In the volume upon Deep-Sea Deposits, issued as one of the reports of the Challenger Expedition, there are published twenty-five analyses of the “red clay.” This sediment is now recognised as the most extensive and important of all the oceanic deposits, for it covers 51,500,000 square miles of the sea-floor, and is characteristic of the greatest depths. It is therefore obviously desirable to know its average composition as exactly as possible, and for that reason the following investigation was undertaken.