The bottom sediments of Lake Monona consist at the top of a black sludge dominantly composed of insoluble matter, of which most is silica, calcium carbonate, and organic matter. Beneath the sludge are light-colored, firm sediments which are composed of 60 to 70 per cent water and 30 to 40 per cent solid matter. The solid matter is dominantly calcium carbonate with a range of 50 to 70 per cent. The other constituents are silica, alumina, iron oxide, magnesium carbonate, and organic matter. The high content of insoluble matter in the sludge is correlated with white occupation of the region.