''The Republic of the Ghetto'' is, to paraphrase James Baldwin, another country - a separate landscape - and thus in a definition based on source and scenario, a foreign architecture.
In the Bronx you can find many once-distinctive structures that are now both devastated and defensive. Focus on the South Side of Chicago, on Watts, on Newark, and you see a grass-in-the-cracks vacant lot, a bolt-it-down-or-you'll-lose-it design that produces the density, texture, and color that create a unique architecture.
This decline is the theme of the exhibition ''Ruins and Revivals: The Architecture of Urban Devastation,'' at New York's Municipal Art Center through Nov. 3.
As guided by photographer Camillo Vergara and social historian Kenneth Jackson, the exhibit offers a fascinating and revealing look at the influence of the life style on surrounding architecture.
In its way, the exhibition raises fundamental questions about the definition of architecture. Eschewing the view that urban design is only facade-deep, this show typifies today's expanded sense of architecture as the creation of place - in which, under the label ''built environment,'' architecture is seen as everything from the miniature golf course to the new capital cut out of the wilderness for Brasilia.
It is an old-new attitude stemming from the inclusiveness of the 50-year-old German Bauhaus, which claimed the creation of all things beautiful as architecture. The late 20th century says that all surroundings, ugly as well as elegant, are, or shape, the architecture in our lives.
That sums up Vergara's view. Architecture, he says, consists of ''how many failed things you see as you walk down the street'' - the defunct businesses that bring to mind things that are built, that thrived, and then faded.
The architecture of the ghetto is unique. In the daytime, it lacks ''street furniture:'' Signs are missing, blocks have no buildings, and mailboxes are ripped away to create a foreign landscape.
At night it is, by and large, a lightless place, dimmed by the buildings, reduced to rubble where strange outlines of decaying shells make evening silhouettes.
Plywood across the windows in vacant buildings, a door painted a very visible red to prevent theft, houses scarred black from fire, spiraling barbed-wire fences: These, in the end, create the poverty-belt dweller's portrait of design as much as Chippendale rooflines outline the homes of the trendy.
The show's authors call this ''welfare'' architecture. Outside, it is shops that service a life style where half the population is unemployed.
Inside, it is furnishings ordered to the specifications of government checks that allow items of a certain price or specific dimensions. The ghetto depicted by Vergara uses the makeshift to guard and enliven simultaneously. A Cyclone fence may protect a home; vandalproof cube glass windows in the shape of a crucifix on the facade will serve religion and protection of property.
In many ways, then, this assemblage of photos and words charts more of what could be called decline and fall than ''Ruin and Revival.''
Part I, The Process of Decay, ''traces patterns of industrial, commercial, and residential abandonment which have helped create a type of devastation as common to America as skyscrapers and commercial strips.''
Part II, Defensive Architecture, shows the solutions designed to stem it, from sealed windows to a defunct apartment's courtyard closed off with a concrete wall.
More positively, Part III deals with revivals - reversals or attempted reversals of the pattern of decline built into the environment.
What the exhibitors call defensive design can produce a conventionally attractive structure. The Medical Center in Milwaukee, for example, where pointed wood roofs, skylights, and careful massing somewhat relieve the barricaded exterior.
More often, defensive design produces either bizarre or depressing adaptations to the social conditions. When a church, the major socializing institution of the ghetto, rolls down a metal barricade across the door or an artist's mural dresses up a building on the South Side of Chicago by depicting a descent into violence and drugs, the result exacerbates rather than heals the environmental malaise.
Some may feel that these structures are ''befores'' simply waiting to be recycled into an ''after.'' But the organizers of this assemblage have a more pessimistic view. If a building or housing project skids, it rarely rises.
Still, the thesis of this striking show is somehow as alive as its tenacious collector, Camillo Vergara.
Look at the mosaic painted above a church, or the Ark, a boat built by a lone ''carpenter'' in an abandoned lot, or examine the survivors, and you have a sense of grit and endurance.
''Even though it's burned out,'' says Vergara, ''there's soul or life. It's not sterile. It may be mean. It may be cruel. It may be depressing,'' he says, ''but it's alive.''
The show will travel to Rutgers in Newark, Dec. 5 through Jan. 20, 1984; the Rutgers campus in Camden, Jan. 30 through March 3; and New Brunswick from March 11 through June 3.