On the other hand, in the glass of the Rhine district, including of course the Moselle, we have a return to the more varied types that we met with in the south. Trèves was the northern rival of Arles; it formed the centre of a rich district, including Lorraine on the one hand and the Rhine provinces on the other, where the manufacture of glass by the third century became an important industry. And this district has for us a special interest, for here more than anywhere else we have some evidence to show that the industry was carried on without interruption throughout the Middle Ages. The museums of Trèves, of Cologne and of Bonn, are above all rich in Roman glass, and the German archæologists have endeavoured—and this has hardly been attempted elsewhere—to arrange this glass in a chronological sequence. They think that they can distinguish the following stages in the industry:—1. Up to 50 A.D. glass was a rarity in the north, but the millefiori and marbled glass of the south was imported to some extent. 2. After the middle of the first century, glass-works were established for the manufacture of large urns and smaller vessels of a ‘_Natur-glas_,’ bluish rather than greenish in tint. 3. In the time of Hadrian (117-130 A.D.) a pure white glass was introduced; this was more liable to decay than the older bluish glass. 4. The period of the greatest development was about 200 A.D. Many kinds of decoration were in fashion, as zig-zag threadings on coloured glass. 5. After 250 A.D. This was the time of the glass with the _Frontinus_ stamp.[53] The prevailing tint is a strong green, no longer bluish; the decoration is given chiefly by engraving and cutting; Christian subjects begin to appear. To this period also belongs glass decorated with coloured medallions of glass paste. I give this scheme of classification under all reserve; the interlarding of a period of white glass between two stages of ‘green glass’ may perhaps be open to criticism, but at all events it is a step in the right direction. It must be borne in mind that this Rhenish glass belongs to the same Romano-Celtic family as that found in France, but, as in the latter country, the Celtic element is scarcely perceptible. The art was an entirely new one, and there was no earlier tradition to influence the work as in the case of the contemporary pottery, armour, or sculpture.