In 1798 the Lancaster, Pa., city hall was completed -- a commendable brick structure with simple, finely handled Georgian details. In 1977 the final touches were put on the Dallas city hall -- a long, concrete structure that is as big, bold and far away as the Lancaster building is little, likable and ready to touch.

In between there is, of course, quite a story, one told with a certain fast-track economy in the photographic exhibition, "America's City Halls," that opened this week at the headquarters of the American Institute of Architects. Fifty buildings are represented by perhaps twice as many photographs in the show--a quick survey of the terrain sure to bring a smile of instant recognition to anyone who pauses, however briefly, to look it over. For who among us does not at all times lug around a sort of mental baggage that includes memories, at once specific and general (and perhaps pleasant and unpleasant by turns), of city halls we have seen and known?

Civic architecture and civic pride go together. More directly than other kinds of buildings, the places where we choose to conduct our public business tell us flat out what collectively we think (or thought) of ourselves, or at least give us back the face we want (or used to want) to show to others. To see this story compressed into a few photographic panels is an experience both wistful and surprising, a look into a little mirror that somehow reflects nearly 200 years of changing architectural tastes and, even more forcefully, vast and almost inconceivable changes in the social landscape.

In this context it is the smallness, the perfect tidiness, of the early city halls that is most impressive. Even when constructed in the early 19th century, as in Wilmington, Del., or Charleston, S.C., these places have an 18th-century scale that today is totally beyond our ken. It is not so much a question of size, though these buildings inevitably have the look of doll's houses, as of proportion: those windows and pediments and roofs with cupolas embody a vernacular architectural style, confidently adapted, that was in no essential way different from the nearby commercial and residential buildings. The sense of proportion was not simply a matter of accepted architectural practice, either; it expressed an undemonstrative sort of rationalism, a self-contained sense of order, that today we can recapture only with mighty doses of self-conscious nostalgia or outright irony.

The great explosion of civic building in the United States did not occur until after the Civil War, when it happened with a vengeance: the towers, oh the towers, that our municipal forefathers built, like church steeples, but bigger, higher, more elaborate: massive vertical piles of masonry with all manner of columns attached, and on top domes and roofs of wildly different proportions. Atop these was almost always a standing civic figure in bronze, a pinnacle and beacon for all to see, be it in Baltimore (1865), Boston (1875) or Philadelphia (1901). Beneath the towers there were those massive buildings, bigger and more decorated than even the richest robber baron could afford, in the Second Empire style, the French Baroque Revival style, the Victorian Gothic style and even, in St. Louis (1904), a late-blooming French Renaissance copy that, though lacking a tower, is as exuberant, and as absurd, a concoction as the others.

I suspect that with no evidence other than the photographs in this show even a visiting extraterrestrial could tell much about the changes in social and economic circumstance that precipitated this outburst of civic building. In any case, these are the kinds of buildings that, more than any of the others, still form a powerful mental image of city hall as a massively ridiculous, but somehow lovable, enclosure of walls behind which sharp politicos and ward heelers make their deals. We love these buildings less for their intrinsic architectural merits--although in a time of vast size and uniformity, their textures and shadings make them seem better than they really are -- than for their associations and their unmasked, unapologetic self-confidence.

In the midst of this eclecticism Henry Hobson Richardson introduced his own revival style--the Romanesque -- and his special gift for muscular stonework and massing makes his Albany city hall (1883), with its statueless off-center tower, stand out. Richardson's manner in itself became a cliche', although no one would sensibly complain today. The Richardsonian-style city hall buildings in Cambridge, Mass. (1889), Lowell, Mass. (1893), Salt Lake City (1894) and Bay City, Mich. (1905), are, like the Old Post Office building in Washington, historical monuments very much worth fighting for. (Happily, all of the 50 buildings in this show are still standing. Only one, in Richmond, is in danger.)

If the eclectic monuments of the gilded age speak ostentatiously of urbanization, industrialization and immigration -- the forces that forever changed the city and brought about a new kind of politics for new kinds of problems -- the classical revival buildings that succeeded them are the architectural equivalents of the patrician reform movement. The realities, of course, may belie the analogy (as many shady deals were doubtless cut behind the Beaux Arts walls as anywhere else). But there is nonetheless a certain standoffish, above-the-messy-fray quality in the elegantly proportioned, white buildings in Trenton, N.J. (1910), and Wilmington (1917), and to a lesser extent those in Chicago (1911) and Pittsburgh (1917).

Whereas the politicians who commissioned all those romantic, eclectic revival buildings mimicked the outlandish acquisitiveness of the industrial empire builders, their successors adopted a more serene architectural stance. It is no accident that the proud towers disappeared; even as the skyscrapers went up around them, the new city structures became, for the most part, long, low monuments of respectability.

This principle lasted longer, a lot longer, in Washington than anywhere else. While the Federal Triangle was being built here, everybody else wanted big, high buildings for city centerpieces. In the 1920s and '30s, the set-back "moderne" skyscraper became the archetypal government office building: huge platforms from which rose towers that were, of course, functional offices (sometimes acre after dreary acre) as well as visual exclamation points. It was the beginning of the modern age in civic, as well as commercial, building, with tremendously expanded needs for bureaucratic space and all that it implies, good and bad, about the new magnitude of city problems and our effectiveness in coping with them.

Compared to this, earlier builders had it pretty easy. That seems one clear message of this show. Much modern municipal building seems awesomely standardized as well as parsimonious, a sort of collective giving up on the part of politicians and architects in the face of overwhelming facts. That it need not be so is suggested in a few contemporary examples in the exhibition--the famous Boston city hall (1969), a concrete beehive in a wide open space; a superslick high-tech example in San Bernadino (1972); and in Dallas (1977), where I.M. Pei's long, low concrete-and-glass building makes an apposite bow to Corbusier's Chandigarh.

There is a lot this exhibition doesn't tell us about the insides of these buildings, what was (and is) around them and the way they worked, but there is a lot that it does tell, too, in a very economical fashion. Unfortunately there is no catalogue; funding for a book covering more than 100 city halls is still being sought. The exhibition was organized jointly by the AIA, the Department of the Interior and the U.S. Conference of Mayors as part of a 50th-anniversary tribute to the Historic American Buildings Survey. It continues through Dec. 30 at the AIA headquarters, 1735 New York Ave. NW, open free 8:30 a.m. to 5 p.m., Mondays through Fridays.