56 



THE OPAL SEA 



Sea 

 troughs. 



down walls of rock mile after mile. There is 

 some such slashed chasm of the sea off the 

 coast of Porto Rico where the soundings give 

 a depth of 27,366 feet, and there is a " trough " 

 in the Pacific, a hundred miles off the Kurile 

 Islands, where soundings show a depth of over 

 five miles. 



These hollows of the sea bed, as we have 

 seen, suggest a correspondence to heights of the 

 land; and yet it must be repeated that they 

 should not be looked upon as inverted Hima- 

 layas. The sea trough is not exactly hewn out 

 of rock. It has no sharp edges; all its sur- 

 faces are worn smooth, not so much by erosion 

 as by terrific pressure; and all its substances 

 are honeycombed and softened by the action of 

 carbonic-acid gas. The bottoms of the pit must 

 be mere sinks of ooze. They cannot have the 

 hard surfaces and sharp fractures of the cliff 

 wall. The chemical action of the underlying 

 water would make it quite impossible. Besides, 

 the dredge keeps repeating the tale of ooze. 



The shore beds, lying off the continents, 

 are more varied than the depths. The cut- 

 tings of rivers, the rush of the tides into 

 gulfs and bays, the pound of the breakers on 

 the coast, help to create many irregularities 

 along the meeting place of land and sea. Then, 



Sinks of 

 ooze. 



