84 
ISLAND 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 ddbris of land to greater 
distances. 1 
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. XXIY. 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.) 
