City Dance "78" wound up at the Warner Theater Friday and Saturday nights with impressive and enthusiastically received performances by six Washington area troupes.

The way the festival has proven out in practice two years in a row is an object lesson in the successful assumption of worthwhile risk. "City Dance" was conceived last year by Nancy Pittman, who has since become dance coordinator for the D.C. Arts Commission and is exploring further famifications of her original idea. In both years, responsibility for the production fell to Betryce Prosterman and a hefty backup staff of the Washington Performing Arts Society. All can take credit for a job well done. Its particulary notable that this year, with far less fanfare and publicity than last, the three-night affair was again virtually sold out.

It was also heartening once again to see that the turnout reflected the ethnic composition of metropolitan Washington to a degree seldom encountered in audiences for other cultural offerings hereabouts. That, of course, was part of the "City Dance" idea. Another part was the hope for "cross-polination" - the thought that the followers of any one company on the festival programs would automatically get a chance to watch and respond to a kind accustomed viewing. Audience reaction from night to night showed plainly that hits too was actually happening.

A further festival aim was simply to demonstrate the viability of the Warner Theater, so strategically located at a downtown crossroads, for dance performances. Enhanced this year by new lobby amenities and re-covered seats, the Warner proved more attractive than ever as a site.

One can judge how important this exposure has been to the companies involved from the fact that the nine troupes seen in the festival were chosen from more than 40 applicants. The mix could scarcely have been broader - Friday and Saturday, for example, the Capitol Ballet (sporting some excellent dancing and a first-rate verision of Stravinsky's "ebony Concerto," by Doris Jones) represented ballet classicism with an admixture of contemporary black idioms; the Dance Exchanged and Choreo 18 exhibited several contrasting strains of modern dance; the Richard Cunningham Company and the Cole-Harrison Troupe fused jazz,tap and show dancing; and the African Heritage Dancers and Drummers - perhaps Washington's most consistently polished and inspiring troup - contributed a fold-enthnic repertoire.

Inevitably, the festival also spotlighted gaps and shortcomings in the Washington dance scene, such as the lack of continuity and coherence in our local season, and our relative isolation from today's esthetic frontiers. But such revelation is desirable fallout - one has to know what's needed in order to know what to aim for.