The vestibule of the cathedral is built of sedimentary limestone containing small amounts of silicates, iron oxides, and calcium sulfates. The stone is at present covered by a grey, cracked patina. Samples have been removed and subjected to: 1) qualitative analysis, 2) quantitative analysis of the sulfates at different depths, together with stratigraphic measurement of carbonates and other substances, 3) comparative analysis of sulfates, 4) testing for soluble or insoluble calcium salts. The results of the analyses are given and show the action of atmospheric pollution to be decisive. Tests involving treatment with acrylic monomers are under way; the protection methods are not defined. -- AATA