The growing of wool and flax for cloth-making became an industry of great importance just after the accession of Henry VII. With the advent of peace, it became possible to manufacture into cloth the fibres that before had been sent for that purpose to Flanders. The utilization of the coal and the iron ore years afterward brought about an economic revolution that was intensified by the invention of the steam-engine and the power-loom. These quickly brought the country into the foremost rank as a manufacturing centre. Moreover, they also demanded the foreign markets that have made the country a maritime power as well--for an insular country must also have the ships with which to carry its merchandise to its markets. The development of the manufactures, therefore, is inseparably connected with that of the mineral and metal industries. From very early times the metal deposits of the country have been a source of power. Copper and tin were used by the aboriginal Britons long before Caesar's reconnaissance of the islands, and it is not unlikely that the Bronze Period was the natural development that resulted from the discovery of these metals. Coal occurs in various fields that extend from the River Clyde to the River Severn. The annual output of these mines at the close of the century was about two hundred and twenty-five million tons. In the past century the inroads upon the visible supply were so great that the output in the near future will be considerably lessened. Not far from one-sixth of the output is sold to consumers in Russia and the Mediterranean countries, but a growing sentiment to forbid any sale of coal to foreign buyers is taking shape. [Illustration: BRITISH ISLES] Iron ores are fairly abundant, but the hematite required for the best Bessemer steel is limited to the region about Manchester and Birmingham. The shortage of this ore has become so apparent within recent years that Great Britain has become a heavy purchaser of ores in foreign markets. The coal in the Clyde basin is employed mainly in the manufacture of railway iron, steamship material, and rolling stock. The manufacture of Bessemer steel is gradually moving to the vicinity of South Wales, at the ports of which foreign pig-iron can be most cheaply landed. In west-central England the several coal-fields form a single centre of manufacture, where are located some of the largest woollen and cotton mills in Europe. It also includes the plants for the manufacture of machinery, cutlery, and pottery.