GREATEST DEPTHS 23 



at no great distance from, the mainland, or in the 

 vicinity of volcanic islands. One of the deepest 

 ' pockets ' yet found is probably that sounded by the 

 American expedition on board the Tuscarora (1873- 

 1875) east of Japan, when bottom was only reached at 

 a depth of 4,612 fathoms. More recently, soundings 

 f 5>035 fathoms have been recorded in the Pacific, in 

 the neighbourhood of the Friendly Islands, and south 

 of these again, one of 5,113 fathoms; but the deepest 

 of all lies north of the Carolines, and attains a depth 

 of 5,287 fathoms. It thus appears that there are 

 1 pockets ' or pits in the sea whose depth below the 

 surface of the water is about equal to the height of the 

 highest mountains taken from the sea-level. Both are 

 insignificant in comparison with the mass of the globe; 

 and it is sometimes said that, were the seas gathered 

 up, and the earth shrunk to the size of an orange, the 

 mountain ranges and abysmal depths would not be 

 more striking than are the small elevations and inter- 

 vening depressions on the skin of the fruit. 



But it is not with these exceptional abysses that we 

 have to do ; they are as rare and as widely scattered 

 as great mountain-ranges on land. It is with the deep 

 sea, as opposed to shoal water and the surface layers, 

 that this article is concerned ; but the depth at which 

 the sea becomes 'deep' is to some extent a matter 

 of opinion. Numerous attempts, headed by that of 

 Edward Forbes, have been made to divide the sea into 

 zones or strata ; and, just as the geological strata are 

 characterized by peculiar species, so, in the main, the 

 various deep-sea zones have their peculiar fauna. 

 These zones, however, are not universally recognized ; 

 and their limits, like those of the zoogeographical 

 regions on land, whilst serving for some groups of 

 animals, break down altogether as regards others. 

 There are, however, two fairly definite regions in the 

 sea ; and the limit between them is the very one for 



