Miners dominated the exploration and development of the range from the 1880s through the early 20th century. For example, mines around the boomtown of Monte Cristo, in the southwest portion of the North Cascades, produced between $1 and $2.7 million in silver and gold. The Holden Mine, on the east side of the main divide, produced 106,000 tons of copper and 600,000 ounces of gold. Discovery of gold by American prospectors on the banks of the Thompson River at its confluence with the Nicoamen River, at the northern tip of the range, helped trigger the Fraser Canyon Gold Rush of 1858-1860 which in turn prompted the declaration of the Colony of British Columbia to affirm British possession of territories north of the 49th Parallel. The Fraser rush led to exploration of the Cascades to the east of the canyon and in the valley of the Similkameen River, with minor rushes in the area of Princeton, British Columbia in 1859 through the early 1860s and the creation of non-native towns (on top of much older native ones) at Boston Bar, Lytton and Hope, as well as Princeton.