TRANSACTIONS OF SECTION £. 791 
T have redrawn the several contour-lines of depth in the great ocean-basins, 
after careful consideration of the most recent data, and these may now be re- 
_ garded as a somewhat close approximation to the actual state of matters, with 
the possible exception of the great Southern and Antarctic Oceans, where there 
are relatively few soundings, but where the projected Antarctic Expeditions should 
soon be at work, On the whole, it may be said that the general tendency of 
recent soundings is to extend the area with depths greater than 1,000 fathoms, 
and to show that numerous volcanic cones rise from the general level of the floor 
of the ocean-basins up to various levels beneath the sea-surface. 
The areas marked out by the contour-lines of depth are now estimated as 
follows :— 
Between the shore and 100 fms. 7,000,000 sq. geo. m. (or 79% of the sea-bed) 
. 100 ,, 1,000 ,, 10,000,000 ,, ,,  (or10% ,, ion i 
“ 1,000 ,, 2,000 ,, 22,000,000 _,, is (or 21% _ ,, is 
ré 2,000 ;, 3,000 ,, 57,000,000 ,, | (or65% ~ MS 
Over 3,000 fathoms, 7,000,000 , 5 (or 7% , » ) 
103,000,000 sq. geo. m. 100 per cent. 
From these results it appears that considerably more than half of the sea 
floor lies at a depth exceeding 2,000 fathoms, or over two geographical miles. It 
is interesting to note that the area within the 100-fathom line occupies 7,000,000 
square geographical miles, whereas the area occupied by the next succeeding 900 
fathoms (viz. between 100 and 1,000 fathoms) occupies only 10,000,000 square 
geographical miles. This points to a relatively rapid descent of the sea-floor along 
the continental slopes between 100 and 1,000 fathoms, and therefore confirms the 
results gained by actual soundings in this region, many of which indicate steep 
inclines or even perpendicular cliffs. Not only are the continental slopes the seat 
of many deposit-slips and seismic disturbances, but Mr. Benest has given good 
reasons for believing that underground rivers sometimes enter the sea at depths 
beyond 100 fathoms, and there bring about sudden changes in deep water. Again, 
the relatively large area covered by the continental shelf between the shore-line 
and 100 fathoms points to the wearing away of the land by current and wave 
action. 
On the Challenger charts all areas where the depth exceeds 3,000 fathoms 
have been called ‘ Deeps,’ and distinctive names have been conferred upon them. 
Forty-three such depressions are now known, and the positions of these are shown 
on the map here exhibited; twenty-four are situated in the Pacific Ocean, three 
in the Indian Ocean, fifteen in the Atlantic Ocean, and one in the Southern and 
Antarctic Oceans, The area occupied by these thirty-nine deeps is estimated at 
7,152,000 square geographical miles, or about 7 per cent. of the total water-surface 
of the globe. Within these deeps over 250 soundings have been recorded, of which 
twenty-four exceed 4,000 fathoms, including three exceeding 5,000 fathoms. 
Depths exceeding 4,000 fathoms (or four geographical miles) have been recorded 
within eight of the deeps, viz. in the North Atlantic within the Nares Deep; in 
the Antarctic within the Ross Deep; in the Banda Sea within the Weber Deep ; 
in the North Pacific within the Challenger, Tuscarora, and Supau Deeps ; and in 
the South Pacific within the Aldrich and Richards Deeps. Depths exceeding 
5,000 fathoms have been hitherto recorded only within the Aldrich Deep of the 
South Pacific, to the east of the Kermadecs and Friendly Islands, where the 
greatest depth is 5,155 fathoms, or 530 feet more than five geographical miles, 
being about 2,000 teet more below the level of the sea than the summit of Mount 
Everest in the Himalayas is above it. The levels on the surface of the lithosphere 
thus oscillate between the limits of about ten geographical miles (more than 
eighteen kilometres). 
Temperature of the Ocean-floor. 
Our knowledge of the temperature on the floor of the ocean is derived from 
observations in the layers of water immediately above the bottom by means of 
