Answer:
One specific cause of the process of "sorting out classes at the workplace" during the period 1865 to 1900 that Blumin referred to is the growth of large-scale corporations and the shift from a mainly agrarian, craft-based economy to an industrial one. The rise of industrialization led to the growth of large companies and factories which in turn led to the formation of a new working class, separate from the previous artisan class, who were concentrated in the urban centers and worked in the factories. This change in the economy, led to a sorting out of classes at the workplace, with the rise of a new working class and a new capitalist class that owns and control these new industrial enterprises.