From the eastern Mediterranean the knowledge spread during the second millennium along the ordinary trade routes which had long been in use. The mineral ores of Spain were exploited in pre-Mycenean times and probably contributed in no small measure to the industrial development of southern Europe. From tribe to tribe, along the Atlantic coasts the traffic in minerals reached the British Isles, where the rich ores were discovered which, in their turn, supplied the markets of the north, the west and the south. Even Ireland was not left untouched by Aegean influence, which reached it, according to G. Coffey[90], by way of the Danube and the Elbe, and thence by way of Scandinavia, though this is a matter on which there is much difference of opinion. Ireland's richness in gold during the Bronze Age made her "a kind of El Dorado of the western world," and the discovery of a gold torc found by Schliemann in the royal treasury in the second city of Troy raises the question as to whether the model of the torc was imported into Ireland from the south[90], or whether (which J. Dechelette[91] regards as less probable) there was already an exportation of Irish gold to the eastern Mediterranean in pre-Mycenean times. Of recent years great strides have been made towards the establishment of a definite chronology linking the historic with the prehistoric periods in the Aegean, in Egypt and in Babylonia, and as the estimates of various authorities differ sometimes by a thousand years or so, the subjoined table will be of use to indicate the chronological schemes most commonly followed; the dates are in all cases merely approximate. It has often been pointed out that there is no reason why iron should not have been the earliest metal to be used by man. Its ores are more abundant and more easily reduced than any others, and are worked by peoples in a low grade of culture at the present day[92]. Iron may have been known in Egypt almost as early as bronze, for a piece in the British Museum is attributed to the fourth dynasty, and some beads of manufactured iron were found in a pre-dynastic grave at El Gerzeh[93]. But these and other less well authenticated occurrences of iron are rare, and the metal was not common in Egypt before the middle of the second millennium. By the end of the second millennium the knowledge had spread throughout the eastern Mediterranean[94], and towards 900 at latest iron was in common use in Italy and Central Europe. CHRONOLOGICAL TABLE. Egypt[95] Babylonia[96] Aegean[97] Greece[98] Bronze Age in Europe[99]