All these things and more I have time to notice while I trudge along above the housetops of the city, those flat roofs named _astrici_ which are such pleasant lounges on the summer evenings when the blue bay is dotted over with white sails and the shadows deepen on the flanks of Vesuvius or the distant line of the Sorrento coast. At length the road approaching the ridge of hill whose point forms the headland of Posilipo drops swiftly, and I find myself in face of a short ascent leading to the mouth of the very ancient grotto by which, these two thousand years and more, those who fared from Naples unto Pozzuoli have saved themselves the trouble of the hill. It is absolutely necessary to pause and consider this hill, to which so much of the rare beauty of Naples is to be attributed. For the moment I set aside its legends and traditions, and turn my attention to the eminence itself. It is a cliff of yellow rock, whose consistency somewhat resembles sandstone, evidently worked without much difficulty, since it has been quarried out into vast cavities. The rock is tufa. It is a volcanic product, and forms the staple not only of the headland, but also of all the site of Naples and the rising ground behind it up to the base of the blue Apennines, which are seen continually towering in the distance beyond Vesuvius. Thus at Naples one may distinguish between the eternal hills and those which have no title to the name. Of the latter is Posilipo, formed as I said out of volcanic ash. That ash was ejected underneath the sea, and having been compacted into rock by the action of the water was upreared by some convulsion long since forgotten. It is an intrusion on the landscape--a very ancient one, certainly--but it has nothing to do with the great mountain-chain which hems in the Neapolitan Campagna and ends at last in the Sorrento peninsula and Capri. All the most fertile plain which lies within that barrier was once beneath the sea, which flowed up to the bases of the mountains. There is no doubt about it. The rock of Posilipo contains shells of fish now living in the bay. From Gaeta to Castellammare stretched one wide inlet of the sea. But underneath the water volcanic ash was being cast out, as it is still at certain spots within the bay; the heaps of ash and pumice stone grew into shoals and reefs, were uplifted into hills, the sea flowed back from its uptilted bed, and the coasts of Naples and of Baiæ assumed some outline roughly similar to that which they possess at present.