The most noticeable feature of the interior is the treatment of the triforium of the eastern portion of the church. This is groined with a succession of transverse barrel-vaults, the effect of which is to give an immense addition of strength to the main walls. They spring from the capitals of a succession of detached shafts which are placed across the triforium, so that the perspective of its interior is singularly picturesque. It was not very long after the erection of the church that the western portion of the triforium was altered, a quadripartite vault being substituted for the barrel vaulting, and wherever this has been done, the thrust has been too great for the principal groining shafts, which have bulged considerably, and are now held in place by iron ties. In the apse, the bays being of necessity much wider on one side than on the other, the ridge of the barrel-vault rises rapidly towards the external wall: and the triforium is lighted by a succession of immense simple circular windows. The internal elevation of one bay of this cathedral is nearly identical with the original design of that of Paris, though simple and (I fancy) rather earlier in date; but from the shortness of the church and the absence of transepts (in which one point it reminds me of the fine church of S. Leu d’Esserent) it has both inside and outside the effect rather of a choir only than of a complete cathedral. There are various additions to the church of later date, which add much to its picturesque character, especially a chapel on the south side, the chapels round the apse, and the sacristies on the north side. The stone roof above the groining of one of these is remarkable. The arrangement of coloured tiles on the roof is one of the best I have seen. The pattern is rather complicated, and is formed with dark tiles (green and black used indiscriminately) on a ground of yellowish tiles.
The church from the apse to the western towers consists of but three bays of sexpartite vaulting, each bay covering two bays of the main arcades. Between the towers is one bay of quadripartite vaulting.
Walking from Mantes across the river to the suburb of Limay, a fine view is obtained of the town and cathedral, which shows here the whole picturesque exaggeration of height as compared with length which distinguishes it. Limay church boasts of nothing save a tower and spire on the south side, of late Romanesque character throughout. The surface of the spire is covered with scalloping, and has spire-lights and fine pinnacles at its base. Some attached shafts against the face of the belfry stage, which seem to serve no purpose, are curious as being probably the type from which some similarly placed shafts in the steeples of the cathedral were derived. Here too, as in the cathedral, a most effective form of label is used, the section of which is a square cut out into diamonds like unpierced dogteeth. We see the same thing in England, and among other examples there is a good one at Lanercost. Its effect is singularly bold and piquant.