The northern part of Park Slope emerged in the late 19th century as an extremely affluent neighborhood. Its wealth of beautiful architecture comes as no surprise.

To the south, Park Slope was a working-class neighborhood, at first mainly Irish but later also Italian and Jewish, home to longshoremen and factory workers.

This area of Park Slope lacks the architectural cohesiveness of the more affluent parts of the neighborhood, but it definitely rewards a careful look, and yields many sweet surprises.

A good place to begin is the motley block of 9th Street between Fourth and Fifth avenues. The 9th Street station of the BMT Fourth Avenue line opened underground in 1915. In 1933 the elevated station on Fourth Avenue at 9th Street opened as part of the IND’s Crosstown Line. The Art Deco station is handsome, and is listed on the National Register of Historic Places, but is desperately ill-maintained, and given the transit authority’s budgetary woes, the station will get even worse before it gets better.

The big Gothic Revival church, St. Thomas Aquinas, on the northeast corner of 9th Street and Fourth Avenue, was designed by William Schickel, a prolific architect of Catholic churches. It was built in 1885-86, at a time when the church served an Irish working-class congregation.

About mid-block on the north side stands one of Park Slope’s most unexpected houses. Built in 1856-57, the Italianate/French Second Empire freestanding villa is the exact contemporary of Edwin Litchfield’s on Prospect Park West at 5th Street. In the mid-19th century, before most of the streets around here were cut through, a mansion district took shape in the countryside near the intersection of the two main roads, Fourth Avenue and 9th Street. Its only remnant is 271 9th St.

This house became, in the 1890s, offices of Charles Higgins’s famous India ink company, whose former factory stands on 8th Street directly behind the house. Built in 1899, the factory has been converted into apartments. The house is once again a private residence.

The white terra-cotta-clad C-Town supermarket on the north side of 9th Street between Fifth and Sixth avenues was, from 1914 to 1967, the RKO Prospect Theatre, which at first presented both vaudeville and films, then later only films. In 1922 comedian Ted Healy invited two fledgling Brooklyn-born performers, Moe and Shemp Howard, to join him onstage for some ad-libbed humor. The act continued at the Prospect under the name Ted Healy and His Stooges. Later, Park Slope-born writer Pete Hamill’s mother worked at the Prospect as a ticket taker.

At the northeast corner of Sixth Avenue and 9th Street stands the very handsome Prospect branch of the Brooklyn Public Library, designed by the extremely talented Brooklyn-born architect Raymond Almirall and built in 1906. This library, one of the countless American libraries built through the Andrew Carnegie bequest, bears Viennese Secessionist flourishes.

A block north, at 8th Street, don’t miss the wonderfully Victorian Public School 39, designed by Samuel Leonard and built in 1876-77 when many of the students would have been the children of Irish immigrants who worked on the Gowanus Canal.

At the northwest corner of Seventh Avenue and 9th Street is the former Acme Hall, a large Romanesque Revival structure built in 1890 to house meeting spaces for fraternal organizations. The building, now offices, also had a bowling alley, a pool hall, and a two-story ballroom. (Thanks to Ruth Edebohls of the Center for the Urban Environment for filling me in on Acme Hall.)

Turn right on Seventh Avenue and walk to 12th Street. The Ansonia Court apartments were carved out of what was once one of the largest clock factories in the world. The Ansonia Clock Company opened on this site in 1879. The present structure dates from 1881. The company grew rapidly, especially as a result of its international business during the great phase of globalization immediately prior to World War I. (Even at our present level of free trade and free movement of people across borders, we’ve still not caught up to where we were then.) The war proved devastating to Ansonia, which went out of business in 1929. (Ansonia sold its clock-making machinery to the Soviet Union, and some Ansonia employees went to the Soviet Union for up to 18 months to train workers there on the use of the machinery.) In 1981 the handsomely utilitarian building was made into luxury cooperative apartments.

At 14th Street take a left. At Eighth Avenue stands the massive structure of the 14th Regiment Armory, designed by William A. Mundell and built in 1895. Like many New York armories, it looks like a medieval fortress with towers and turrets and Romanesque corbeling.

The building, with an impressive 70,000-square-foot drill hall, has in part been renovated by the city into a $16 million community athletic facility scheduled to open in early 2009 and to be operated by the nearby Prospect Park YMCA.

Note the doughboy statue in front of the main entrance on Eighth Avenue. The sculptor Anton Scaaf created this bronze soldier in 1923. World War I is commemorated throughout the working-class neighborhoods of New York — Red Hook, the West Village (yes, it once was working-class), Hell’s Kitchen, etc. — not by likenesses of specific heroes, or by allegories, but by the anonymous foot soldier meant to stand for all the immigrants’ sons who died for their new country.

fmorrone@nysun.com