xi The Bed of the Oceans 201 



Orkney, where the Old Man of Hoy is a magnificent stack 

 450 feet in height. 



267. Origin of the Continental Shelf. The action of 

 waves and tidal currents usually ceases to be perceptible at 

 the depth of 100 fathoms. Beach deposits swept seaward 

 by the waves assist in scooping out and deepening the 

 shore, the final result being, possibly, to eat inward along 

 the top of the wall of the world ridge until a depth of 100 

 fathoms is attained. The continental shelf is widest on the 

 margins of the oldest continents exposed to the heaviest 

 waves, and may be compared to the line which some 

 chemical solutions etch on the glass bottles containing 

 them. Harder masses resisting the attacks of the waves 

 remain as islands or shoals on the continental shelf. 

 Where currents sweeping mud and sand to and fro are 

 checked by some inflection of the coast -line, sandbanks 

 are formed. In many cases it is possible that the con- 

 tinental shelf is the end of a low plain submerged by 

 subsidence ; in others a low plain may be an upheaved 

 continental shelf, and probably wave action is only one of 

 the factors at work. Long furrows of great depth cross it 

 in some places. These grooves and submarine canons 

 ( 326) have a peculiar interest, because they seriously 

 detract from the usefulness of the continental shelf as a 

 guide to sailors groping their way to land by means of the 

 sounding-line in foggy weather. 



268. Marine Deposits. Immense quantities of sedi- 

 ment are carried down by rivers into the sea (331). M. 

 de Lapparent calculates the amount as 33 times greater than 

 all the sand, gravel, and pebbles worn off by tidal and 

 solar energy acting through waves and currents on the 

 coasts. Countless myriads of plants and animals living in 

 the water affect the substance in solution ( 222), forming 

 shells or skeletons which at their death fall to the bottom, 

 producing various kinds of deposits. Sea-water acts chemi- 

 cally on substances exposed to it, producing a further series 

 of changes. In all parts of the ocean not precipitous nor 

 swept by strong currents, the original rock is covered with 

 a mantle of deposits of various thickness, to which the 



