The arches of the east and south sides of the crossing are of a different character from the above, the arches being fully moulded. The piers (Fig. 998) do not differ much from those of the older sides, and are all of an early form of design. The same character is kept up in the inserted piers of the north arch, although these must undoubtedly be later than the original piers. The mouldings seem all to have been imitated from early forms (such as we find in the choir arch of the nunnery).[39] The changes in the plans of the piers and mouldings, common in the central districts of the mainland, would be long in reaching this distant spot, and the old forms appear to have been, therefore, adhered to. Such continuations or imitations of early forms are very common in Scotland. It should be observed that the outer or central shaft of each pier has a fillet on its central line, which indicates (especially here) a somewhat advanced date (see Fig. 996). The bases are of a very late type, as also the caps, both of which differ from those of the earlier piers. From these facts we infer that the south and east sides of the crossing have been rebuilt at a late period, possibly at the time when the tower was erected. That part of the edifice is always justly regarded as being very late, and the rectangular windows and their tracery clearly support that view. If the above supposition is correct, then the insertion of the additional piers and arch under the north tower arch is explained. That addition would be made at the time the tower was built, in order to make sure that the support was sufficient to carry it. Similar extra strengthening was not necessary in the case of the west arch, as it already had sufficient piers and supports; and the additions to the piers of the north arch, which [Illustration: FIG. 998.--Iona Cathedral. South-East Pier of Crossing.] previously were small, simply made them of similar strength to those of the west arch. We therefore conclude that the original north and west arches and piers of the crossing are old, perhaps of the thirteenth century; while the east and south arches and piers have been rebuilt, and the additional piers and arch added to the north opening, probably when the tower was erected. Let us now turn to the choir. When we consider the forms of the tracery of the three windows of the presbytery (see Figs. 983 and 984, and the north window in Fig. 999), the thickness of the mullions, and the number and smallness of their mouldings, there cannot be much difficulty in assigning these windows to a late date. The forms and decorations of the arches of the sedilia (see Fig. 986) also clearly indicate a late period. These are exceedingly good of their kind, and it is a misfortune that the Celtic ornament with which they are covered is now so far decayed as to be scarcely legible; but there can be no question as to the lateness of their date.