Each phase of the regional school of Gothic can be found in Angers. In the tower of St. Aubin, a vestige of an ancient abbey named after a VI-century bishop of the city, is a ribbed cupola, typical of the incubating period of the school.[185] It is more a cupola than a Gothic vault. The stones are laid horizontally in concentric rings, and the ribs are more decorative than structural, being in part embedded in the infilling. The abbot who erected it ruled from 1127 to 1154. The First Period of the Gothic of Anjou is represented at Angers by a masterpiece of elemental force--the nave of the cathedral. Three huge so-called domical vaults, truly Gothic in construction, span the sixty-foot unaisled nave of St. Maurice. The stones are laid parallel with the groin line of each triangular panel between the intersecting ribs. Those diagonals are needlessly heavy, for the builders were still experimenting. The crown of each vault section is ten feet higher than the framing arches--wall arch and transverse arch. The exceptional span of Angers' three massive vaults is due to a reconstruction of the nave undertaken in the XII century, at which time the side aisles of the Romanesque cathedral were eliminated and the entire width of the edifice thrown into an unobstructed hall. Mr. John Bilson, the eminent English archæologist, belittles the influence of the cupola church in Angevin Gothic, the shape of whose vaults he attributes to a structural cause. He thinks that the extreme width of Angers' nave made it essential to raise the keystone above the crowns of transverse and wall arches in order to prevent its settling. The diagonals were made more obtuse than the equilateral framing arches lest they might tower too high. Given the form adopted for the arches, the _bombé_ vault web resulted inevitably. Arch curves determine the forms of a vault. None the less is M. Berthelé's account of the Plantagenet school sound both ethnically and æsthetically. The Angevin architect chose to persist in the use of _bombé_ vaults over narrow spans where there was no structural need to raise the keystone.