EXCLUSIVE NEW TO MARKET FIELDSTON HISTORIC DISTRICT  Classic 6-Bd. Center-Hall Colonial with Patio & Spacious, Grassy Grounds    This classic 6-bedroom Colonial Revival-style house with 3 full and 2 half bathrooms was built in 1923. It boasts spacious, grassy grounds, secluded by a tall, natural rock outcropping, and enjoys an inviting flagstone patio - perfect for al fresco dining and entertaining. The property also has mature trees, manicured lawns, abundant shrubbery and terraced rock gardens.    Standout features include: an entry vestibule; a center hall; living room with wood-burning fireplace; formal dining room; windowed kitchen with polished granite countertops, radiant-heat flooring, 2 sinks abundant cabinetry and stainless-steel appliances (Sub-Zero refrigerator, 4-burner GE Monogram cooktop; 2 Bosch dishwashers, 2 GE Monogram wall ovens, warming drawer, Jenn-Air microwave); family room/office with custom built-ins and separate entrance; bright western sunroom; master bedroom suite; air-conditioning throughout; finished basement with recreation room, powder room and laundry and storage rooms; long private driveway; attached 2-car garage; 0.40-acre lot in landmark district.    Conveniently located for well-known private schools (Horace Mann School, Fieldston School and Riverdale Country School) and transportation to Manhattan (including express buses).