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174 slight difficulty in establishing a division in these sedimentary rocks, according to the order in which they have been deposited; and the difficulty has not been diminished by comparing them with rocks of determined epoch, which might be contemporary with ours. For us, who have not had the advantage of seeing in their natural position any of the various sedimentary formations (except those of the kingdom of Naples), the difficulty has been still greater, nor can we flatter ourselves that we have surmounted it. Meanwhile, without entering into discussions which would be foreign to the principal aim of our work, but keeping to what appears to us to agree with our observations, we prefer to divide our neptunian rocks into three series, that is, three distinct formations. In the first series we shall include all those calcareous rocks which are particularly characterized by Nummulites, Nerineæ, and those organic forms of which we have no example in the fauna of the present epoch, and which paleontologists, uncertain of their real nature, have denominated rudimentary. (Rudisti.) The greater part of our Apennine mountains being formed of this calcareous rock, we shall retain for it exclusively the name of (Calcarea Apennina) Apennine limestone. The rocks of the second series, very varied in their mineralogical composition, agree in being distinctly stratified, in being almost entirely destitute of animal fossils, and in sometimes containing a great quantity of vegetable fossils of the Fucoid order. Although it is not easy to find Fuciform impressions everywhere in rocks of this formation, nothing better indicates their character than the presence of these plants; for the absence or extreme rarity of animal fossils is a negative characteristic which in exceptional cases may be affirmed of every species of rock. The last series comprehends marls, limestone, and sandstone, abounding in marine fossils, the greater number of which belong to species which now exist in our seas. To these rocks we shall confine the name sub-Apennine, although others elsewhere have designated some of the rocks of the preceding series with this denomination. They undoubtedly belong to the supercretaceous period; and if the igneous phenomena of Epomeo in the Phlegrean region, as we have shown elsewhere, have