historic structures in its cities and towns. The boom also drove many extensions of the railway network in the mountains, including such lines as the Denver, South Park and Pacific, which built an early narrow-gauge line to Leadville. Likewise the extension of the railroad network up the Roaring Fork Valley to the previously failed mining town of Aspen in the late 1880s made the extraction of silver ore there economically feasible, and saved the town from near extinction. The government purchases of silver were subsequently nearly doubled by the 1890 Sherman Silver Purchase Act, further extending the boom into the early 1890s. The repeal of the act in 1893 resulted in a collapse of silver prices, bringing about an end to the boom. After 1893, many mining camps became ghost towns. The accompanying collapse in statewide economic activity was ameliorated somewhat by the simultaneous emergence of agriculture, previously derided as not feasible, as a large component of the state economy. The working conditions inside the mines were often very dangerous. Silicosis, which at the time was incurable, ruined miners' lungs quickly. Many other hazards existed. Apart from the lanterns or tallow candles the miners carried, the mines were otherwise completely dark. Miners at the time were also subject to the threat of tunnel collapse, flooding, and the lack of oxygen in the deeper areas of the mines. Often the miners brought caged canaries down with them; when the bird passed out, it indicated that the oxygen levels were dangerously low in the area. Mines were commonly very small and tightly spaced to save on the cost, effort, and time it would take to expand the tunnels, and so resulted in the use of people of smaller stature and even children. See also Leadville Miners' Strike Leadville mining district Silver mining in Colorado Gold mining in Colorado References Further reading Carl Ubbelohde, Maxine Benson, and Duane A. Smith, A Colorado History, (1965) [1]"Colorado Mines", [2]"Leadville Mines”. Aspen Daily Times, August 31, 1899. [3]"Henderson, Charles W.. Mining in Colorado: a history of discovery, development and production. Washington: G.P.O., 1926 [4]"Graff, Marshall Conant. A history of Leadville, Colorado. : , 1920. [5]Whitney, Joel Parker. Silver mining regions of Colorado With some account of the different processes now being introduced for working the gold ores of that territory.. New York: D. Van Nostrand, 1865. Colorado Mining Boom Economic history of the United States Mining