The stratification, which may be concentric or excentric, appears to be due to a difference in density of the various layers.
The rocks are largely volcanic, the stratification being complex.
If, in addition to this horizontal stratification, the atmosphere varies similarly in vertical planes, then the object would be magnified both horizontally and vertically.
In the case of thin, flat organs such as leaves, the whole organ may be spread out in the plane of stratification, leaving its impress on the overlying and underlying layers.
No satisfactory collection has been made of the Celtic inscriptions of Cisalpine Gaul, though many are scattered about in different museums. For our present purpose it is important to note that the archaeological stratification in deposits like those of Bologna shows that the Gallic period supervened upon the Etruscan.