The paper gives physical and chemical analyses of the sediments of Lake Mendota. The sediments over the bottom are very fine-grained and are composed of 60 to 80 per cent carbonate, mostly calcium, about 20 per cent silica, and small percentages of alumina and iron oxide. When first deposited the sediments are black soupy oozes rich in organic matter. Elimination of the organic matter lay micro-organisms changes these to light-colored marls. These deposits are made in depths to a maximum of 80 feet.