In attempting to set limits to the earlier periods of Sumerian history, it is still impossible to do more than form a rough and approximate estimate of their duration. For in dealing with the chronology of the remoter ages, we are, to a great extent, groping in the dark. The material that has been employed for settling the order of the early kings, and for determining their periods, falls naturally into three main classes. The most important of our sources of information consists of the contemporary inscriptions of the early kings themselves, which have been recovered upon the sites of the ancient cities in Babylonia.[1] The inscriptions frequently give genealogies of the rulers whose achievements they record, and they thus enable us to ascertain the sequence of the kings and the relative dates at which they reigned. This class of evidence also makes it possible to fix certain points of contact between the separate lines of rulers who maintained an independent authority within the borders of their city-states. A second class of material, which is of even greater importance for settling the chronology of the later Sumerian epoch, comprises the chronological documents drawn up by early scribes, who incorporated in the form of lists and tables the history of their own time and that of their predecessors. The system of dating documents which was in vogue was not a very convenient one from the point of view of those who used it, but it has furnished us with an invaluable summary of the principal events which took place for long periods at a time. The early dwellers in Babylonia did not reckon dates by the years of the reigning king, as did the later Babylonians, but they cited each year by the event of greatest importance which took place in it. Such events consisted in the main of the building of temples, the performance of religious ceremonies, and the conquest of neighbouring cities and states. Thus the dates upon private and official documents often furnish us with historical information of considerable importance.