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Yet even as the federal government did little to check the breadth of the new slavery, the economic logic of the system weakened. Crude industrial enterprises to which slave labor lent itself so effectively for fifty years were being eclipsed by modern technologies and business strategies. Mechanized coal mining—using hydraulic digging tools, electric lights, modern pumps, and transportation—made obsolete the old manual labor mines of Alabama, packed with thousands of slave workers and mules.
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Douglas A. Blackmon (Slavery by Another Name: The Re-Enslavement of Black Americans from the Civil War to World War II)