THE WORK OF THE OCEAN 295 



forms of life, and, through these changes, (5) influence the processes 

 of gradation, especially at the contact of sea and land. 



Vulcanism affects the sea-bottom much as it affects the land. 

 At the volcanic centers, where the great body of extruded matter 

 accumulates, mounds and mountains are built up, and most of the 

 mountain peaks of the sea-bottom had a volcanic origin. Where 

 volcanic cones are built up near the surface of the sea, they often 

 furnish a home for shallow-water life, such as polyps. Wherever 

 they are built up so as to be within the reach of waves, gradational 

 processes are stimulated. 



The number of acti 3 volcanoes on islands is about 200, but the 

 number of active vents beneath the sea is unknown. A few sub- 

 marine eruptions have been observed, but those observed are prob- 

 ably but a small percentage of those which have taken place in his- 

 toric time, for slight eruptions in deep water might not be seen at 

 the surface, even if observers were near. 



Oceanic volcanoes affect both the temperature and the composi- 

 tion of the sea-water. Both the increase of temperature and the 

 solution of volcanic gases increase the solvent power of water, 

 and both the change in temperature and composition affect the life 

 of the adjacent waters. The decay of organisms destroyed during 

 eruptions generates gases, and these gases may lead to further 

 chemical changes. Volcanoes in the sea have furnished much of 

 the material now found on the bottom of the ocean. Some of this 

 material is very fine, like volcanic dust, and some of it is much 

 coarser. Both the very fine material arid the pumice may be dis- 

 tributed far from the volcanoes which emit them. As a matter of 

 fact, they are found nearly everywhere on the bottom of the deep 

 sea, though not in uniform abundance. From these considerations 

 it is apparent that the effects of oceanic volcanoes on the sea-water 

 are considerable, when long periods of time are considered. 



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 predom- 

 inates and degradation is subordinate. On the land, degradation 

 is greatest, on the whole, where the land is highest, while aggrada- 

 tion is of consequence only where the land is low, OP where steep 



