Which of the old to save?

MOST tourists think of Florence as the ultimate Renaissance city. But its even more distant, well-buried past has been slowly coming to light in recent years in a way that has provoked a heated local controversy: Which part of Florence's great past is most worth preserving. An interim agreement, probably as good as can be found, is delaying a real choice. Florence's medieval Piazza della Signoria has for centuries been the center of the city's social and political life. The Palazzo Vecchio, overlooking the piazza, still functions as the main city hall. Tourists line up to gaze at Botticelli and Cimabue masterpieces in the nearby Uffizi Gallery. But over the years the gray flagstone surface of the square has become cracked and chipped; asphalt patches no longer suffice.

When city public works officials proposed a repaving job, a squawk went up from Tuscan archaeological superintendent Francesco Nicosia and the city's cultural ministry. Mr. Nicosia said the repaving would bury forever such under-the-square treasures as the great baths of the city's Roman period, prize foundations of medieval churches and homes, and such evidence of trade across the Mediterranean as pieces of Greek pottery and Sardinia products dating back to 3,000 BC. Mr. Nicosia dreams of establishing an underground museum there one day.

Others, such as Florence Public Works Superintendent Paolo Cappelletti, who takes a daily dawn stroll across the square for peace of mind, insist that a museum with its necessary emergency exits and light ducts would change the very nature of the piazza.

The dispute is now on hold, in a very Italianesque compromise. A temporary agreement reached earlier this year allows both some repaving and some continued archaeological exploration. For the time being, Florentines get the best of both worlds: a window on the full breadth of their city's past.

It is the lament of the world's archeologists that the world's great civilizations literally are built on top of one another.

All of Mexico City, for instance, sits above the ruined pyramids and temples of the Aztec capital of Tenochtitl'an. Many of the finest remains of that civilization were uncovered when workers put in underground power lines and Mexico City's subway. The serpentine shape of the subway in Rome follows the course of archeological treasures discovered during construction.

Sometimes well-funded development projects gain the upper hand in the name of economic progress and human need. When Egypt's Aswan Dam was built, many Nubian monuments were flooded. The most important ones were moved - after a massive international fund-raising effort.

Rarely is much thought given to potential underground treasures in construction projects, particularly in the third world. Occasionally publicity and protests from an aroused public make a difference. After several recent press reports spotlighted the potential loss of Mayan sites along the Mexico-Guatemala border where a new dam is planned, Mexican President Miguel de la Madrid a few days ago pledged that any new dam would be built in a way that ensures no damage to the sites or border ecology.

It would be useful if developers, particularly in archaeologically rich areas, meet a new requirement: They should prepare a ``cultural impact statement'' before proceeding with their plans. And, since cultural resources do translate into economic assets, funds to help rescue treasures should be anticipated in any development package.