Industrially and commercially they virtually form a part of Troy.
Other minerals that have been discovered but have not been industrially developed are gypsum, lignite and cement rock.
Minerals are fairly abundant in number, but few are present in sufficient quantity to be industrially important.
Minneapolis is the chief flour-making centre of the world, and the cities at the " Head of the Lakes " (Duluth, Minnesota, and Superior, Wisconsin, considered industrially as one place) constitute the second largest centre.
Lead chromate, PbCrO 4, is prepared industrially as a yellow pigment, chrome yellow, by precipitating sugar of lead solution with potassium bichromate.