This is so called Ruhrgebeit (Ruhr region). Ruhr is mining area and many Poles, Czech's, Slovaks and Yugoslavians went there to work in the mines. But Poles were most numerous. This migration is called Ostflucht. My great grand-father also worked in Ruhrgebeit and Dutch mines. People with Polish ancestry are called Ruhrpolen. Famous Ruhrpole was Erich Kempka who was Hitlers driver.

In 1900, the main concentrations of the Polish minority were:

    District of Gelsenkirchen (Westfalia) 13.1 %
    District of Bochum (Westfalia) 9.1 %
    District of Dortmund (Westfalia) 7.3 %
    City of Gelsenkirchen (Westfalia) 5.1 %