Hence, we have certain complex structures, buildings of gradation and transition, which may be Roman at the base, Gothic in the middle, and Greco-Roman at the top. This is caused by the fact that it took six hundred years to build such a fabric. This variety is rare. The donjon-keep at Étampes is a specimen. But monuments of two formations are more frequent. Such is Notre-Dame at Paris, a structure of the pointed arch, its earliest columns leading directly to that Roman zone, of which the portals of Saint-Denis and the nave of Saint-Germain des Prés are perfect specimens. Such is the charming semi-Gothic chapter-house of Boucherville, where the Roman layer reaches midway. Such is the cathedral of Rouen, which would be wholly Gothic if the tip of its central spire did not dip into the zone of the Renaissance. [Footnote: This part of the spire, which was of timber, happens to be the very part which was burned by lightning in 1823.] However, all these gradations and differences affect the surface only of an edifice. Art has but changed its skin. The construction itself of the Christian church is not affected by them. The interior arrangement, the logical order of the parts, is still the same. Whatever may be the carved and nicely-wrought exterior of a cathedral, we always find beneath it, if only in a rudimentary and dormant state, the Roman basilica. It rises forever from the ground in harmony with the same law. There are invariably two naves intersecting each other in the form of a cross, the upper end being rounded into a chancel or choir; there are always side aisles, for the processions and for chapels, a sort of lateral galleries or walks, into which the principal nave opens by means of the spaces between the columns. This settled, the number of chapels, doors, steeples, and spires may be modified indefinitely, according to the fancy of the century, the people, and the art. The performance of divine service once provided for and assured, architecture acts its own pleasure. Statues, stained glass, rose-windows, arabesques, denticulations, capitals, and bas-reliefs,--it combines all these flowers of the fancy according to the logarithm that suits it best. Hence the immense variety in the exteriors of those structures within which dwell such unity and order. The trunk of the tree is fixt; the foliage is variable. The Louvre By Grant Allen [Footnote: From "Paris."]