As time progressed, these detached lancets were brought together into groups, the wall space was gradually thinned off, so as to form mullions between them, and frequently all are enclosed within a single arch. This naturally led to the panel or shield in the arch being perforated with a circle, trefoil, or other aperture, and toward the close of the period there were indications of the introduction of the earlier form of tracery. We have fine examples of the first arrangement at Dunblane, and of the embryo tracery at Glasgow. As in the twelfth century a large extension was given to monastic churches and parish churches, so it is to the thirteenth century that we are indebted for the commencement and partial erection of most of our cathedrals. The style of this period is fully represented in our cathedrals by the splendid crypt and choir of Glasgow, the beautiful nave of Dunblane, the fine, but now dilapidated, transept of Dryburgh, and the aisleless choir of Brechin. In Elgin Cathedral we have a noble example of the later design of the period, and the choir and western portal of Kirkwall form a worthy completion to that fine old Norman edifice. Dornoch once possessed the small and elegant cathedral of the North, erected by St. Gilbert, which has now, however, been almost completely rebuilt. The conventual establishments of the thirteenth century, although chiefly of older foundation, still contain a large proportion of the work of this period. Arbroath Abbey comprises in its ruins some good transitional and early pointed work. Holyrood Abbey, besides a good deal of the transition style, had one of the finest west doorways of the period, and still retains a very striking north-west tower. The first pointed architecture is also well represented at the abbeys of Dundrennan, Pluscardine, and Inchmahome, besides smaller specimens in other monasteries. Kilwinning Abbey, now reduced to a portion of the south transept, shows some beautiful work. The Maison Dieu at Brechin is also very pure first pointed, and at Culross, Restennet, Inchcolm, and Coldingham are many interesting fragments of this date. The thirteenth century parish churches are, for the most part, of small dimensions and simple design. They generally consist of a single oblong chamber, without choir or aisles, and are unprovided with towers. Of these simple buildings a considerable number are to be found in the Western Highlands and Islands. They are usually of the plainest description, the windows being simple lancets, with wide internal splay, though they have occasionally a double window with a mullion, and a single plain doorway. A large number of these chapels will be described, but, from the absence of any distinctive features connected with them, it will be found difficult to assign most of them to any definite period.