Page:A Treatise on Geology, volume 1.djvu/276

 260 may be reduced to the above general type; the lower beds being more argillaceous.

The Italian tertiaries constitute a triple series, but the lower and upper terms appear only at particular points.

The relation of these tertiary to the subjacent cretaceous groups has again been the subject of an elaborate investigation by Murchison. Taking the limestone of La Spezia and Carrara (of Lower Jurassic age) to be the oldest of the Italian secondaries, we have above the ammonitico rosso and the cretaceous series, this latter being well exhibited on the flanks of the Venetian Alps and about Nice. In these localities they are covered conformably by tertiary accumulations, characterized by nummulites, and devoid of types of secondary fossils. In such sections it is only by the sequence and combinations of the forms of life that the separation of tertiary and secondary strata can be safely attempted, nor can a hard line be drawn where nature has employed the softening pencil.