The great private ballrooms are no more, so the fashionables give dances and receptions at the Colony Club (not to be confused with the Colony Restaurant). For women only, this is one of the last stands of exclusiveness--so snazzy that non-members, guests, must enter by the side door. Only those who belong may use the Park Avenue portal or the main elevator within. The Union and Knickerbocker Clubs are as snooty as the day they were founded. To increase income they instituted ladies' nights, when members may invite wives, daughters and guests to dine. The Metropolitan Club, founded by the first J.P. Morgan for the rich but outré rejected by the Union, has let down its bars, as have other smart clubs. The Horse Show was another brilliant event in the golden age, but blue-blooded humans don't patronize it as enthusiastically as before. Fashionable women who do now affect sports togs. Those who cling to dressy traditions are stared at curiously. In the eras of rubberneck wagons, visitors got their money's worth. Men shouted through megaphones untruthful descriptions of famous mansions. Provincials thrilled when they reached Fifth Avenue and 57th Street. The four corners were sites of baronial citadels. On the northwest corner stood the great château of Cornelius Vanderbilt, with its iron-grilled garden in the rear. On the northeast was the white marble residence of Mrs. Hermann Oelrichs. On the southeast was the somber castle of Collis P. Huntington, and on the southwest the Moorish-windowed retreat of William C. Whitney. Today, commercial buildings have replaced them all. The great double mansion of Mrs. William Astor and her son, John Jacob, dominated upper Fifth Avenue at 65th Street. Its walls beheld entertainments that rivaled those of oriental potentates. Its most luxurious feature was a huge bathtub, carved from a single block of Carrara marble, with gold spigots shaped like dolphins' heads. Mrs. Jack Astor, née Ava Willing, America's most celebrated society beauty, now Lady Ribblesdale, laved herself there in eau de cologne. The architecturally impressive Temple Emanu-el now stands where all this was. The white mansion of Mrs. Oliver Belmont, at 51st Street and Madison Avenue, is now the Administration Building of the Catholic Archdiocese. The brownstone Jay Gould town house, at Fifth Avenue and 47th Street, where shouting thousands clamored for the life of the Wall Street operator on historic Black Friday, is a furniture store. The famous Bertie Goelet home, a block away, was recently razed. The largest private residence in the city, the brownstone palace at 640 Fifth Avenue, where Mr. Cornelius Vanderbilt entertained royalty and the other elect, was sold to the English Astors and has since been demolished. Its famed marble mantelpieces and interior decorations were auctioned to Hollywood producers. An office building was erected on its site. Mrs. Vanderbilt, undaunted, carries on in a smaller house at 1048 Fifth Avenue.