350 PHYSIOGRAPHY 



sources. Some of them were thrown out from volcanoes, some 

 were blown out from the land, some were probably derived from 

 the shells, etc., of marine life, and still others came from meteors. 

 On the lands there is rock (conglomerate) composed of cemented 

 gravel, rock (sandstone) composed of cemented sand, rock (shale) 

 composed of cemented mud, and rock (limestone) composed of 

 material derived from shells, corals, etc. None of these seem to be 

 deep-sea oozes cemented together, and none correspond to the red 

 clay of the very deep sea. In the lands, therefore, there are rocks 

 corresponding to all the sediments now making in the shallow 

 water of the sea, but none corresponding to those of the very deep 

 waters. This suggests (1) that most lands have been, at some time, 

 beneath the sea, and (2) that, so far as now known, no part of the pres- 

 ent continents was ever at the bottom of the deep ocean. 



RELATION OF THE SEA TO THE REST OF THE EARTH 



The ocean is of great importance to the rest of the earth, in 

 ways which have been pointed out already. By way of summary 

 they may be brought together here. 



f. Waves change the coast-lines; they wear away the land in 

 some places and build new land in others. On the whole, destruc- 

 tion exceeds building, so far as the land is concerned. The ocean 

 therefore tends to extend itself at the expense of the land. 



2. Oceans modify the climate of the land, affecting both tem- 

 perature and precipitation. The effect on temperature comes 

 from the fact that water is heated and cooled more slowly than 

 land is. The air over the sea, therefore, has a lesser range of tem- 

 perature than that over the land, and blowing to the land tends 

 to carry the temperature of the sea over to it. Winds from the 

 ocean make the lands to which they blow less cold in winter and 

 less hot in summer than they would be if there were no ocean. 



The climatic effect of the sea on the land is felt most on the west 

 sides of the continents in the zones of westerly winds, and on the 

 east sides of the continents in the zones of easterly (trade) winds. 

 The cold currents of the sea have much less effect than warm ones 

 on the climate of the land, because they lie along the east sides of 



