The sediments of fresh-water lakes are deposited in response to the various environmental factors present in lakes that have sedimentary control. Among these are the origins of the lake basins, size and depth of basins, relief of drainage areas, extents of shallow water adjacent to shores, degree of vegetable protection over drainage areas, characters of surrounding rock and soil terranes, climatic conditions, and organisms dwelling in lakes. The plan of lake deposits is considered and the characters of the sediments deposited in lakes are described in detail. Deposits of different lakes have individual characteristics and this is the case even in lakes which are close together. Rates of deposition vary with lakes. Deposits over the deep parts of almost every lake contain a high content of matter of organic origin. Profound changes take place in lake sediments after their deposition. Organic matter is eliminated, the content of carbonates may increase or decrease, and there are other important changes. There is much formation of combustible gas, thought to be methane, and a paraffine wax has been extracted. Bacteria and other micro-organisms are responsible for these changes.