3^6 NATURE AND MAN. 



with in the real oceanic area (as distinguished from those which, 

 like the British Isles, are really outlying parts of the slightly 

 sunken corner of the platform which rises into continental land in 

 their vicinity; or which, like the great islands of the Malayan 

 Archipelago, are the " survivals " of a continental platform more 

 deeply submerged) are all of Volcanic origin, having been pro- 

 jected upwards from beneath, instead of having gone down from 

 above. This may be stated with confidence in regard to all those 

 which consist of inorganic rocks ; and since it is equally true of 

 those coral islands whose rock basis shows itself above the surface, 

 the same maybe fairly presumed in regard to the submerged peaks 

 on which those "atolls" rest, above whose level platforms no 

 rocky base now rises. These volcanic vents are generally found 

 on upward bulgings of median portions of the depressed ocean- 

 floors ; whilst, on the other hand, the volcanoes which rise from 

 the elevated land-platforms are for the most part thrown up near 

 their oceanic margins; and Professor Dana gives mechanical 

 reasons for both these classes of facts, deduced from consideration 

 of the mode in which the horizontal thrust will be exerted in the 

 two areas respectively. The " crumpling " of the elevated portions 

 of the crust which throws up mountain ridges, produces at the 

 same time equivalent depressions, and these will be filled by sea- 

 water if it has access to them, as is the case with the enormously 

 deep pit-holes found in various parts of the Malayan area; or 

 with fresh waters where, being cut off from the sea, they are sur- 

 rounded by a mountainous region affording a large supply of it, as 

 in deep lake-basins of Switzerland ; or they may remain almost 

 empty for want of water, like the deeply depressed valley of the 

 Jordan ; or may be partly filled, like the Caspian. And thus the 

 distribution of land and water over different parts of the Con- 

 tinental platforms may have been greatly changed from time to 

 time, and groups or chains of islands may have been raised and 

 again submerged in the Oceanic area, without making any such 

 essential changes in the Map of the World as Sir Charles Lyell 

 supposed to have taken place over and over again. 



Now this view of the permanence of the great original division 

 of the crust of the earth into elevated and depressed areas, and of 

 the non-conversion of any considerable part of a continental plat- 



