The famous pavement of the Duomo--a thing unique of its kind--might well have paved the first terrace of Dante's Mountain of Purgation. The tradition that this work was originally designed by Duccio (from which it would follow that Dante himself may have seen its first beginnings) is now almost entirely rejected. Documentary evidence proves that it was not begun until the year 1369, shortly after the resumption of work upon the Duomo Vecchio. The greater part of it was laid down after Giovanni da Spoleto[77] in 1396 had begun publicly to expound the _Divina Commedia_ in the Studio of Siena, and we can readily imagine that the men under whose superintendence it was done had in their minds those superb _terzine_ in which the divine poet describes the figured scenes over which his feet passed to meet that _creatura bella_, the Angel of Humility, whose face was like the morning star.[78] With one solitary exception--the rout of the Assyrians after the death of Holofernes--the subjects shown here are not the same as those on Dante's _duro pavimento_. Instead of the examples of the punishment of pride, we have here a series of scenes which can hardly be said to be dominated by one idea, but which in the main (a few scenes standing apart, unconnected with the general scheme), through symbol, type and prophecy, lead up to the Sacrifice of Isaac before the High Altar, as mystically representing the Atonement of Calvary, renewed daily in the Sacrifice of the Mass. In the earliest of these _commessi_ and _graffiti_, white and black marbles alone are used; later, coloured marbles are employed as well, both in shading and in the decorative portions. Executed at various dates, for the most part in the fifteenth and sixteenth centuries, they have been frequently altered and restored, while in some instances modern copies have been substituted for the originals. Save in the season of the feast of the Assumption, the central portions are kept covered.