86 ISLAND LIFE 



coast line at which such depths are reached is seldom less 

 than twenty miles, and is very frequently more than a 

 hundred, while in some cases such shallow seas extend 

 several hundred miles from existing continents. The great 

 depth of a thousand fathoms is often reached at thirty 

 miles from shore, but more frequently at about sixty or a 

 hundred miles. Round the entire African coast for 

 example, this depth is reached at distances varying from 

 forty to a hundred and fifty miles (except in the Red Sea 

 and the Straits of Mozambique), the average being' about 

 eighty miles. 



Now the numerous specimens of sea-bottoms collected 

 during the voyage of the Challenger show that true shore- 

 deposits — that is, materials denuded from the land and 

 carried down as sediment by rivers — are almost always 

 confined within a distance of 50 or 100 miles of the coast, the 

 finest mud only being sometimes carried 150 or rarely 200 

 miles. As the sediment varies in coarseness and density it 

 is evident that it will sink to the bottom at unequal 

 distances, the bulk of it sinking comparatively near shore, 

 while only the very finest and almost impalpable mud will 

 be carried out to the furthest limits. Beyond these limits 

 the only deposits (with few exceptions) are organic, con- 

 sisting of the shells of minute calcareous or siliceous 

 organisms with some decomposed pumice and volcanic dust 

 which floats out to mid-ocean. It follows, therefore, that 

 by far the larger part of all stratified deposits, especially 

 those which consist of sand or pebbles or any visible frag- 

 ments of rock, must have been formed within 50 or 100 

 miles of then existing continents, or if at a greater distance, 

 in shallow inland seas receiving deposits from more sides 

 than one, or in certain exceptional areas where deep ocean 

 currents carry the debris of land to greater distances.^ 



^ In his Preliminary Report on Oceanic Deposit, Mr. Murray says : — "It 

 has been found that the deposits taking place near continents and islands 

 have received their chief characteristics from the presence of the debris 

 of adjacent lands. In some cases these deposits extend to a distance of 

 over 150 miles from the coast." (Proceedings of the Royal Society, 

 Vol. XXIV. p. 519.) 



' ' The materials in suspension appear to be almost entirely deposited 

 within 200 miles of the land." (Proceedings of the Royal Society of Edin- 

 burgh, \^1Q-11, p. 253.) 



