That the geographical change from a promontory into a real island did not take place during the last two thousand years, is proved by the description which Diodorus Siculus, a little before the Christian era, gives of St. Michael’s Mount. “The inhabitants of the promontory of Belerium,” he says (lib. v. c. 22), “were hospitable, and, on account of their intercourse with strangers, eminently civilized in their habits. These are the people who work the tin, which they melt into the form of astragali, and then carry it to an island in front of Britain, called _Ictis_. This island is left dry at low tide, and they then transport the tin in carts from the shore. Here the traders buy it from the natives, and carry it to Gaul, over which it travels on horseback in about thirty days to the mouths of the Rhone.” That the Island of Ictis, described by Diodorus, is St. Michael’s Mount, seems, to say the least, very probable, and was at last admitted even by the late Sir G. C. Lewis. In fact, the description which Diodorus gives answers so completely to what St. Michael’s Mount is at the present day, that few would deny that if the Mount ever was a “Hoar rock in the wood,” it must have been so before the time of which Diodorus speaks, that is, at least before the last two thousand years. The nine apparent reasons why St. Michael’s Mount cannot be the Ictis of Diodorus, and their refutation, may be seen in Mr. Pengelly’s paper “On the Insulation of St. Michael’s Mount,” p. 6, seq. Mr. Pengelly proceeded to show that the geological change which converted the promontory into an island may be due to two causes. First, it may have taken place in consequence of the encroachment of the sea. This would demand a belief that at least 20,000 years ago Cornwall was inhabited by men who spoke Cornish. Secondly, this change may have taken place by a general subsidence of the land, and this is the opinion adopted by Mr. Pengelly. No exact date was assigned to this subsidence, but Mr. Pengelly finished by expressing his decided opinion that, subsequent to a period when Cornwall was inhabited by a race speaking a Celtic language, St. Michael’s Mount was “a hoar rock in the wood,” and has since become insulated by powerful geological changes. In a more recent paper read at the Royal Institution (April 5, 1867), Mr. Pengelly has somewhat modified his opinion. Taking for granted that at some time or other St. Michael’s Mount was a peninsula and not yet an island, he calculates that it must have taken 16,800 years before the coast line could have receded from the Mount to the present cliffs. He arrived at this result by taking the retrocession of the cliffs at ten feet in a century, the distance between the Mount and the mainland being at present 1,680 feet.