Photographs such as those of the Monuments Historiques answer all the just purposes of underground travel. The Aquilon is one's first lesson in Transition architecture because it is dated (1112); and the crypt of Saint-Denis serves almost equally well because the Abbe Suger must have begun his plans for it about 1122. Both have the same arcs doubleaux and arcs-formerets, though in opposite arrangement. Both show the first heavy hint at the broken arch. There are no nervures--no rib-vaulting,--and hardly a suggestion of the Gothic as one sees it in the splendid crypt of the Gros Fillers close at hand, except the elaborately intersecting vaults and the heavy columns; but the promenoir above is an astonishing leap in time and art. The promenoir has the same arrangement and columns as the Aquilon, but the vaults are beautifully arched and pointed, with ribs rising directly from the square capitals and intersecting the central spacings, in a spirit which neither you nor I know how to distinguish from the pure Gothic of the thirteenth century, unless it is that the arches are hardly pointed enough; they seem to the eye almost round. The height appears to be about fourteen feet. The promenoir of Abbot Roger II has an interest to pilgrims who are going on to the shrine of the Virgin, because the date of the promenoir seems to be exactly the same as the date which the Abbe Bulteau assigns for the western portal of Chartres. Ordinarily a date is no great matter, but when one has to run forward and back, with the agility of an electric tram, between two or three fixed points, it is convenient to fix them once for all. The Transition is complete here in the promenoir, which was planned as early as 1115. The subject of vaulting is far too ambitious for summer travel; it is none too easy for a graduate of the Beaux Arts; and few architectural fields have been so earnestly discussed and disputed. We must not touch it. The age of the "Chanson de Roland" itself is not so dangerous a topic. Our vital needs are met, more or less sufficiently, by taking the promenoir at the Mount, the crypt at Saint-Denis, and the western portal at Chartres, as the trinity of our Transition, and roughly calling their date the years 1115-20, To overload the memory with dates is the vice of every schoolmaster and the passion of every second-rate scholar. Tourists want as few dates as possible; what they want is poetry. Yet a singular coincidence, with which every classroom is only too familiar, has made of the years--15 a curiously convenient group, and the year 1115 is as convenient as any for the beginning of the century of Transition. That was the year when Saint Bernard laid the foundations of his Abbey of Clairvaux. Perhaps 1115, or at latest 1117, was the year when Abelard sang love-songs to Heloise in Canon Fulbert's house in the Rue des Chantres, beside the cloister of Notre Dame in Paris. The Abbe Suger, the Abbe Bernard, and the Abbe Abelard are the three