These stairs ascend above the roofs of the side aisles to the upper part of the buttress piers, from which open passages, with breast-walls on either side, lead above the buttress piers to the angles at the base of the dome. There were two flights of steps leading to the platform of the dome: one of these on the south-east, which Salzenberg shows dotted in Plate viii., is still quite preserved, though injured at the upper end; remains too can still be traced of the north-west stair. A door now built up, on the north side of the south-east stair, and remains of vaulting in the north-west stair, seem to show that other passages must have existed. The roof of the cylindrical vaulting at the west end of the nave is reached by means of stairs in the small round towers, which flank it on the outside.[259] These turrets can also be reached from the roof of the narthex. Another passage runs along under the narthex roof at the west (Salz., Plate ix.), which has an opening close to the upper surface of the vaulting, and from thence any part above the nave can be reached. Probably this was formerly used for the lighting of the church. To reach the cornice at the foot of the dome there was an opening in the wall under one of the dome windows. _Decorative Work._--All the constructional forms were shown boldly on the outside without any adornment; the west front of the narthex next to the atrium was alone covered with slabs of Proconnesian marble, some of which are still preserved, but the upper wall surfaces were perfectly plain. In the interior the whole of the walls are plated with rare variegated marbles, and the vaults are covered with glass mosaic. Two chief masses of colour in the nave are separated horizontally by a cornice, and another cornice forms the springing for the vaulting. There are also cornices at the foot of the dome, and around the walls of the aisles. All these are of carved white marble in simple profiles. The lower range of arch spandrils between the piers of the nave is formed of slabs of white marble completely covered with carving: the upper spandrils above the gynaeceum arches have sectile work of coloured marbles. The carving is sharply cut, but conforms very closely to the general surfaces; according to the old descriptions it was gilt, and remains of colour still extant show some of the leaf-ornament coloured with a dark red.[260]