82 SEA AND LAND 



Accepting this view as to the tolerable })ermanence of land 

 and sea with the greater assurance, because it has slowly won 

 its way to belief over the previous opinions and prejudices of 

 students, we are prepared to fintl that the conditions of the 

 ocean-bottom are in man)' regards entirely unlike those of the 

 land. It is easy to see that the shape of the land is mainly 

 determined b)' a contest between the down-wearing action of 

 the water which falls upon its surface as rain or snow, or is 

 swung against it in tides and waves, and the uprising movement 

 which lifts the mass of the continent and wrinkles the under- 

 lying rocks into mountain folds. Rivers and glaciers have 

 battled with these ascending masses of strata, they have 

 carved out the valleys and gorges which fret the land in 

 every direction ; even the plains which appear to feel little of 

 this erosive action generally owe their horizontal aspect to 

 the fact that the wearinof assents have done their most effec- 

 tive work in these areas. We readily perceive that all this 

 mighty machinery of flowing rivers and beating waves has no 

 place in the depths of the sea. It is true that the ocean has 

 great streams upon its surface, some of which, for certain 

 parts of their courses, move with the speed of the larger 

 rivers; but these swift currents are superficial things; they 

 rarely if ever touch the bottom except where they come upon 

 the shores of continents or islands, but flow upon a base of 

 deep-lying nearly motionless water. While everywhere upon 

 the land, e\en in the most arid regions, there are occasional 

 rains, and for a time the torrents do their appointed work of 

 wearing away the surface of the earth, the deeper ocean-lloors 

 are practically always the seat of deposition, that is, they are 

 receiving contributions of sediment from the rivers of the 

 land, from the waves and tides of the shores, from the vast 



