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 x] ARCHITECTURE 309 Roughly speaking, the period of Romanesque ex- tends from the breaking up of Charlemagne's Empire to the end of the twelfth century. It was a time when Germanic peoples collectively were rapidly ad- vancing in civilization ; but as yet there was no large growth of towns. Monasteries contained the largest and wealthiest assemblages of people. Their needs, and the endeavor to replace the perishable wooden roofs of the antique basilica with a roof of stone, de- termined the development of Romanesque. The parts of the basilica used by the clergy were extended, the ground plan reaching the form of a Latin cross; the choirs were enlarged, or sometimes doubled, and like- wise the transepts; crypts and clock towers were added ; piers and pillars began to replace the antique columns. Toward the end of the period, pillars and piers were breaking into clusters corresponding to the strains which they supported. They had still a two- fold function, that of a pier supporting a vertical weight and that of a buttress counteracting a thrust. Yet the spirit of architectonic analysis is beginning to distinguish these two functions, and is approaching a corresponding division of these structural elements into pillars and buttresses. The Roman vault was cast, a rigid block of brick and concrete. Yet in Gaul Roman builders soon substituted a vault of stone, as may be seen at Nimes in the " Baths of Diana." Everywhere the Romanesque vault was to be a living arch of stones. The antique basilica had a vaulted apse, which first of all the Romanesque archi- tects constructed of stone. Next in order to replace the flat wooden roof of the antique basilica, they succeeded in vaulting the side aisles and then the central nave.