X.] EARTHQUAKES. 197 ravine by insensible degrees1. Again, the formation of alluvium about the mouths of rivers, which we have noticed as being brought prominently before the minds of the Greeks from an early period by their observation of the Delta in Egypt, led to further speculations; and first Aristotle, and afterwards Polybius, affirmed that owing to this cause the Palus Maeotis was rapidly filling up, so that vessels of less draught than those previously in use were required to navi- gate it*. Polybius adds that, on a smaller scale, the same thing was taking place in the Black Sea. Finally, Posi- donius remarked on the inrush of the tide into the estuaries and the lower courses of rivers, a phenomenon which he had noticed at the mouth of the Thames during his visit to Britain8. The frequent occurrence of earthquake shocks, to which throughout its history Greece has been much ex- f T * ,. , Earthquakes posed, naturally attracted the attention of the and Volcanic Greeks, and caused them to speculate on the Action* changes which might have been produced on the face of the earth by such convulsions. As early as the time of Aeschylus we find that the idea prevailed that Sicily had been separated from Italy by such an agency, and that the name of Rhegium, or "The Rent," was derived from this circumstance4. Herodotus expresses a similar opinion with regard to the disruption of Olympus and Ossa, and the formation of the vale of Tempe5. Among the earlier philosophers we meet with two theories that were put forward to account for these movements. The first of these, which is attributed to Anaximenes, referred them to fractures of the crust of the earth, which 1 Polyb., 4. 70. 7; iroiet 5e /cai rb vapdvav fyupto Kal 8v of W) 'Pijyto? ic«fXi}