The spiral is the only decorative motive used in Christian Celtic art that can be proved to have been borrowed from the Pagan Celtic art of the preceding period. Although spiral ornament appears to be so complicated when it is completed, the geometrical process of setting it out is simplicity itself. All that it is necessary to do is to fill in the surface to be decorated with circles of any size, leaving about the same distance between each; then connect the circle with =S=- or =C=-shaped curves; and, lastly, fill in the circles with spirals working from the tangent points, where the =S= or =C= curves touch the circles, inwards to the centre. As the size of the circles is a matter of no importance, a surface of irregular shape may be covered with spiral ornament just as easily as one of symmetrical shape. In the flamboyant ornament of the Pagan Celtic period we have the same =S=- and =C=-shaped curves, but the circles were occupied either by a disc of enamel (as on the bronze shield from the Thames), or by raised conchoids (as on the gold necklet from Limavady, Co. Londonderry). In the spiral ornament of the Christian Celtic period closely coiled spirals like those of the bronze age were substituted for the discs of enamel or raised conchoids. The background of the spirals, however, retained several of the prominent features of the repoussé metalwork, the effect of the light shining on the raised trumpet-shaped expansions of the =S= and =C= curves being imitated in black and white or coloured by almond-like dots. In the later and less refined spiral ornament of the Christian period this background disappears altogether, and the spirals are made all the same size and placed close together. [Illustration: Tree key-pattern, Meigle, Perthshire] As the spiral was the earliest decorative motive in Christian Celtic art, so it was also the first to disappear, and its disappearance marks the decadence of the style. We have in a previous chapter traced the spiral motive from the Pagan metalwork through the enamelled disc ornaments of the bronze bowls of the Transition period to the illuminated MSS. of Christian times. Spiral ornament in its best form is to be found in the following MSS.:— The Lindisfarne Gospels A.D. 720. The Book of Kells 8th century. The Gospels of St. Chad ” ” The Book of Durrow ” ” The Book of Armagh A.D. 750-808. The Gospels of Willibrod A.D. 739. The Gospels of St. Gall 8th or 9th century. The Gospels of MacRegol A.D. 820. The Gospels of Stockholm A.D. 871. The Vespasian A. i. Psalter 8th century. [Illustration: Spiral ornament, with key-pattern border, from the Book of Kells] In metalwork spiral ornament is less common than in the MSS., there being good examples on the Ardagh Chalice, the Tara Brooch, the Hunterston Brooch, the Monymusk Reliquary, and the Athlone Crucifixion Plaque.