T he lecturer began by pointing out that the carbonate of lime, of which the hard parts of most corals consist, has been derived in the first instance from the decomposition of such eruptive rocks as basalt and dolerite. The lime is dissolved out of these rocks chiefly by the chemical action of the humus acids, which are formed by the action of bacteria upon the carbon compounds present in dead vegetable matter. One of the essential constituents of a coral is therefore drawn in the first place from the atmosphere, just as the other constituent, the lime, is derived from the heated interior of the globe. The carbonate of lime thus liberated is carried seawards from the land in the form of the soluble bicarbonate, which, at the zone where rivers enter the sea, is next converted into the sulphate of lime. In this form it is eventually diffused throughout the waters of the ocean, and from them it is continually being extracted by living organisms, by whose vital agencies it is then converted into the solid bicarbonate, of which the hard structures of marine organisms are usually built up. The supply of sulphate of lime carried into the sea by rivers varies greatly according to local conditions, and within wide limits, but any extra demand upon that supply, such as must arise in those parts of the sea where great masses of coral reef are being built up, is sooner or later met by the supply carried thither by