HAS THE EXTENT OF LAND SURFACE BEEN INCREASING? 213 



by recent liydrographic explorations, namely, that the mean depth of the 

 ocean is much greater than the mean elevation of the land above the sea- 

 level. This, taken in connection -with the fact of the much larger area of 

 ocean than of land on the globe, means, of course, that certain regions of 

 the earth have assumed a slightly elevated position with reference to the 

 arbitrary datum-line above mentioned ; and that cei'tain other much more 

 extensive regions have, synchronously with that elevation of the higher 

 smaller portion of the earth, become very considerably depressed. This fact 

 adds great strength to the argument in favor of the joermancncy of the con- 

 tinents derived from the superior simplicity of that view as compared with 

 the opposite one. 



As to what has happened over the subsiding areas of the earth's surfoce 

 during the geological ages we know little with certainty beyond the fact 

 of the subsidence itself, because they have been for the most part deeply 

 covered by water. The areas of elevation, on the other hand, we know to 

 have been subject to disturbances, very different in character and amount, 

 however, in different regions. The larger part of the land of the globe has 

 not been raised to a very great elevation above the sea-level ; certain other 

 regions of limited extent, on the other hand, have been ; and in a few places 

 an elevation has been attained just about as great as the greatest depth of 

 the deepest known portions of the ocean. These loftiest areas belong to the 

 great mountain ranges of the globe, and are the culminating points of certain 

 regions which appear to have been rising during a long succession of geo- 

 logical ages, since they have accumulated on their flanks stratified masses 

 largely formed from the detritus brought down from the adjacent higher 

 regions, and which from the character of the fossil remains which they con- 

 tain are known to have occupied a long succession of ages in their formation. 

 Other ranges, of le.ss elevation, are found to belong to regions over which the 

 elevatory force has long ago ceased to act. But the fact that the great 

 mountain chains — the Himalayas, the Andes, the Cordilleras of North 

 America, the Alps — all have included within their masses strata of various 

 ages, down to and including the Tertiary, is proof that, on the whole, the 

 tendency of high regions has been to rise still higher. This is another fact 

 rendering probable the correctness of the view that the continents and the 

 oceans have not interchanged positions. This theory cannot, however, be 

 considered as having been proved with certainty' by alignments drawn from 

 the class of facts to which our attention has thus far been turned. 



