THE WORK OF THE OCEAN. 333 



places, however, as at various points in and about Italy, the configura- 

 tion of the coast is influenced by the building of volcanoes. Indirectly, 

 vulcanism influences the shape of coast-lines, for the resistance of igne- 

 ous rock is often different from that of the rock with which it is asso- 

 ciated, and under the influences of the forces of gradation it may come 

 to form projecting points or reentrants, 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 in- 

 crease of temperature and the solution of volcanic gases increase the 

 capacity of the water for mineral matter, and both the change in tem- 

 perature 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. 



Gradation. — The gradational processes of the land and the sea are 

 in striking contrast. On the land, degradation predominates, and 

 aggradation is subordinate. In the sea, aggradation predominates, 

 and degradation is subordinate. On the land, degradation is, on the 

 whole, greatest where the land is highest, while aggradation is of con- 

 sequence only where the land is low, or where steep slopes give place 



