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Show 160 OLDER PLIOCENE PERIOD. [Ch. XII. of the places where I h ave seen 1't , a border formation near the juncti·O n of the terti· ary anc1 secon dary rocks. In some cases ' as near t h e town of S.te ntla , we see sand and calcareo. us gravel rest·m g 1· mme d1' a t e1 y on the Apennine li.m estone, Without the m· terventw· n of any blue marl · AlternatiOns are there seen of b ed s conta·m ·m g fl uv I''a tile shells ' with others filled exclusively wi· t1 1 marm· e spec·i es,. and I observed oyste.r s attached to many of the pebbles of limestone. 'l~his locahty appears . to have been a point where a river, flowmg from the Apenmnes, 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 Subapen.nine beds, which is seen for eleven miles continuously 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 torrents. to have gained u~on the bed of a sea where blue marl had prevwusly been deposited. The upper arenaceous group above described sometimes passes into a calcareous sandstone, as at San Vignone. It contains lapidified shells more frequently than the marl, owing probably to the more free percolation of mineral waters, which often dissolve and carry away the original component elements of fossil bodies and substitute others in their place. In some cases the shells imbedded in this group are silicified, as at San Vitale, near Parma, from whence I saw two species, one freshwater and the other marine (Limnea palustris, and Cytherea con· centrica, Lamk. ), both recent and perfectly converted into flint. On the other hand, the shells of Monte ·Mario, near Home, which are probably referrible to the same formation, are changed into calcateous spar, the form being preserved notwithstandinO' the crystallization of the carbonate of lime. 0 • Mode of formation of the Subapennine beds.-The tertiary strata above described have resulted from the waste of the secondary rocks which now form the Apennines, and which Ch. XII.] S VDAPENNINE STRATA, II OW FORMED. lGl had become dry land before the older Pliocene beds were deposited. In the territory of Placentia we have an opportunity of observing the kind of sediment which the rivers are now bringing down from the Apennines. The tertiary marl of that district being too calcareous to be used for bricks or pottery, a substitute is obtained, by conveying· into tanks the turbid waters of the rivers Braganza, Parma, Taro and Enza. In the course of a year a deposit of brown clay, much re. sembling some of the Subapennine marl, is procured, several feet in thickness, divided into thin laminre of different shades of colour. In regard to the sand and gravel, we see yellow sand thrown down by the Tiber near Rome, and by the Arno, at Florence. The northern part of the Apennines consists of a grey micaceous sandstone with an argillaceous base, alternating with .shale, from the degradation of which brown clay and sand would result. If a river flow through such strata, and some one of its tributaries drains the ordinary limestone of the Apennines, the clay will become marly by the intermixture of calcareous matter. 'rhe sand is frequently yellow from being stained by oxide of iron, but this colour is by no means constant. The similarity in composition of the tertiary strata in the basins of the Po, Arno, and Tiber, is merely such as might be expected to arise from theit· having been all derived from the disintegration of the same continuous chain of secondary rocks. But it does not follow that the latter rocks were all upheaved and exposed to degradation at the same time. The correspondence of the tertiary groups consists in their being all alike composed of marl, clay, and sand; but we might say the same of the London and Hampshire basins, although the English and Italian groups, thus compared, belong nearly to the two opposite extremes of the tertiary series. The similarity in mineral character of the lacustt·ine deposit of the Upper Val d'Arno, and the marine Subapennine hills of northem Italy, ought, we think, to serve as a caution Vor .. lli. l'ri |