A third class of material for settling the chronology has been found in the external evidence afforded by the early historical and votive inscriptions to which reference has already been made, and by tablets of accounts, deeds of sale, and numerous documents of a commercial and agricultural character. From a study of their form and material, the general style of the writing, and the nature of the characters employed, a rough estimate may sometimes be made as to the time at which a particular record was inscribed, or the length of a period covered by documents of different reigns. Further, in the course of the excavations undertaken at any site, careful note may be made of the relative depths of the strata in which inscriptions have been found. Thus, if texts of certain kings occur in a mound at a greater depth than those of other rulers, and it appears from an examination of the earth that the mound has not been disturbed by subsequent building operations or by natural causes, it may be inferred that the deeper the stratum in which a text is found the earlier must be the date to be assigned to it. But this class of evidence, whether obtained from palaeographical study or from systematic excavation, is sometimes uncertain and liable to more than one interpretation. In such cases it may only be safely employed when it agrees with other and independent considerations, and where additional support is not forthcoming, it is wiser to regard conclusions based upon it as provisional. The three classes of evidence that have been referred to in the preceding paragraphs enable us to settle the relative order of many of the early rulers of Babylonia, but they do not supply us with any definite date by means of which the chronology of these earlier ages may be brought into relation with that of the later periods of Babylonian history. In order to secure such a point of connection, reliance has in the past been placed upon a notice of one of the early rulers of Babylonia, which occurs in an inscription of the last king of the Neo-Babylonian empire. On a clay cylinder of Nabonidus, which is preserved in the British Museum, it is stated that 3200 years elapsed between the burial of Narâm-Sin's foundation-memorial in the temple of the Sun-god at Sippar, and the finding of the memorial by Nabonidus himself when digging in the temple's foundations.[4] Now Narâm-Sin was an early king of Akkad, and, according to later tradition, was the son of the still more famous Sargon I. On the strength of the figure given by Nabonidus, the approximate date of 3750 B.C. has been assigned to Narâm-Sin, and that of 3800 B.C. to his father Sargon; and mainly on the basis of these early dates the beginning of Sumerian history has been set back as far as 5000 and even 6000 B.C.[5]