Rancho San Marcos was a 35,573-acre (143.96 km2) Mexican land grant in present-day Santa Barbara County, California given in 1846 by Governor Pio Pico to Nicolas A. Den and Richard S.
His younger brother Richard Den came to Santa Barbara in 1843, and was a doctor who practiced in the Pueblo de Los Angeles.
With the cession of California to the United States following the Mexican–American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored.
[7][8] Richard Somerset Den (1821–1895), a native of County Kilkenny, Ireland, had followed his brother Nicolas, to Santa Barbara in 1843.
Richard Den practiced medicine in Los Angeles with the exception of a brief sojourn in 1848–1850 prospecting for gold in Calaveras County, and a decade's absence (1854–1866) to administer Rancho San Marcos, until 1895.