242 J. Starlcie Gardner — Permanence of Continents and Oceans. 



than the present 1000-fathom line. Again, the geohigical evidence, 

 as I have pointed out in the Popular Science Review, is far from 

 being so favourable to Mr. Wallace's view, as he supposed. 



Apart from the regions of less depth, which I think may have 

 been more or less land during the Tertiary period, there is some 

 reason to believe in the general permanence of the Oceans over the 

 areas where they are now deepest. It is perfectly certain that the 

 causes which lead to elevation and subsidence must react upon each 

 other, and if these were exclusively the result of shrinkage, there would 

 be no reason why the sea-bottom at the greatest depths should not 

 have come to the surface. With a layer fluid under a given pressure, 

 resting upon solid, and sensitive to any increase or decrease of pressure, 

 the chief effects of elevation and subsidence could he explained.^ 



Many persons have been struck with the ahnost universal 

 tendency to depression exhibited in areas occupied by deltas and 

 estuaries. This fact has frequently been alluded to in the Geolo- 

 gical Magazine, and elsewhere, and has been most clearly ex- 

 pressed by Dr. Charles Eicketts,- that this subsidence is directly 

 produced by the accumulation of sediment. However insignificant, 

 some cause must initiate movement in the earth's crust, and as an 

 incautious shout may bring down an avalanche, so even an accumu- 

 lation of a few feet of clay over several square miles may create a 

 disturbing re-adjustment, and eventually lead to a downward tendency. 

 Supposing a sediment, 60 feet in depth and entirely submerged, to 

 have displaced an equivalent of sea-watei', we should have an in- 

 creased pressure per square yard (taking the mean density of the 

 materials composing a delta at 170 lbs. per cubic foot) of rather more 

 than 25,000 lbs., or about 34,848,000 tons per square mile. When 

 we see that deltas have accumulated to depths of perhaps even 

 beyond 1000 feet, and extend, as in the Mississippi, to 19,450 square 

 miles, it is easy to realize how vast a force is present.^ 



The hypothesis that added weight leads to subsidence may also 

 to some extent be sustained by the continuous depression of Coral 

 Islands. Great accumulations of ice in the Glacial period seem to 

 have been accompanied by subsidence, and even Greenland at the 

 present day may be sinking under its ice-cap. 



To apply the theory to a wider field, we frequently observe signs 

 of subsidence on the sea-coast. We meet on every shore with 

 vestiges of submerged land vegetation and with traditions of sub- 

 mergence since historic times. Though raised beaches exist, it 

 should be remembered that these are local and always rendered con- 

 spicuous, while depressed beaches seldom or never attract attention. 

 Forests have been depressed beneath the sea-level and no trace 

 of them has ever come to light, except at low spring-tides and in 



' According to Lyell, all known rocks would fuse under a pressiu'e of from 20 to 

 25 miles, whilst greater pressure would reconvert them to solids with a high specific 

 gravity. 



2 Geol. Mag. 1872, Vol. IX. p. 119. 



^ See Permanence of Continents, J. S. Gardner, Popular Science Eeview, 1881, 

 p. 125. 



