I know this is not very original of me but it's a place to start. This is Wikipedia's definition of Tonalism.
Tonalism (1880 to 1915) is an artistic style that emerged in the 1880s when American artists began to paintlandscape forms with an overall tone of colored atmosphere or mist. Dark, neutral hues, such as gray, brown or blue, would usually dominate such compositions. During the late 1890s American art critics began to use the term "tonal" to describe these works. Two of the leading painters associated with this style are George Inness and James McNeill Whistler.
Tonalism is also sometimes used to describe American landscapes derived from the French Barbizonstyle, which employs an emphasis on mood and shadow. Tonalism, in both its forms, was eclipsed by the popularity of Impressionism and European modernism.