Churches of one region are contrasted with those of another. The material employed is considered, since the stone of a province causes richness or poverty of sculpture: thus, Brittany's granite and Auvergne's lava mean an undeveloped sculpture compared with the fine white limestone districts of the Oise, or in Normandy and Poitou. When practicable, excavations under an edifice can give data concerning previous churches on the site. M. Jules Quicherat was the first to teach that the history of the Middle Ages architecture was the history of the architect's fight against the weight and push of the vaulting.[15] Once the right path was blazed, many an able pioneer helped clear the new road--such students as Viollet-le-Duc, de Caumont, Woillez, Prosper Merimée, de Dion, Coutan, de Beaurepaire, Grandmaison, Révoil, Rupricht-Robert, Félix de Verneilh, Anthyme Saint-Paul, Louis Courajod, Buhot de Kersers. At the École des Chartes, Robert de Lasteyrie occupied with distinction the chair held by Quicherat for thirty years, and his pupils, Camille Enlart and Eugène Lefèvre-Pontalis, in their turn, are passing on the high tradition to a younger school. M. Enlart, the director of the museum of comparative sculpture at the Trocadéro, is an authority on Romanesque architecture, and has initiated the study of the spread of Gothic architecture in mediæval Italy, Spain, the North, and the Levant.[16] M. Lefèvre-Pontalis has written a host of erudite monographs; one learns to accept his decisions as final, in so far as the ever-expanding realm of knowledge can be final. He directs the invaluable publications called the _Congrès Archéologique de France_ and the _Bulletin Monumental_, and he edits those excellent short studies known as the _Petites Monographies des grands édifices de la France_, which are convenient pocket guides for the serious tourist.[17] Each year is producing final monographs on the chief churches of France. M. Georges Durand has rendered fitting tribute to Amiens. M. de Farcy has identified himself with Angers, René Merlet with Chartres, Lucien Broche with Laon, and Lucien Bégule with Lyons. MM. Brutails has specialized on Gascony, the Thollier and H. du Ranquet on Auvergne, Labande on Provence, Berthelé on Plantagenet Gothic, André Rhein on Poitou and Anjou, Émile Bonnet on Hérault, Charles Porée on Burgundy, and Louis Demaison on Champagne. Other able students are MM. Bouet, Louis Serbat, Marcel Aubert, Ernest Rupin, Jules de Lahondès, René Fage, Amédée Boinet, Jean Virey, Robert Triger, and Louis Régnier.