Medieval writers did not add much to what the Greeks and Romans had said in regard to the causes of vulcanism. In general they accepted the theories of Isidore and Pliny. Sicily, a typical volcanic region, was supposed to be cavernous and full of sulphur and bitumen strata which, when ignited and kept burning by the air, throw off smoke, vapor, and flames and, when a strong wind blows upon them, vomit forth masses of sand and rocks.[1031] Gervase of Tilbury elaborates confusedly on this theme,[1032] saying that there are many fires and earthquakes in Sicily because beneath that land there is a mighty abyss, the bottom of which is unknown to man. Near at hand are immense caverns and broad caves, wherein winds are conceived from the whirling of the waters, for mountains and waters create winds—mountains by offering an obstacle to the air. Though he does not say so specifically, we may conclude that Gervase believed that earthquakes and other volcanic phenomena were caused by these winds trying to escape from the interior of the earth. This was certainly the opinion of Michael Scot, who pointed out that masses of sulphur and other white-hot rocks (_petre calidissime_) are made to burn by the native heat of the earth’s interior and by the winds which enter the earth’s crust in remote regions (_in extremis partibus_) and force their way downward through passages, tubes, and caverns. These winds are volatilized and given explosive force by contact with the sulphur and hot rocks. When they burst forth again into the atmosphere they have all the attributes of fire and flame—sparks, ashes, cinders—and are supposed by many people to be genuine fire, though as a matter of fact they are by nature quite different because the waters ever present in the subterranean cavities fail to extinguish them. So intense is the heat produced by the sulphur and other combustible materials that the world would be entirely consumed by the winds that would blow over them if they were on the earth’s surface. Hence it is a great mercy of God that he has hidden them away in the depths of the ground and has thus made impossible the destruction of the world by this cause and that he has permitted men to dwell and cultivate their fields on the mountains beneath which such evil forces are buried.[1033] Alexander Neckam defined a volcano as a subterranean fire which, though bound to the earth with one foot, seeks to spring aloft with the other. He believed that volcanic rocks contain gases within them which when kindled produce eruptions.[1034]