84 



ISLx\ND LIFE. 



[part I. 



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, consisting 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 con- 

 sist of sand or pebbles or any visible fragments of rock, must 

 have been formed within 50 or 100 miles of then existing con- 

 tinents, 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.^ 



If we now examine the stratified rocks found in the very centre 

 of all our great continents, we find them to consist of sandstones, 

 limestones, conglomerates, or shales, which must, as we have 

 seen, have been deposited within a comparatively short distance 

 of a sea-shore. Professor Archibald Geikie says : — " Among the 



1 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, 1876-77, p. 253.) 



