transition from the dry-built and rough stone structures to buildings cemented with mortar, and having the stones dressed. To these were added chancels in the ninth and tenth centuries, having radiating chancel arches, which are invariably semi-circular, and have inclined jambs. The church of St. Kevin at Glendalough presents a good example of a chancel added to a primitive single-chambered church. Ornament was gradually introduced, but the Irish characteristic of the stone roof, supported on an arch, was retained in small structures up to the twelfth century. As time progressed the original overlapping form of arch was superseded by the true radiating arch. In the case of the larger churches, however, the roof seems generally to have been constructed with wooden rafters and shingles.[19] The radiating arch appears to have been introduced about the same time as the chancel, and was by degrees applied both to doors and windows, but the sloping form of the jambs continued in use till the introduction of the Norman style.[20] The religious enthusiasm which pervaded the Irish monasteries was very great, and displayed itself in the numerous offshoots and missions which they sent out, not only to the neighbouring countries of England and Scotland, but also to Gaul, Switzerland, Germany, and Italy. Rude and primitive as were their dwellings, the Celtic monks excelled in several departments of art and literature. Their chronicles of events are almost our only guide to the history of the country in those early times, and the writings and illuminations of their religious books are marvels of beautiful caligraphy and design. The forms and features of their drawings and illuminations are of a marked and special character, and are found prominently displayed not only in their MSS., but on all objects of Celtic production, such as gold and silver ornaments and shrines, and the sculptured crosses and architectural enrichments of a somewhat later date.[21]