For three hundred years the Roman sword secured order and peace without Britain and within, and with peace and order came a wide and rapid prosperity. Commerce sprang up in ports amongst which London held the first rank; agriculture flourished till Britain became one of the corn-exporting countries of the world; the mineral resources of the province were explored in the tin mines of Cornwall, the lead mines of Somerset or Northumberland, and the iron mines of the Forest of Dean. But evils which sapped the strength of the whole Empire told at last on the province of Britain. Wealth and population alike declined under a crushing system of taxation, under restrictions which fettered industry, under a despotism which crushed out all local independence. And with decay within came danger from without. For centuries past the Roman frontier had held back the barbaric world beyond it, the Parthian of the Euphrates, the Numidian of the African desert, the German of the Danube or the Rhine. In Britain a wall drawn from Newcastle to Carlisle bridled the British tribes, the Picts as they were called, who had been sheltered from Roman conquest by the fastnesses of the Highlands. It was this mass of savage barbarism which broke upon the Empire as it sank into decay. In its western dominions the triumph of these assailants was complete. The Franks conquered and colonized Gaul. The West-Goths conquered and colonized Spain. The Vandals founded a kingdom in Africa. The Burgundians encamped in the border-land between Italy and the Rhone. The East-Goths ruled at last in Italy itself. [Sidenote: Conquests of Jute and Saxon]