If there were men to observe, the legend would follow the subsidence; and Phoenician or other voyagers would naturally bear it back to the Eastern Mediterranean, to Plato or the sources from which Plato derived it. In any such case the submergence would most likely be exaggerated and made a great catastrophe, but there were special reasons why the exaggeration should be enormous in this particular story. It is the office of a myth or legend to explain. We see that in Plato’s time the Atlantic Ocean was believed, in part at least, to be no longer navigable, and with some modifications this idea persisted far down into the Middle Ages, involving at least a conviction of abnormal obstacles hardly to be overcome. The account of Critias is: “Since that time the sea in those quarters has become unnavigable; vessels cannot pass there because of the sands which extend over the site of the buried isle.” This item differs from the other features of the narration put into his mouth by Plato, in that it related to a present and continuing condition and in a way challenged investigation--which would have to be at a distant and ill-known region but was not really impracticable. It must be evident that Plato would not have written thus unless he relied on the established general repute of that part of the ocean for difficulty of navigation. REPORTS OF OBSTRUCTION TO NAVIGATION IN EARLY TIMES We get further light on this matter of obstruction from the Periplus of Scylax of Caryanda, the greater part of which must have been written before the time of Alexander the Great. Probably we may put down the passage as approximately of Plato’s own period. He begins on the European coast at the Strait of Gibraltar, makes the circuit of the Mediterranean, and ends at Cerne, an island of the African Atlantic coast, “which island, it is stated, is twelve days’ coasting beyond the Pillars of Hercules, where the parts are no longer navigable because of shoals, of mud, and of seaweed.”[31] “The seaweed has the width of a palm and is sharp towards the points, so as to prick.”[32] Similarly, when Himilco, parting from Hanno, sailed northward on the Atlantic about 500 B. C., he found weeds, shallows, calms, and dangers, according to the poet Avienus, who professes to repeat his account long afterward and is quoted by Nansen, with doubts inclining to acceptance. It reads: No breeze drives the ship forward, so dead is the sluggish wind of this idle sea. He [Himilco] also adds that there is much seaweed among the waves, and that it often holds the ship back like bushes. Nevertheless, he says that the sea has no great depth, and that the surface of the earth is barely covered by a little water. The monsters of the sea move continually hither and thither, and the wild beasts swim among the sluggish and slowly creeping ships.[33] Avienus also has the following: