San Marcos (săn mär`kəs). 1 City (1990 pop. 38,974), San Diego co., S Calif., a northern suburb of San Diego; settled 1880s, inc. 1963. Citrus fruit, avocados, flowers, and ornamental plants are grown, poultry and cattle are raised, and there is dairying. Plastic products, apparel and accessories, electronics equipment, dental equipment, printing plate, and plumbing products are manufactured. California State Univ. San Marcos is there. 2 City (1990 pop. 38,974), seat of Hays co., S central Tex., on the San Marcos River; inc. 1877. Metal and plastic products, business forms, machinery, and transportation and lighting equipment are manufactured, and there is tourism. The city is situated on the Balcones fault, where prehistoric earthquakes split the earth, releasing spring water to the surface (the San Marcos River was thus formed) and creating underground caves. Attractions include Aquarena Springs, with glass-bottomed boats, and Wonder World Cave. A branch of Texas State Univ. is there.