1880.] on Land and Sea in relation to Geological Time. 271 



the Land above the sea-level with that of the Water which occupies 

 the Ocean-basins, a far greater clisiiroportion shows itself. For the 

 average elevation of the whole Land of the globe certainly does not 

 exceed 1000 feet ; — that of Asia and Africa being somewhat above 

 that amount, while that of America (North and South), Europe, and 

 Australia is considerably below it. On the other hand, the average 

 depth of the Ocean-basins is now known to be rather over than under 

 2^ miles, and may be taken (for the convenience of a round number) 

 at 13,000 feet. Thus the average depth of the ocean being thirteen 

 times as much as the average height of the land, and the area of the 

 sea being 2 J times that of the land, the total volume of the Ocean-ivafer 

 is just thirty-six times that of the Land above the sea-level. 



The Northern hemisphere is pre-eminently the land hemisphere, 

 and the Southern the water hemisphere ; and the distribution of the 

 two components of their respective surfaces, so far from being 

 " capricious " (Lyell), is found to have a remarkable symmetry. It 

 is between lat. 30° and 70° that Water most predominates in the Southern 

 hemisphere ; the Southern Ocean forming a continuous girdle around 

 it between Cape Horn, lat. 56° S., and the Antarctic continental 

 platform. On the other hand, it is between lat. 30° and 70° that 

 Land most predominates in the Northern hemisphere, girdling nine- 

 tenths of its circumference between lat. 60° N. and the Arctic 

 Ocean. 



The great land-masses of the Northern hemisphere send down 

 three extensions into the Southern, viz. South Africa, South America, 

 and the Papuo-Australian continent ; which last may be considered as 

 the southward extension of the Asiatic, being connected with it by a 

 nearly continuous though partly submerged continental platform, of 

 which the peninsula and archipelago of Malaya are the most elevated 

 portions. It is further remarkable that each of these southward 

 extensions is almost entirely detached from its northern land-mass 

 by an intervening sea;— South from North America by the Gulf of 

 Mexico and Caribbean Sea ; Africa from Europe and Asia by the 

 Mediterranean and Eed Seas ; and the Malayan continental platform 

 from south-east Asia by the shallow Yellow Sea, and by those smaller 

 seas, some of them remarkable for their depth, that lie among the great 

 islands of the Malay Archipelago — the interrwption in each case coin- 

 ciding with an area of great Volcanic activity. 



On the other hand, the vast Oceanic area of the Southern hemi- 

 sphere sends three great extensions northwards ; the Pacific, the 

 Atlantic, and the Indian Oceans, of which the two former are pro- 

 longed as far as the North Polar area. 



But the existing borders of these Oceans by no means correspond 

 with the borders of their real basins. The deep-sea soundings of the 

 ' Challenger ' have brought out this remarkable fact — that the ocean- 

 floors present a uniformity of level which corresponds with that of 

 our most level and extensive Continental plains ; so that in long 

 section-lines the differences of depth (when represented on true pro- 



