poggibonsi, in Tuscany, there is a great range of conglomerate of the Suba- pennine beds, which is seen for eleven miles continu- ously from Casciano to the south of Barberino. The pebbles are chiefly of whitish limestone with some sandstone. On receding from the older Apennine rocks, the conglomerate passes into yellow sand and sandstone, with shells, the whole overlying blue marl. In such cases we may suppose the deltas of rivers and b -i