It is important to meditate often and with ardor and respect on the documents which the ancestors have left us.--ST. THOMAS AQUINAS. The Era of the Great Cathedrals was inaugurated by Notre Dame of Paris, the most imposing Gothic church hitherto attempted. The popular voice has chosen to group it among the chief four, with Chartres, Rheims, and Amiens--all four of them dedicated to Our Lady, though in a special way Notre Dame of the capital seems to have appropriated the name. Of the four, the cathedral of Paris was the first built, and traits of the Romanesque epoch lingered in it, such as the tribune galleries over the side aisles, the division of its interior wall into four vertical stories, and the Byzantine feeling of its earlier sculpture. The piers were massive single columns of true majesty. In the sixth pier of the nave, counting from the east, an experiment was tried when an engaged shaft was added to its front. The seventh pier (c. 1192) marks a date in the development of Gothic structure since with it was made the type of ground support which was to predominate in the XIII century--four engaged shafts around a central pillar. When the middle core was made elliptical, as at Beauvais, the type pier was achieved. [Illustration: _Notre Dame of Paris. View from the South_] Notre Dame of Paris used the sexpartite system which calls for alternating ground supports. Either the uniform piers here were laid before a sexpartite vault was thought of, or else the architect preferred them for æsthetic reasons, and in this case he certainly was right. Double aisles about both nave and choir differentiate the interior of Notre Dame of Paris from that of the average cathedral. The far-stretching aisles of this church compose vistas of unsurpassed picturesqueness and variety of perspective. Some have said that the central nave is not sufficiently wide for such a stretch of lateral aisles, and have found a certain monotony in the clearstory, tribune, and pier arcade being of equal height. Originally, beneath the clearstory were small circular unglassed apertures giving on the rafters over the tribune. Those oculi were done away with during the XIII century, when the clearstory windows were lengthened for the better lighting of the church. During his able restoration of Notre Dame, M. Viollet-le-Duc found hidden under the pavement some of the discarded window frames, and he took the liberty (which many regret) of replacing a few in the bays near the transept, thus marring the uniformity of the interior.