420 NATURAL SCIENCE. august. 



the purpose of indicating the isolation of the continents from each 

 other throughout the equatorial and most of the temperate zones, 

 there is very little to choose between the 1,000-fathom or the 

 2,000-fathom boundary. The latter, however, allows more scope for 

 possible land extensions between the three southern continents and 

 the Antarctic lands, which, during mild epochs, and by means 

 of intervening islands, may, perhaps, have served as a means of 

 communication between these continents. All that is necessary to 

 maintain, therefore, is that existing continents with their included 

 seas, and their surrounding oceanic waters as far as the 

 1,500-fathom, or in some extreme cases the 2,000-fathom line, 

 mark out the areas within which the continental lands of the globe 

 have been built up ; while the oceanic areas beyond the 2,000-fathom 

 line, constituting, according to Mr, Murray's data, 71 percent, of the 

 whole ocean, have almost certainly been ocean throughout all known 

 geological time.- 



It will now be seen that this is a problem which deals with the 

 very broadest contrasts of the earth's surface, and that its funda- 

 mental data are on so vast a scale as not to be materially affected by 

 the smaller details of physical geography, or by differences of opinion 

 as to the exact meaning of certain terms. Whether a particular 

 island is more correctly classed as oceanic or continental, whether a 

 certain portion of the ocean should be placed within the oceanic or 

 the continental area, and whether certain rocks were formed in very 

 deep or in comparatively shallow water, are of slight importance, 

 except in so far as they may throw light on the real question, which 

 is, whether the vast expanses of ocean beyond the 1,000-fathom line, 

 forming about 92 per cent, of the whole oceanic area, have ever been 

 occupied, or extensively bridged over, by continental land. It is 

 towards the solution of this great problem that I now propose to 

 submit certain general considerations which appear to me to lie at 

 the root of the whole matter. 



Comparison of Oceanic and Continental Masses. — In the paper already 

 referred to, Mr. John Murray has carefully estimated both the area 

 of the land and of the water on the earth's surface, and their bulk as 

 deduced from the best available data. Taking the whole area of the 

 globe as loo, he finds the land surface to be 28, the water surface 

 72. But the mean height of the land above sea-level is 2,250 feet, 

 while the mean depth of all the oceans and seas is 12,456 feet. In 

 this estimate, however, all the inland seas and shallow coast waters 

 are included, and as these, at least as far as the loo-fathom line, are 

 universally admitted to be within the " continental area," we omit 

 them in our estimate of the mean depth of the oceans proper, which 

 are thus brought to something over 15,000 feet, or nearly seven times 

 as much as the mean height of the land. 



2 Mr. Murray gives his results for oceans and inland seas together. The 

 above refers to the oceans only as usually understood; but the difference is not great. 



