332 GEOLOGY, 



The ragged coast of Maine is another example, though glaciation as well 

 as subsidence has been operative here. From the present configura- 

 tion of coast-Hnes, it has been inferred that the present is, on the whole, 

 an era of continental depression/ River valleys, the lower ends of 

 which are embayed, are sometimes found to be continuous with sub- 

 merged valleys beyond the coast-line (Fig. 298). Submerged river 

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



Bays may be developed by local subsidence as well as by the sub- 

 merging of valleys, though decisive examples are not readily cited. 

 Bays may also be produced by uplift of the surface en either side of 

 an area which does not change its level. For example, upHft on either 

 side of the Gulf of California has probably been one element, though 

 probably not the only one, in the development of this indentation. The 

 general outline of a great bay produced by coastal warping might be 

 regular, though it would be Hkely to be marked by small irregularities 

 where the streams enter. It is not to be understood that all, or even 

 most, bays are due to local diastrophism. 



Diastrophism, then, as it affects the ocean-bottoms and the ocean- 

 borders, may make the water of any ocean shallower or deeper; it may 

 cause the emergence or submergence of land; it may make coast-hnes 

 regular or irregular; it may shift the habitat of life, and through these 

 changes may greatly influence the processes of gradation, which are 

 especially active along 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 accumu- 

 lates, mounds and mountains are built up. Most of the mountain 

 peaks of the sea-bottom, whether their crests are islands, or whether 

 they are wholly submerged, have had a volcanic origin. The rock 

 material ejected from submarine vents is probably less widely distributed 

 than that from vents on land, and so far forth, the volcanic cones 

 in the oceans are steeper than those on land. 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 built up so as to be within 

 the reach of waves, gradational processes are stimulated. 



The processes of vulcanism do not commonly influence coasts of 

 continents directly, for few volcanoes He immediately on coasts. In 



* J. Geikie. Earth Sculpture, p. 329. 



