The processes of vulcanism do not commonly influence coasts of continents directly, for few volcanoes lie immediately on coasts. In places, however, as at various points in and about Italy, the configuration of the coast is influenced by the building of volcanoes. Indirectly, vulcanism influences the shape of coast-lines, for the resistance of igneous rock is often different from that of the rock with which it is associated, and under the influences of the forces of gradation it may come to form projecting points or reëntrants, as the case may be. The number of active volcanoes on islands is about 200, or about two-thirds of all now known. Since the area of the sea is about three times that of the land, the known active volcanoes in the sea are rather less numerous per unit area than those on the land. The number of active vents beneath the sea is altogether unknown. A few submarine eruptions have been observed, and those observed are probably but a small percentage of those which have taken place in historic time. Slight eruptions in deep water might not manifest themselves at the surface in an unequivocal way, even were observers stationed near them. Volcanic cones which fail to reach the surface are known, and the forms of many sea-bottom mountain peaks are such as to make it probable that they are volcanic. These phenomena, as well as the numerous volcanic islands, give some indication of the importance of submarine eruptions in past time. Ocean volcanoes, and especially submarine volcanoes, affect both the temperature and the composition of the sea-water. Both the increase of temperature and the solution of volcanic gases increase the capacity of the water for mineral matter, and both the change in temperature and composition affect the life of the adjacent waters. The destruction of life during eruptions occasions the generation of the products of organic decomposition, and these stimulate further chemical changes. The diffusion of affected waters occasions chemical changes wherever they go. The effects of oceanic volcanoes on the sea-water are, therefore, appreciable, when long periods of time are considered. The deposition of the finer parts of volcanic discharges will be considered in connection with the deposits of the deep sea.