Ch. XII.] SUBAPENNINE STRATA, HOW FORMED. 161 



liad become dry land before the older Pliocene beds were 

 deposited. In the territory of Placentia we have an oppor- 

 tunity 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 laminse 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. The 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 their 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 corre- 

 spondence 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 lacustrine de- 

 posit of the Upper Val d'Arno, and the marine Subapennine 

 hills of northern Italy, ought, we think, to serve as a caution 

 VOL. III. M 



