172 WORK OF THE OCEAN 



submerged valleys beyond the coast-line (Fig. 175). Submerged 



river valleys show that the surface in which they lie was once land. 



j The effects of diastrophism'in the ocean and about its borders, 



jmay (i) make the water of any ocean, or of any part of it, shallower 



/or deeper; (2) cause the emergence or submergence of land; (3) make 



-coast-lines regular or irregular; (4) shift the habitat of many forms 



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



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



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

 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 

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

 ever they are built up so as to be within the reach of waves, grada- 

 tional processes are stimulated. 



The number of active 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 take place, for eruptions 

 in deep water may not be seen at the surface. 



Oceanic volcanoes affect both the temperature and the composi- 

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

 volcanic gases increase the solvent power of water, and both the 

 change in temperature and composition affect the life of the adja- 

 cent waters. Volcanoes in the sea have furnished much of the 

 sediment now found on the bottom of the ocean. Some of it is very 

 fine, like volcanic dust, and some of it is coarse. Both the fine and 

 the coarse are distributed far from the volcanoes which emit them, 

 are found indeed nearly everywhere on the bottom of the deep sea, 

 though not in uniform abundance. It is therefore clear 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, or where steep 

 slopes give place to gentle ones. In the sea, degradation is vir- 



