TRANSITION FROM CELTIC TO NORMAN ARCHITECTURE. Having in the preceding chapters dealt with the churches in Scotland which may be regarded as of Celtic descent, we now turn to those in which the Roman or Norman influence begins to make itself apparent. A few structures exist in Scotland in which both the Celtic and Norman influences can be traced, and which are interesting as examples of the transition from the one style to the other. We have noted in the foregoing descriptions the gradual introduction in the later Celtic churches of features characteristic of Norman work, such as round arched openings and upright jambs, not to mention the pointed and more advanced details of later examples. But the gradual approach of Norman architecture, and its encroachment on the Celtic forms, is more particularly noticeable in some of the early towers in the East of Scotland. In that part of the country of which the Frith of Tay forms the centre, and which was then the central region of Pictavia, there exists a series of early ecclesiastical towers, eight in number. Six of these towers are situated south of the Tay--three of them being on the south side of the Ochil Hills, and three on the north side. Those on the south side are the tower of St. Regulus at St. Andrews and the towers at Markinch and Dunblane; while the towers on the north side of the Ochils are at Abernethy, Muthill, and Dunning. The towers on the north side of the Tay are those at Brechin and Restennet. These places are all known to have been the sites of Culdee establishments, which were ultimately absorbed into the Roman system. The towers of Abernethy and Brechin are round towers, similar in design to the round towers of Ireland.[136] These two towers contain many features which indicate their Irish derivation, while one of them--that at Abernethy--shows the impress of the Norman influence in its details. The other towers above mentioned (which are described under their respective heads) are all square in plan; but that of Restennet shows considerable affinity with Celtic modes of construction. The Church of St. Regulus probably marks the next step in the Norman advance, while the remaining towers, although simple in details, are quite distinctly Norman in their features. The Round Tower of Brechin is the one which approaches most nearly to the Celtic or Irish forms. Its masonry is built with large stones, but they are not laid in regular courses. The doorway is some feet above the ground, and is ornamented with details quite Irish in character. This tower will be more fully described under the head of Brechin Cathedral. ABERNETHY ROUND TOWER, PERTHSHIRE.