Ch. XII.] SUBAPENNINE STRATA, HOW FORMED. 
161 
had 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 laminae of different shades 
of colour. 
In regard to the sand and gravel, we see yellow sand 
thrown down by the Tiber near Home, 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 
